Copyright © 2021-2026 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU General Public License, Version 3 or any later version published by the Free Software Foundation.  A copy of the license is included in the section entitled ``GNU General Public License''.

The SFrame format
*****************

This manual describes Version 3 of the SFrame file format.  SFrame stands for Simple Frame.  The SFrame format keeps track of the minimal necessary information needed for generating stack traces:

  - Canonical Frame Address (CFA).

  - Frame Pointer (FP).

  - Return Address (RA).

The reason for existence of the SFrame format is to provide a simple, fast and low-overhead mechanism to generate stack traces.

Introduction
************

Overview
========

The SFrame stack trace information is provided in a loaded section, named .sframe.  When available, the .sframe section appears in segment of type PT_GNU_SFRAME.  An ELF SFrame section will have the type SHT_GNU_SFRAME.

The SFrame format is currently supported only for select ABIs, namely, AMD64, AAPCS64, and s390x.

A portion of the SFrame format follows an unaligned on-disk representation. Some data structures, however, (namely the SFrame header and the SFrame function descriptor index) have elements at their natural boundaries.  All data structures are packed, unless otherwise stated.

The contents of the SFrame section are stored in the target endianness, i.e., in the endianness of the system on which the section is targeted to be used. An SFrame section reader may use the magic number in the SFrame header to identify the endianness of the SFrame section.

Addresses in this specification are expressed in bytes.  The use of term `data word' in this document is colloquial; it should not be understood to correlate with the architectural machine word or any specific hardware data width.

The rest of this specification describes the current version of the format, SFRAME_VERSION_3, in detail.  Additional sections outline the major changes made to each previously published version of the SFrame stack trace format.

This document is intended to be in sync with the C code in sframe.h. Please report discrepancies between the two, if any.

Changes from Version 2 to Version 3
===================================

The following is a list of the changes made to the SFrame stack trace format since Version 2 was published.  Note that SFrame Version 2 had up to two Errata.

  * Terminology improvements and renames for readability

       - Use the terminology `PC offset' in place of `Addr' for function start PC offset consistently.

       - Make a distinction between SFrame FDE Type (e.g., SFRAME_FDE_TYPE_DEFAULT, SFRAME_FDE_TYPE_FLEX) vs SFrame FDE PC Type (i.e., SFRAME_FDE_PCTYPE_MASK, SFRAME_FDE_PCTYPE_INC).

       - Instead of using the term `info word', use a more precise term `info byte' in specification for the info bytes in SFrame FDE and SFrame FRE.

       - Use term `data word' instead of `offset' to convey the functional role of the variable-length array of bytes trailing the SFrame FRE header.  With the introduction of flexible FDE type, the interpretation of those bytes is not always as an offset.

       - Rename SFRAME_FRE_OFFSET_<N>B to SFRAME_FRE_DATAWORD_<N>B.

  * Reorganize the SFrame function descriptor entry into two distinct structures:

       - SFrame function descriptor index

       - SFrame function descriptor attribute

     Rename structure members as a consequence.

  * Narrow the width of sfda_func_num_fres to uint16_t and remove padding field sfde_func_padding2.

  * Increase the width of the sfdi_func_start_offset to int64_t.  This field is renamed from the sfde_func_start_address in SFrame Version 2 specification.

  * Signal frames are marked with one bit in sfda_func_info.

  * Addition of a new function info byte sfda_func_info2 in SFrame function descriptor attribute structure to store additional information about the stack trace data for the function.

  * Reserve 5-bits for FDE types.  Define two FDE types: default FDE type SFRAME_FDE_TYPE_DEFAULT, and flexible FDE type SFRAME_FDE_TYPE_FLEX.

  * Define a new FDE type SFRAME_FDE_TYPE_FLEX to convey stack trace information for specific cases, e.g., when CFA is non-SP/FP based, or when FP/RA recovery is REG-based.

  * An SFrame FDE of type SFRAME_FDE_TYPE_DEFAULT with no FREs is used to indicate an outermost frame.

  * On s390x, use FDE type SFRAME_FDE_TYPE_FLEX to encode FP/RA recovery from REG, instead of encoding DWARF register number in the SFrame FRE variable-length data of FDE type SFRAME_FDE_TYPE_DEFAULT.

Changes from Version 1 to Version 2
===================================

The following is a list of the changes made to the SFrame stack trace format since Version 1 was published.

  * Add an unsigned 8-bit integral field to the SFrame function descriptor entry to encode the size of the repetitive code blocks.  Such code blocks, e.g., pltN entries, use an SFrame function descriptor entry of type SFRAME_FDE_PCTYPE_MASK.

  * Add an unsigned 16-bit integral field to the SFrame function descriptor entry to serve as padding.  This helps ensure natural alignment for the members of the data structure.

  * The above two imply that each SFrame function descriptor entry has a fixed size of 20 bytes instead of its size of 17 bytes in SFrame format Version 1.

  * [Errata 1] Add a new flag SFRAME_F_FDE_FUNC_START_PCREL, as an erratum to SFrame Version 2, to indicate the encoding of the SFrame FDE function start address field:

       - if set, sfde_func_start_offset field contains the offset in bytes to the start PC of the associated function from the field itself.

       - if unset, sfde_func_start_offset field contains the offset in bytes to the start PC of the associated function from the start of the SFrame section.

  * [Errata 1] Add a new ABI/arch identifier SFRAME_ABI_S390X_ENDIAN_BIG for the s390 architecture (64-bit) s390x ABI.  Other s390x-specific backward compatible changes including the following helper definitions have been incrementally added to SFrame Version 2 only:

       - SFRAME_S390X_SP_VAL_OFFSET: SP value offset from CFA.

       - SFRAME_V2_S390X_OFFSET_IS_REGNUM: Test whether FP/RA offset is an encoded DWARF register number.

       - SFRAME_V2_S390X_OFFSET_ENCODE_REGNUM: Encode a DWARF register number as an FP/RA offset.

       - SFRAME_V2_S390X_OFFSET_DECODE_REGNUM: Decode a DWARF register number from an FP/RA offset.

       - SFRAME_FRE_RA_OFFSET_INVALID: Invalid RA offset value (like SFRAME_CFA_FIXED_RA_INVALID).  Used on s390x as padding offset to represent FP without RA saved.

       - SFRAME_S390X_CFA_OFFSET_ADJUSTMENT: CFA offset (from CFA base register) adjustment value.  Used to enable use of 8-bit SFrame offsets on s390x.

       - SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR: CFA offset alignment factor. Used to scale down the CFA offset to improve the use of 8-bit SFrame offsets.

       - SFRAME_V2_S390X_CFA_OFFSET_ENCODE: Encode CFA offset (i.e., apply CFA offset adjustment and then scale down by CFA offset alignment factor).

       - SFRAME_V2_S390X_CFA_OFFSET_DECODE: Decode CFA offset (i.e., scale up by CFA offset alignment factor and then revert CFA offset adjustment).

  * [Errata 1] An ELF SFrame section has the type SHT_GNU_SFRAME.

  * [Errata 2] An offset count of zero in the SFrame FRE info byte indicates that the return address (RA) is undefined for the range of PCs covered by the SFrame FRE.  A stack tracer may use this as indication that an outermost frame has been reached and the stack trace is complete.

SFrame Version 1 is now obsolete and should not be used.

SFrame Section
**************

The SFrame section consists of an SFrame header, starting with a preamble, and two other sub-sections, namely the SFrame function descriptor entry (SFrame FDE) sub-section, and the SFrame frame row entry (SFrame FRE) sub-section.

SFrame Preamble
===============

The preamble is a 32-bit packed structure; the only part of the SFrame section whose format cannot vary between versions.

     typedef struct sframe_preamble
     {
       uint16_t sfp_magic;
       uint8_t sfp_version;
       uint8_t sfp_flags;
     } ATTRIBUTE_PACKED sframe_preamble;

Every element of the SFrame preamble is naturally aligned.

All values are stored in the endianness of the target system for which the SFrame section is intended.  Further details:

Offset   Type   Name   Description
0x00
uint16_t
sfp_magic
The magic number for SFrame section: 0xdee2.  Defined as a macro SFRAME_MAGIC.

0x02
uint8_t
sfp_version
The version number of this SFrame section.  See SFrame Version, for the
set of valid values.  Current version is
SFRAME_VERSION_3.

0x03
uint8_t
sfp_flags
Flags (section-wide) for this SFrame section.  See SFrame Flags, for the
set of valid values.

SFrame Magic Number and Endianness
----------------------------------

SFrame sections are stored in the target endianness of the system that consumes them.  A consumer library reading or writing SFrame sections should detect foreign-endianness by inspecting the SFrame magic number in the sfp_magic field in the SFrame header.  It may then provide means to endian-flip the SFrame section as necessary.

SFrame Version
--------------

The version of the SFrame format can be determined by inspecting sfp_version.  The following versions are currently valid:

Version Name   Number   Description
SFRAME_VERSION_1
1   First version, obsolete.
SFRAME_VERSION_2
2   Second version.
SFRAME_VERSION_3
3   Third version, under development.

This document describes SFRAME_VERSION_3.

SFrame Flags
------------

The preamble contains bitflags in its sfp_flags field that describe various section-wide properties.

The following flags are currently defined.

Flag   Version   Value   Meaning
SFRAME_F_FDE_SORTED   All   0x1   Function Descriptor
Entries are sorted on PC.
SFRAME_F_FRAME_POINTER   1-2   0x2
All functions in the object file preserve frame pointer.
SFRAME_F_FDE_FUNC_START_PCREL   2-3   0x4
The sfdi_func_start_offset field in the SFrame FDE is an offset in
bytes to the function's start address, from the field itself.  If unset, the
sfdi_func_start_offset field in the SFrame FDE is an offset in bytes to
the function's start address, from the start of the SFrame section.

The purpose of SFRAME_F_FRAME_POINTER flag was to facilitate stack tracers to reliably fallback on the frame pointer based stack tracing method, if SFrame information is not present for some function in the SFrame section.

Further flags may be added in future.  Bits corresponding to the currently undefined flags must be set to zero.

SFrame Header
=============

The SFrame header is the first part of an SFrame section.  It begins with the SFrame preamble.  All parts of it other than the preamble (see SFrame Preamble) can vary between SFrame file versions.  It contains metadata that apply to the section as a whole, and offsets to the various other sub-sections defined in the format.  As with the rest of the SFrame section, all values are stored in the endianness of the target system.

The two sub-sections tile the SFrame section: each section runs from the offset given until the start of the next section.  An explicit length is given for the last sub-section, the SFrame Frame Row Entry (SFrame FRE) sub-section.

     typedef struct sframe_header
     {
       sframe_preamble sfh_preamble;
       uint8_t sfh_abi_arch;
       int8_t sfh_cfa_fixed_fp_offset;
       int8_t sfh_cfa_fixed_ra_offset;
       uint8_t sfh_auxhdr_len;
       uint32_t sfh_num_fdes;
       uint32_t sfh_num_fres;
       uint32_t sfh_fre_len;
       uint32_t sfh_fdeoff;
       uint32_t sfh_freoff;
     } ATTRIBUTE_PACKED sframe_header;

Every element of the SFrame header is naturally aligned.

The sub-section offsets, namely sfh_fdeoff and sfh_freoff, in the SFrame header are relative to the end of the SFrame header; they are each an offset in bytes into the SFrame section where the SFrame FDE sub-section and the SFrame FRE sub-section respectively start.

The SFrame section contains sfh_num_fdes number of fixed-length array elements in the SFrame FDE sub-section.  Each array element is of type SFrame function descriptor entry; each providing a high-level function description for the purpose of stack tracing.  More details in SFrame Function Descriptor Entries.

Next, the SFrame FRE sub-section, starting at offset sfh_fre_off, describes the stack trace information for each function.  For each function, the SFrame FRE sub-section contains the SFrame FDE attribute data and sfh_num_fres number of variable-length array elements.  Each array element is of type SFrame frame row entry.  See SFrame Frame Row Entries.

SFrame header allows specifying explicitly the fixed offsets from CFA, if any, from which FP or RA may be recovered.  For example, in AMD64, the stack offset of the return address is CFA - 8.  Since these offsets are expected to be in close vicinity to the CFA in most ABIs, sfh_cfa_fixed_fp_offset and sfh_cfa_fixed_ra_offset are limited to signed 8-bit integers.

The SFrame format has made some provisions for supporting more ABIs/architectures in the future.  One of them is the concept of the auxiliary SFrame header.  Bytes in the auxiliary SFrame header may be used to convey further ABI-specific information.  The sframe_header structure provides an unsigned 8-bit integral field to denote the size (in bytes) of an auxiliary SFrame header.  The auxiliary SFrame header follows right after the sframe_header structure.  As for the calculation of the sub-section offsets, namely sfh_fdeoff and sfh_freoff, the end of SFrame header must be the end of the auxiliary SFrame header, if the latter is present.

Putting it all together:

Offset   Type   Name   Description
0x00
sframe_
 preamble
sfh_preamble
The SFrame preamble. See SFrame Preamble.

0x04
uint8_t
sfh_abi_arch
The ABI/arch identifier.  See SFrame ABI/arch Identifier.

0x05
int8_t
sfh_cfa_fixed_fp_offset
The CFA fixed FP offset, if any.

0x06
int8_t
sfh_cfa_fixed_ra_offset
The CFA fixed RA offset, if any.

0x07
uint8_t
sfh_auxhdr_len
Size in bytes of the auxiliary header that follows the
sframe_header structure.

0x08
uint32_t
sfh_num_fdes
The number of SFrame FDEs in the section.

0x0c
uint32_t
sfh_num_fres
The number of SFrame FREs in the section.

0x10
uint32_t
sfh_fre_len
The length in bytes of the SFrame FRE sub-section.

0x14
uint32_t
sfh_fdeoff
The offset in bytes to the SFrame FDE sub-section.

0x18
uint32_t
sfh_freoff
The offset in bytes to the SFrame FRE sub-section.

SFrame ABI/arch Identifier
--------------------------

SFrame header identifies the ABI/arch of the target system for which the executable and hence, the stack trace information contained in the SFrame section, is intended.  There are currently four identifiable ABI/arch values in the format.

ABI/arch Identifier   Value   Description

SFRAME_ABI_AARCH64_ENDIAN_BIG
1   AArch64 big-endian

SFRAME_ABI_AARCH64_ENDIAN_LITTLE
2   AArch64 little-endian

SFRAME_ABI_AMD64_ENDIAN_LITTLE
3   AMD64 little-endian

SFRAME_ABI_S390X_ENDIAN_BIG
4   s390x big-endian

The presence of an explicit identification of ABI/arch in SFrame may allow stack trace generators to make certain ABI/arch-specific decisions.

SFrame FDE
==========

SFrame function descriptor entry is a conceptual entity which contains the function-level metadata necessary for stack tracing through the function.  It is composed of two physical entities: the SFrame function descriptor index (SFrame FDE index) and the SFrame function descriptor attribute (SFrame FDE attribute).  Both SFrame FDE index and SFrame FDE attribute are fixed-length structures, albeit with different alignment guarantees.

The SFrame FDE Index
--------------------

The SFrame FDE index entries are stored in a sub-section of their own, forming a searchable index.  If the SFrame header flag SFRAME_F_FDE_SORTED is set, then the entries are sorted by sfdi_func_start_offset, allowing for efficient binary search.  Typically (as is the case with GNU ld) a linked object or executable will have the SFRAME_F_FDE_SORTED set.  This makes the job of a stack tracer easier as it may then employ a binary search scheme to look for the stack trace information pertinent to a given PC.

     typedef struct sframe_func_desc_idx
     {
       int64_t sfdi_func_start_offset;
       uint32_t sfdi_func_size;
       uint32_t sfdi_func_start_fre_off;
     } ATTRIBUTE_PACKED sframe_func_desc_idx;

Each entry of the SFrame function descriptor index is naturally aligned.  The following table describes each component of the SFrame FDE index entry:

Offset   Type   Name   Description
0x00
int64_t
sfdi_func_start_offset
Signed 64-bit integral field specifying the offset to the start address of
the described function.  If the flag SFRAME_F_FDE_FUNC_START_PCREL,
see SFrame Flags, in the SFrame header is set, the value encoded in the
sfdi_func_start_offset field is the offset in bytes to the function's
start address from the sfdi_func_start_offset field itself.  Otherwise,
it is the offset in bytes from the start of the SFrame section.

0x08
uint32_t
sfdi_func_size
Unsigned 32-bit integral field specifying the size of the function in
bytes.

0x0c
uint32_t
sfdi_func_start_fre_off
Unsigned 32-bit integral field specifying the offset to the start of the
function's stack trace data (SFrame FREs).  This offset is relative to the
beginning of the SFrame FRE sub-section.

The SFrame FDE Attribute
------------------------

The SFrame FDE attribute structure provides information about the SFrame FRE entries that follow: their number and their encoding.  The SFrame FDE attributes are stored at the beginning of each function's stack trace data within the SFrame FRE sub-section.  Because these structures are interleaved with variable-length FREs, their elements are not guaranteed to be at naturally aligned boundaries.

     typedef struct sframe_func_desc_attr
     {
       uint16_t sfda_func_num_fres;
       uint8_t sfda_func_info;
       uint8_t sfda_func_info2;
       uint8_t sfda_func_rep_size;
     } ATTRIBUTE_PACKED sframe_func_desc_attr;

Following table describes each component of the SFrame FDE attribute:

Offset   Type   Name   Description
0x00
uint16_t
sfda_func_num_fres
Unsigned 16-bit integral field specifying the total number of SFrame FREs
used for the function.

0x02
uint8_t
sfda_func_info
Unsigned 8-bit integral field specifying the SFrame FDE info byte.

0x03
uint8_t
sfda_func_info2
Additional unsigned 8-bit integral field specifying the SFrame FDE info byte.

0x04
uint8_t
sfda_func_rep_size
Unsigned 8-bit integral field specifying the size of the repetitive code
block for which an SFrame FDE of type SFRAME_FDE_PCTYPE_MASK is used.  For
example, in AMD64, the size of a pltN entry is 16 bytes.

sfda_func_info and sfda_func_info2 are the SFrame FDE Info Bytes, containing information like the FRE type and their encoding, and the FDE type for the function.  See The SFrame FDE Info Bytes.

The SFrame FDE attribute has some currently unused bits in the SFrame FDE info bytes, that may be used for the purpose of extending the SFrame file format specification for future ABIs.  See The SFrame FDE Types subsection.

The SFrame FDE Info Bytes
-------------------------

The SFrame FDE Attribute contains two distinct bytes, sfda_func_info and sfda_func_info2.  Together these are referred to as the SFrame FDE info bytes.  These bytes contain vital information necessary to:

  - read and interpret SFrame FRE data, e.g., the number and size of each SFrame FRE offset,

  - PC Type for SFrame FDE,

  - type of SFrame FDE,

  - size of repeat block, if PC Type is SFRAME_FDE_PCTYPE_MASK.

The first info byte sfda_func_info is a bitfield split into four parts. From MSB to LSB:

Bit offset   Name   Description

7
signal_p
Signal frame.

6
unused
Unused bit.

5
pauth_key
(For AArch64) Specify which key is used for signing the return addresses
in the SFrame FDE.  Two possible values:

SFRAME_AARCH64_PAUTH_KEY_A (0), or

SFRAME_AARCH64_PAUTH_KEY_B (1).

Unsed in AMD64, s390x

4
fde_pctype
Specify the SFrame FDE PC Type.  Two possible values:

SFRAME_FDE_PCTYPE_MASK (1), or

SFRAME_FDE_PCTYPE_INC (0).

See The SFrame FDE PC Types.

0--3
fre_type
Choice of three SFrame FRE types. See The SFrame FRE Types.

The second info byte sfda_func_info2 is a bitfield split into two parts. From MSB to LSB:

Bit offset   Name   Description
7--5
unused
Unused bits.

4--0
fde_type
Specify the SFrame FDE type.  Two possible values:

SFRAME_FDE_TYPE_DEFAULT (0), or

SFRAME_FDE_TYPE_FLEX (1).

See The SFrame FDE Types.

The SFrame FDE PC Types
.......................

The SFrame format defines two types of FDE PC types.  The choice of which SFrame FDE PC type to use is made based on the instruction patterns in the relevant program stub.

An FDE of PC type SFRAME_V3_FDE_PCTYPE_INC contains FREs whose PCs are to be interpreted as the address of a single instruction, measured in bytes and relative to the beginning of the function.

In contrast, a FDE of PC type SFRAME_V3_FDE_PCTYPE_MASK contains FREs whose PCs are to be interpreted as masks that identify several instructions. This is useful for cases where a small pattern of instructions in a program stub is used repeteadly for a specific functionality, like PLT entries and trampolines.

Name of SFrame FDE PC Type   Value   Description

SFRAME_V3_FDE_PCTYPE_INC
0   Stacktracers perform a

(PC >= FRE_START_ADDR) to look up a matching FRE.

SFRAME_V3_FDE_PCTYPE_MASK
1    Stacktracers perform a

(PC % REP_BLOCK_SIZE

>= FRE_START_ADDR)
to look up a matching FRE.  REP_BLOCK_SIZE is the size in bytes of the
repeating block of program instructions and is encoded via
sfde_func_rep_size in the SFrame FDE.

The SFrame FDE Types
....................

The SFrame format defines two types of Function Descriptor Entries (FDEs) to encode stack trace information.  The choice of FDE type determines how the data in the variable-length Frame Row Entries (FREs) is interpreted.  The FDE type is encoded in the lower 5 bits of the sfda_func_info2 field in the SFrame FDE attribute.

Name   Value   Description
SFRAME_FDE_TYPE_DEFAULT
0
The default FDE type.

CFA is recovered using the Stack Pointer (SP) or Frame Pointer (FP) plus a
signed offset.  Return Address (RA) and Frame Pointer (FP) are recovered using
the CFA plus a signed offset (or a fixed register for specific
architectures like s390x).

The variable-length array of bytes trailing each SFrame FRE are interpreted
according to the ABI/arch-specific rules for the target architecture.  More
details in Default FDE Type Interpretation.

SFRAME_FDE_TYPE_FLEX
1
The flexible FDE type.

Used for complex cases such as stack realignment (DRAP), non-standard CFA base
registers, or when RA/FP recovery requires dereferencing or non-CFA base
registers.

The variable-length array of bytes may be interpreted as pairs of Control Data
and Offset Data (or Padding Data), allowing for complex recovery rules (e.g.,
DRAP on AMD64, Stack Realignment).  More details in Flexible FDE Type Interpretation.

Currently, five bits are reserved in the sfda_func_info2 for indicating SFrame FDE types.  In future, other ABIs/architectures may add even arch-specific FDE types.  Each distinct FDE type may define a different layout, encoding, and interpretation of the variable-length data words trailing each SFrame FRE.

The SFrame FRE Types
....................

A real world application can have functions of size big and small.  SFrame format defines three types of SFrame FRE entries to efficiently encode the stack trace information for such a variety of function sizes.  These representations vary in the number of bits needed to encode the start address offset in the SFrame FRE.

The following constants are defined and used to identify the SFrame FRE types:

Name   Value   Description

SFRAME_FRE_TYPE_ADDR1
0
The start address offset (in bytes) of the SFrame FRE is an unsigned
8-bit value.

SFRAME_FRE_TYPE_ADDR2
1
The start address offset (in bytes) of the SFrame FRE is an unsigned
16-bit value.

SFRAME_FRE_TYPE_ADDR4
2
The start address offset (in bytes) of the SFrame FRE is an unsigned
32-bit value.

A single function must use the same type of SFrame FRE throughout.  The identifier to reflect the chosen SFrame FRE type is stored in the fre_type bits in the SFrame FDE info byte, see The SFrame FDE Info Bytes.

SFrame FRE
==========

The SFrame frame row entry sub-section contains the core of the stack trace information.  An SFrame frame row entry (FRE) is a self-sufficient record containing SFrame stack trace information for a range of contiguous (instruction) addresses, starting at the specified offset from the start of the function.

Each SFrame FRE encodes the information to recover the CFA, FP and RA (as specified by the ABI or the FDE type) for the respective instruction addresses. To encode this information, each SFrame FRE is followed by S*N bytes, where:

  - S is the size of each data word in the variable-length array of data words trailing the SFrame FRE, and

  - N is the number of data words trailing the SFrame FRE.

NB: The term `data word' is used throughout this specification in a colloquial sense to denote a discrete unit of information within an SFrame Frame Row Entry (FRE). It is intended to describe the semantic role of the data rather than its physical size. Consequently, `data word' should not be understood to correlate with the architectural machine word size or any specific hardware data width; the actual size of a data word in the SFrame format is variable and is defined in the SFrame FRE info byte.

The entities S, N are encoded in the SFrame FRE info byte, via the fre_dataword_size and the fre_dataword_count respectively. More information about the precise encoding and range of values for S and N is provided later in the The SFrame FRE Info Word.

It is important to underline here that although the canonical interpretation of these data words is as stack offsets (to recover CFA, FP and RA) for default FDE type, these bytes may be used by future ABIs/architectures to convey other information on a per SFrame FRE basis.

In summary, SFrame file format, by design, supports a variable length array of bytes at the tail end of each SFrame FRE.  To keep the SFrame file format specification flexible yet extensible, the interpretation of these bytes is specific to ABI/arch or FDE type.  More details about the precise interpretation are covered in the section Interpretation of SFrame FREs.

Next, the definitions of the three SFrame FRE types are as follows:

     typedef struct sframe_frame_row_entry_addr1
     {
       uint8_t sfre_start_address;
       sframe_fre_info sfre_info;
     } ATTRIBUTE_PACKED sframe_frame_row_entry_addr1;

     typedef struct sframe_frame_row_entry_addr2
     {
       uint16_t sfre_start_address;
       sframe_fre_info sfre_info;
     } ATTRIBUTE_PACKED sframe_frame_row_entry_addr2;

     typedef struct sframe_frame_row_entry_addr4
     {
       uint32_t sfre_start_address;
       sframe_fre_info sfre_info;
     } ATTRIBUTE_PACKED sframe_frame_row_entry_addr4;

For ensuring compactness, SFrame frame row entries are stored unaligned on disk.  Appropriate mechanisms need to be employed, as necessary, by the serializing and deserializing entities, if unaligned accesses need to be avoided.

sfre_start_address is an unsigned 8-bit/16-bit/32-bit integral field denoting the start address of a range of program counters, for which the SFrame FRE applies.  The value encoded in the sfre_start_address field is the offset in bytes of the range's start address, from the start address of the function.

Further SFrame FRE types may be added in future.

The SFrame FRE Info Word
------------------------

The SFrame FRE info byte is a bitfield split into four parts.  From MSB to LSB:

Bit offset   Name   Description
7
fre_mangled_ra_p
Indicate whether the return address is mangled with any authorization bits (signed RA).

5-6
fre_dataword_size
Size of data word in bytes.  Valid values are:

SFRAME_FRE_DATAWORD_1B,

SFRAME_FRE_DATAWORD_2B, and

SFRAME_FRE_DATAWORD_4B.

1-4
fre_dataword_count
Being a 4-bit sized field, a max value of 15 is allowed.  Typically, a
value of up to 3 is sufficient for most ABIs to track all three of CFA, FP and
RA.  A value of zero indicates that the return address (RA) is undefined.  A
stack tracer may use this as indication that an outermost frame has been
reached and the stack trace is complete.

0
fre_cfa_base_reg_id
Distinguish between SP or FP based CFA recovery.

Name   Value   Description

SFRAME_FRE_DATAWORD_1B
0
All data words following the fixed-length FRE structure are 1 byte
long.

SFRAME_FRE_DATAWORD_2B
1
All data words following the fixed-length FRE structure are 2 bytes
long.

SFRAME_FRE_DATAWORD_4B
2
All data words following the fixed-length FRE structure are 4 bytes
long.

Interpretation of SFrame FREs
*****************************

Each SFrame Frame Row Entry (FRE) provides information about a PC range within some function, encoded using a variable number of bytes (see SFrame Frame Row Entries).  The interpretation of these bytes depends on the FDE type used to represent stack tracing information for the function.

Default FDE Type Interpretation
===============================

If the FDE type is SFRAME_FDE_TYPE_DEFAULT, the interpretation of the FRE bytes is ABI/arch-specific.  Typically, these bytes are interpreted as a sequence of (signed integer) stack offsets.

The following sections describe the specific interpretation rules for currently supported architectures.

AMD64
-----

Irrespective of the ABI, the first stack offset is always used to locate the CFA, by interpreting it as: CFA = BASE_REG + offset1.  The identification of the BASE_REG is done by using the fre_cfa_base_reg_id field in the SFrame FRE info byte.

In AMD64, the return address (RA) is always saved on stack when a function call is executed.  Further, AMD64 ABI mandates that the RA be saved at a fixed offset from the CFA when entering a new function.  This means that the RA does not need to be tracked per SFrame FRE.  The fixed offset is encoded in the SFrame file format in the field sfh_cfa_fixed_ra_offset in the SFrame header.  See SFrame Header.

Hence, the second stack offset (in the SFrame FRE), when present, will be used to locate the FP, by interpreting it as: FP = CFA + offset2.

Hence, in summary:

Offset ID   Interpretation in AMD64
1   CFA = BASE_REG + offset1
2   FP = CFA + offset2

AArch64
-------

Irrespective of the ABI, the first stack offset is always used to locate the CFA, by interpreting it as: CFA = BASE_REG + offset1.  The identification of the BASE_REG is done by using the fre_cfa_base_reg_id field in the SFrame FRE info byte.

In AArch64, the AAPCS64 standard specifies that the Frame Record saves both FP and LR (a.k.a the RA).  However, the standard does not mandate the precise location in the function where the frame record is created, if at all.  Hence the need to track RA in the SFrame stack trace format.  As RA is being tracked in this ABI, the second stack offset is always used to locate the RA, by interpreting it as: RA = CFA + offset2.  The third stack offset will be used to locate the FP, by interpreting it as: FP = CFA + offset3.

Given the nature of things, the number of stack offsets seen on AArch64 per SFrame FRE is either 1 or 3.

Hence, in summary:

Offset ID   Interpretation in AArch64
1   CFA = BASE_REG + offset1
2   RA = CFA + offset2
3   FP = CFA + offset3

s390x
-----

A stack tracer implementation must initialize the SP to the designated SP register value, the FP to the preferred FP register value, and the RA to the designated RA register value in the topmost stack frame of the callchain.  This is required, as either the SP or FP is used as CFA base register and as the FP and/or RA are not necessarily saved on the stack.  For RA this may only be the case in the topmost stack frame of the callchain.  For FP this may be the case in any stack frame.

Irrespective of the ABI, the first stack offset is always used to locate the CFA.  On s390x the value of the offset is stored adjusted by the s390x-specific SFRAME_S390X_CFA_OFFSET_ADJUSTMENT and scaled down by the s390x-specific SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR, to enable and improve the use of signed 8-bit offsets on s390x. s390x-specific helpers SFRAME_V2_S390X_CFA_OFFSET_ENCODE and SFRAME_V2_S390X_CFA_OFFSET_DECODE are provided to perform or undo the adjustment and scaling.  The CFA offset can therefore be interpreted as: CFA = BASE_REG + offset1 - SFRAME_S390X_CFA_OFFSET_ADJUSTMENT or CFA = BASE_REG + (offset1 * SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR) - SFRAME_S390X_CFA_OFFSET_ADJUSTMENT. The identification of the BASE_REG is done by using the fre_cfa_base_reg_id field in the SFrame FRE info byte.

The (64-bit) s390x ELF ABI does not mandate the precise location in a function where the return address (RA) and frame pointer (FP) are saved, if at all. Hence the need to track RA in the SFrame stack trace format.  As RA is being tracked in this ABI, the second stack offset is always used to locate the RA stack slot, by interpreting it as: RA = CFA + offset2, unless the offset has a value of SFRAME_FRE_RA_OFFSET_INVALID.  RA remains unchanged, if the offset is not available or has a value of SFRAME_FRE_RA_OFFSET_INVALID. Stack tracers are recommended to validate that the "unchanged RA" pattern, when present, is seen only for the topmost stack frame.  The third stack offset is used to locate the FP stack slot, by interpreting it as: FP = CFA + offset3. FP remains unchanged, if the offset is not available.

In leaf functions the RA and FP may be saved in other registers, such as floating-point registers (FPRs), instead of being saved on the stack.  To represent this in the SFrame stack trace format, SFrame FDE of type SFRAME_FDE_TYPE_FLEX may be used.

Given the nature of things, for default type FDEs, the number of stack offsets seen on s390x per SFrame FRE is either 1, 2, or 3.

Hence, in summary:

Offset ID   Interpretation in s390x
1   CFA = BASE_REG + offset1
2   RA stack slot = CFA + offset2

RA not saved if (offset2 == SFRAME_FRE_RA_OFFSET_INVALID)
3   FP stack slot = CFA + offset3

The s390x ELF ABI defines the CFA as stack pointer (SP) at call site +160.  The SP can therefore be obtained using the SP value offset from CFA SFRAME_S390X_SP_VAL_OFFSET of -160 as follows: SP = CFA + SFRAME_S390X_SP_VAL_OFFSET

Future ABIs must specify the algorithm for identifying the appropriate SFrame FRE stack offsets in this chapter.  This should inevitably include the blueprint for interpreting the variable number of bytes at the tail end of the SFrame FRE for the specific ABI/arch.

Flexible FDE Type Interpretation
================================

Flexible FDEs (SFRAME_FDE_TYPE_FLEX) are used in cases where the most common default recovery rules implied by SFRAME_FDE_TYPE_DEFAULT are insufficient.  Common use cases include:

  * DRAP (Dynamically Realigned Argument Pointer): Where the CFA is based on a register other than SP or FP, or requires dereferencing.

  * Stack Realignment: Where strict alignment requirements (e.g., AVX512) force dynamic stack adjustments.

  * Register-based RA/FP Locations: Where the Return Address or Frame Pointer is transiently saved in a general-purpose register and/or requires a dereference rule.

For flexible FDE types, the variable-length bytes trailing an SFrame FRE can be interpreted as one of the following:

 1. Control Data: Encodes the base register number, a dereference flag, and a register-mode flag.  A value of 0 is reserved as the padding data word.

 2. Offset Data: Encodes the signed offset to be added to the base.

For each tracked entity (CFA, RA, FP), the SFrame FRE carries a pair of data words to specify the respective recovery rule.  The pair of data words appear in the order: CFA, RA, FP.  These data words obey the fre_dataword_size defined in the FRE info byte (i.e., they are 1, 2, or 4 bytes wide).

Given the nature of things, since CFA is always tracked, the first two data words pertain to CFA recovery.  If RA recovery rule is unspecified (because the RA can be recovered from its default location), a single padding data word is used instead of the pair of Control data word and Offset data word if FP recovery rule is to be specified using the subsequent data words.

Following is the order of information for specifying the recovery rule for a tracked entity in a flexible FDE.

Encoding of Data Word 1 (Control Data)
......................................

The first data word of the pair is an unsigned integer of size fre_dataword_size.  It is used as a bitfield that describes register/control data for the tracked entity.  From LSB to MSB:

Bit Offset   Name   Description
0
reg_p
Register-based Location Rule

If 1, the base is a DWARF register (encoded in bits 3+).
If 0, the base is the CFA (used for RA/FP recovery).
1
deref_p
Dereference Flag

If 1, the location of the value is the address (Base + Offset), i.e.,
value = *(Base + Offset).

If 0, the value is Base + Offset.
2
unused
Unused bit.
3+
regnum
The DWARF register number used as the base.  Effective only if
reg_p is 1.

A value of 0 (i.e., regnum = 0, deref_p = 0, reg_p = 0) in the Control Data Word is used to indicate that no further data words follow for the tracked entity.  This is to convey an absence of recovery rule for the respective tracked entity (which means that fixed offsets sfh_cfa_fixed_fp_offset or sfh_cfa_fixed_ra_offset apply if used for the ABI/arch).  Note that, using a value of 0 as padding data word, does mean that currently, e.g., for RA, the rule RA = CFA + 0 cannot be encoded.  NB: RA = CFA + 0 is distinct from RA = *(CFA + 0).  The former should not be needed for any ABI, and the latter is representable (regnum = 0, deref_p = 1, reg_p = 0).

Encoding of Data Word 2 (Offset Data)
.....................................

The second data word of the pair is a signed integer of width fre_dataword_size.  It is used as a offset for the respective tracked entity (CFA, FP or RA).

Recovery Rules
..............

The value of the tracked entity (CFA, RA, or FP) is calculated using the following logic:

         Base   = (reg_p == 1) ? Register[regnum] : CFA;
         Addr   = Base + Offset2;
         Value  = (deref_p == 1) ? *Addr : Addr;

Examples:

  * CFA = *(RBP - 8): (Typical DRAP pattern on AMD64)

      Data Word 1: (RBP << 3) | (1 << 1) | 1 (Reg RBP, deref_p=True, reg_p=True)

      Data Word 2: -8

  * FP = *(RBP + 0):

      Data Word 1: (RBP << 3) | (1 << 1) | 1 (Reg RBP, deref_p=True, reg_p=True)

      Data Word 2: 0

  * RA = *(CFA - 8): (Standard RA recovery on AMD64)

      Data Word 1: (0 << 3 | (1 << 1) | 0) (reg_p=False, implies Base=CFA, deref_p=True by implication of standard stack save)

      Data Word 2: -8

If the FDE type is SFRAME_FDE_TYPE_FLEX, the FRE bytes are interpreted using a universal encoding scheme designed to handle complex recovery rules (such as DRAP or non-standard RA locations).

Generating Stack Traces using SFrame
************************************

Using some C-like pseudocode, this section highlights how SFrame provides a simple, fast and low-overhead mechanism to generate stack traces.  Needless to say that for generating accurate and useful stack traces, several other aspects will need attention: finding and decoding bits of SFrame section(s) in the program binary, symbolization of addresses, to name a few.

In the current context, a frame is the abstract construct that encapsulates the following information:

  - program counter (PC),

  - stack pointer (SP), and

  - frame pointer (FP)

With that said, establishing the first frame should be trivial:

         // frame 0
         frame->pc = current_IP;
         frame->sp = get_reg_value (REG_SP);
         frame->fp = get_reg_value (REG_FP);

where REG_SP and REG_FP are are ABI-designated stack pointer and frame pointer registers respectively.

Next, given frame N, generating stack trace needs us to get frame N+1.  This can be done as follows:

          // Get the PC, SP, and FP for frame N.
          pc = frame->pc;
          sp = frame->sp;
          fp = frame->fp;
          // Populate frame N+1.
          int err = get_next_frame (&next_frame, pc, sp, fp);

where given the values of the program counter, stack pointer and frame pointer from frame N, get_next_frame populates the provided next_frame object and returns the error code, if any.

In the following pseudocode for get_next_frame, the sframe_* functions fetch information from the SFrame section.  Note that the stack tracer must retrieve the FDE type to decide how to interpret the FRE data words.

         fre = sframe_find_fre (pc, &fde_type);
         if (fre && fde_type == SFRAME_FDE_TYPE_DEFAULT)
             // Whether the base register for CFA tracking is REG_FP.
             base_reg_val = sframe_fre_base_reg_fp_p (fre) ? fp : sp;
             // Get the CFA stack offset from the FRE.
             cfa_offset = sframe_fre_get_cfa_offset (fre);
             // Get the fixed RA offset or FRE stack offset as applicable.
             ra_offset = sframe_fre_get_ra_offset (fre);
             // Get the fixed FP offset or FRE stack offset as applicable.
             fp_offset = sframe_fre_get_fp_offset (fre);

             cfa = base_reg_val + cfa_offset;
             next_frame->sp = cfa [+ SFRAME_S390X_SP_VAL_OFFSET on s390x];

             ra_stack_loc = cfa + ra_offset;
             // Get the address stored in the stack location.
             next_frame->pc = read_value (ra_stack_loc);

             if (fp_offset is VALID)
                 fp_stack_loc = cfa + fp_offset;
                 // Get the value stored in the stack location.
                 next_frame->fp = read_value (fp_stack_loc);
             else
                 // Continue to use the value of fp as it has not
                 // been clobbered by the current frame yet.
                 next_frame->fp = fp;

For SFrame FDE of type SFRAME_FDE_TYPE_FLEX, read the set of data words and apply the recovery rules accordingly.

         if (fre && fde_type == SFRAME_FDE_TYPE_FLEX)
             // Get the base register, offset, and deref_p for CFA tracking.
             // The first FRE offset (index 0) is the CFA Control Data.
             cfa_reg_data = sframe_fre_get_offset (fre, 0);
             cfa_offset = sframe_fre_get_offset (fre, 1);

             // Get the RA reg, offset, and deref_p.
             // The third FRE data word (index 2) is the RA Control Data.
             ra_reg_data = sframe_fre_get_udata (fre, 2);
             if (ra_reg_data != SFRAME_FRE_RA_OFFSET_INVALID)
                     ra_offset = sframe_fre_get_offset (fre, 3);
                     fp_tracking_p = fre.num_offsets > 3;
                     fp_data_index = 3;
             else
                     fp_tracking_p = fre.num_offsets > 4;
                     fp_data_index = 4;

             // Get the FP reg, offset, and deref_p (if present).
             if (fp_tracking_p)
                  fp_reg_data = sframe_fre_get_udata (fre, fp_data_index);
                  fp_offset = sframe_fre_get_fp_offset (fre);

             // Safety check for topmost frames:
             // If recovery requires non-standard registers (not SP/FP),
             // it is only valid if we are at the top of the stack
             // (where those registers haven't been clobbered).
             cfa_base_reg = SFRAME_V3_FLEX_FDE_OFFSET_REG_NUM (cfa_reg_data);
             if (!topmost_frame_p && (cfa_base_reg != REG_FP
                                      && cfa_base_reg != REG_SP))
                  return ERR_SFRAME_UNSAFE_UNWIND;

             // Apply rules to recover CFA and RA
             cfa = sframe_apply_rule (cfa_reg_data, cfa_offset, cfa, 1);
             ra = sframe_apply_rule (ra_reg_data, ra_offset, cfa, 0);

             if (fp_tracking_p)
                 next_frame->fp
                    = sframe_apply_rule (fp_reg_data, fp_offset, cfa, 0);
             else
                 next_frame->fp = fp;

             next_frame->sp = cfa;
             next_frame->pc = ra;
         else
             ret = ERR_NO_SFRAME_FRE;

The sframe_apply_rule helper function abstracts the logic of interpreting the Control Data and Offset Data pair for flexible FDEs:

         // Apply SFrame V3 Flex FDE recovery rule.
         // reg_data: The Control Data (Data word 1)
                      containing reg_p, deref_p, regnum.
         // offset:   The Offset (Data word 2).
         // cfa:      The current CFA value (used as base if reg_p is 0).
         // cfa_p:    Bool indicating if we are currently recovering the
                      CFA itself.

         sframe_apply_rule (reg_data, offset, cfa, cfa_p)
             reg_p = SFRAME_V3_FLEX_FDE_OFFSET_REG_P (reg_data);

             // Determine Base Address:
             // If reg_p is set, read from the specific DWARF register.
             // If reg_p is clear, use the CFA (unless we are recovering the
             // CFA itself, in which case reg_p MUST be set).
             if (reg_p)
                  reg_num = SFRAME_V3_FLEX_FDE_OFFSET_REG_NUM (reg_data);
                  base_loc = get_reg_value (reg_num);
             else
                  base_loc = cfa;

             // CFA recovery must always specify a base register.
             assert (!cfa_p || reg_p);

             // Add the displacement
             loc = base_loc + offset;

             // Dereference if required
             deref_p = SFRAME_V3_FLEX_FDE_OFFSET_REG_DEREF_P (reg_data);
             value = deref_p ? read_value (loc) : loc;

             return value;