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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 2 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, known as the .sframe section.  When available, the .sframe section appears in a new segment of its own, PT_GNU_SFRAME.

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

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 entry) 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 rest of this specification describes the current version of the format, SFRAME_VERSION_2, in detail.  Additional sections outline the major changes made to each previously published version of the SFrame stack trace format.

The associated API to decode, probe and encode the SFrame section, provided via libsframe, is not accompanied here at this time.  This will be added later.

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 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_TYPE_PCMASK.

  * 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.

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_2.

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   Current version, under development.

This document describes SFRAME_VERSION_2.

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

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

The following flags are currently defined.

Flag   Versions   Value   Meaning
SFRAME_F_FDE_SORTED   All   0x1   Function Descriptor
Entries are sorted on PC.
SFRAME_F_FRAME_POINTER   All   0x2
All functions in the object file preserve frame pointer.

The purpose of SFRAME_F_FRAME_POINTER flag is 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.

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 things 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 a subsequent section. See SFrame Function Descriptor Entries.

Next, the SFrame FRE sub-section, starting at offset sfh_fre_off, describes the stack trace information for each function, using a total of 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 three 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

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
==========

The SFrame function descriptor entry sub-section is an array of the fixed-length SFrame function descriptor entries (SFrame FDEs).  Each SFrame FDE is a packed structure which contains information to describe a function's stack trace information at a high-level.

The array of SFrame FDEs is sorted on the sfde_func_start_address if the SFrame section header flag sfp_flags has SFRAME_F_FDE_SORTED set.  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 binary search schemes to look for the pertinent SFrame FDE.

     typedef struct sframe_func_desc_entry
     {
       int32_t sfde_func_start_address;
       uint32_t sfde_func_size;
       uint32_t sfde_func_start_fre_off;
       uint32_t sfde_func_num_fres;
       uint8_t sfde_func_info;
       uint8_t sfde_func_rep_size;
       uint16_t sfde_func_padding2;
     } ATTRIBUTE_PACKED sframe_func_desc_entry;

Every element of the SFrame function descriptor entry is naturally aligned.

sfde_func_start_fre_off is the offset to the first SFrame FRE for the function.  This offset is relative to the end of the SFrame FDE sub-section (unlike the sub-section offsets in the SFrame header, which are relative to the end of the SFrame header).

sfde_func_info is the SFrame FDE "info word", containing information on the FRE type and the FDE type for the function see The SFrame FDE Info Word.

Apart from the sfde_func_padding2, the SFrame FDE has some currently unused bits in the SFrame FDE info word, see The SFrame FDE Info Word, that may be used for the purpose of extending the SFrame file format specification for future ABIs.

Following table describes each component of the SFrame FDE structure:

Offset   Type   Name   Description
0x00
int32_t
sfde_func_start_address
Signed 32-bit integral field denoting the virtual memory address of the
described function.

0x04
uint32_t
sfde_func_size
Unsigned 32-bit integral field specifying the size of the function in
bytes.

0x08
uint32_t
sfde_func_start_fre_off
Unsigned 32-bit integral field specifying the offset in bytes of the
function's first SFrame FRE in the SFrame section.

0x0c
uint32_t
sfde_func_num_fres
Unsigned 32-bit integral field specifying the total number of SFrame FREs
used for the function.

0x10
uint8_t
sfde_func_info
Unsigned 8-bit integral field specifying the SFrame FDE info word.
See The SFrame FDE Info Word.

0x11
uint8_t
sfde_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_TYPE_PCMASK is used.  For
example, in AMD64, the size of a pltN entry is 16 bytes.

0x12
uint16_t
sfde_func_padding2
Padding of 2 bytes.  Currently unused bytes.

The SFrame FDE Info Word
------------------------

The info word is a bitfield split into three parts.  From MSB to LSB:

Bit offset   Name   Description
7--6
unused
Unused bits.

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).

Ununsed in AMD64.

4
fdetype
Specify the SFrame FDE type.  Two possible values:

SFRAME_FDE_TYPE_PCMASK (1), or

SFRAME_FDE_TYPE_PCINC (0).

See The SFrame FDE Types.

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

The SFrame FDE Types
--------------------

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

An SFrame FDE of type SFRAME_FDE_TYPE_PCINC is an indication that the PCs in the FREs should be treated as increments in bytes.  This is used fo the the bulk of the executable code of a program, which contains instructions with no specific pattern.

In contrast, an SFrame FDE of type SFRAME_FDE_TYPE_PCMASK is an indication that the PCs in the FREs should be treated as masks.  This type is useful for the cases where a small pattern of instructions in a program stub is used repeatedly for a specific functionality.  Typical usecases are pltN entries and trampolines.

Name of SFrame FDE type   Value   Description

SFRAME_FDE_TYPE_PCINC
0   Stacktracers perform a

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

SFRAME_FDE_TYPE_PCMASK
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 FRE Types
--------------------

A real world application can have functions of size big and small.  SFrame format defines three types of SFrame FRE entries to effeciently 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 fretype bits in the SFrame FDE info word, see The SFrame FDE Info Word.

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 stack offsets to recover the CFA, FP and RA (where applicable) for the respective instruction addresses.  To encode this information, each SFrame FRE is followed by S*N bytes, where:

  - S is the size of a stack offset for the FRE, and

  - N is the number of stack offsets in the FRE

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

It is important to underline here that although the canonical interpretation of these bytes is as stack offsets (to recover CFA, FP and RA), 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 number of stack offsets at the tail end of each SFrame FRE.  To keep the SFrame file format specification flexible yet extensible, the interpretation of the stack offsets is ABI/arch-specific.  The precise interpretation of the FRE stack offsets in the currently supported ABIs/architectures is covered in the ABI/arch-specific definition of the SFrame file format, see ABI/arch-specific Definition.

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 identifies the start address of the 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 start address of the SFrame FRE, 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 word 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_offset_size
Size of stack offsets in bytes.  Valid values are:

SFRAME_FRE_OFFSET_1B,

SFRAME_FRE_OFFSET_2B, and

SFRAME_FRE_OFFSET_4B.

1-4
fre_offset_count
A max value of 15 is allowed.  Typically, a value of upto 3 is sufficient
for most ABIs to track all three of CFA, FP and RA.

0
fre_cfa_base_reg_id
Distinguish between SP or FP based CFA recovery.

Name   Value   Description

SFRAME_FRE_OFFSET_1B
0
All stack offsets following the fixed-length FRE structure are 1 byte
long.

SFRAME_FRE_OFFSET_2B
1
All stack offsets following the fixed-length FRE structure are 2 bytes
long.

SFRAME_FRE_OFFSET_4B
2
All stack offsets following the fixed-length FRE structure are 4 bytes
long.

ABI/arch-specific Definition
****************************

This section covers the ABI/arch-specific definition of the SFrame file format.

Currently, the only part of the SFrame file format definition that is ABI/arch-specific is the interpretation of the variable number of bytes at the tail end of each SFrame FRE.  Currently, these bytes are only used for representing stack offsets (for all the currently supported ABIs).  It is recommended to peruse this section along with see SFrame Frame Row Entries for clarity of context.

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. Any further provisions, e.g., using the auxiliary SFrame header, etc., if used, must also be outlined here.

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 word.

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 word.

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

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.

         fre = sframe_find_fre (pc);
         if (fre)
             // 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;

             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;
         else
             ret = ERR_NO_SFRAME_FRE;