Re: [PATCH v13 00/18] unwind_deferred: Implement sframe handling

[Indu Bhagat <ibhagatgnu@gmail.com>]

On Tue, Jan 27, 2026 at 7:32 AM Jens Remus <jremus@linux.ibm.com> wrote:
>
> This is the implementation of parsing the SFrame V3 stack trace information
> from an .sframe section in an ELF file.  It's a continuation of Josh's and
> Steve's work that can be found here:
>
>    https://lore.kernel.org/all/cover.1737511963.git.jpoimboe@kernel.org/
>    https://lore.kernel.org/all/20250827201548.448472904@kernel.org/
>
> Currently the only way to get a user space stack trace from a stack
> walk (and not just copying large amount of user stack into the kernel
> ring buffer) is to use frame pointers. This has a few issues. The biggest
> one is that compiling frame pointers into every application and library
> has been shown to cause performance overhead.
>
> Another issue is that the format of the frames may not always be consistent
> between different compilers and some architectures (s390) has no defined
> format to do a reliable stack walk. The only way to perform user space
> profiling on these architectures is to copy the user stack into the kernel
> buffer.
>
> SFrame [1] is now supported in binutils (x86-64, ARM64, and s390). There is
> discussions going on about supporting SFrame in LLVM. SFrame acts more like
> ORC, and lives in the ELF executable file as its own section. Like ORC it
> has two tables where the first table is sorted by instruction pointers (IP)
> and using the current IP and finding it's entry in the first table, it will
> take you to the second table which will tell you where the return address
> of the current function is located and then you can use that address to
> look it up in the first table to find the return address of that function,
> and so on. This performs a user space stack walk.
>
> Now because the .sframe section lives in the ELF file it needs to be faulted
> into memory when it is used. This means that walking the user space stack
> requires being in a faultable context. As profilers like perf request a stack
> trace in interrupt or NMI context, it cannot do the walking when it is
> requested. Instead it must be deferred until it is safe to fault in user
> space. One place this is known to be safe is when the task is about to return
> back to user space.
>
> This series makes the deferred unwind user code implement SFrame format V3
> and enables it on x86-64.
>
> [1]: https://sourceware.org/binutils/wiki/sframe
>
>
> This series applies on top of the tip perf/core branch:
>
>   git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git  perf/core
>
> The to be stack-traced user space programs (and libraries) need to be
> built with the recent SFrame stack trace information format V3, as
> generated by the upcoming binutils 2.46 with assembler option --gsframe.
> It can be built from source from the binutils-2_46-branch branch:
>
>   git://sourceware.org/git/binutils-gdb.git  binutils-2_46-branch
>
> Namhyung Kim's related perf tools deferred callchain support can be used
> for testing ("perf record --call-graph fp,defer" and "perf report/script").
>
>
> Changes since v12 (see patch notes for details):
> - Rebase on tip perf/core branch (d55c571e4333).
> - Add support for SFrame V3, including its new flexible FDEs.  SFrame V2
>   is not supported.
>
> Changes since v11 (see patch notes for details):
> - Rebase on tip master branch (f8fdee44bf2f) with Namhyung Kim's
>   perf/defer-callchain-v4 branch merged on top.
> - Adjust to Peter's latest undwind user enhancements.
> - Simplify logic by using an internal SFrame FDE representation, whose
>   FDE function start address field is an address instead of a PC-relative
>   offset (from FDE).
> - Rename struct sframe_fre to sframe_fre_internal to align with
>   struct sframe_fde_internal.
> - Remove unused pt_regs from unwind_user_next_common() and its
>   callers. (Peter)
> - Simplify unwind_user_next_sframe(). (Peter)
> - Fix a few checkpatch errors and warnings.
> - Minor cleanups (e.g. move includes, fix indentation).
>
> Changes since v10:
> - Support for SFrame V2 PC-relative FDE function start address.
> - Support for SFrame V2 representing RA undefined as indication for
>   outermost frames.
>
>
> Patches 1, 4, 11, and 17 have been updated to exclusively support the
> latest SFrame V3 stack trace information format, that is generated by
> the upcoming binutils 2.46 release.  Old SFrame V2 sections get rejected
> with dynamic debug message "bad/unsupported sframe header".
>
> Patches 7 and 8 add support to unwind user (sframe) for outermost frames.
>
> Patches 12-15 add support to unwind user (sframe) for the new SFrame V3
> flexible FDEs.
>
> Patch 16 improves the performance of searching the SFrame FRE for an IP.
>

Thanks Jens for your work on this. Apart from some of those minor 
renames you are already planning on doing (as you mentioned in the 
meeting today), the SFrame related bits look OK to me.

Reviewed-by: Indu Bhagat <ibhagatgnu@gmail.com>

> Regards,
> Jens
>
>
> Jens Remus (7):
>   unwind_user: Stop when reaching an outermost frame
>   unwind_user/sframe: Add support for outermost frame indication
>   unwind_user: Enable archs that pass RA in a register
>   unwind_user: Flexible FP/RA recovery rules
>   unwind_user: Flexible CFA recovery rules
>   unwind_user/sframe: Add support for SFrame V3 flexible FDEs
>   unwind_user/sframe: Separate reading of FRE from reading of FRE data
>     words
>
> Josh Poimboeuf (11):
>   unwind_user/sframe: Add support for reading .sframe headers
>   unwind_user/sframe: Store .sframe section data in per-mm maple tree
>   x86/uaccess: Add unsafe_copy_from_user() implementation
>   unwind_user/sframe: Add support for reading .sframe contents
>   unwind_user/sframe: Detect .sframe sections in executables
>   unwind_user/sframe: Wire up unwind_user to sframe
>   unwind_user/sframe: Remove .sframe section on detected corruption
>   unwind_user/sframe: Show file name in debug output
>   unwind_user/sframe: Add .sframe validation option
>   unwind_user/sframe/x86: Enable sframe unwinding on x86
>   unwind_user/sframe: Add prctl() interface for registering .sframe
>     sections
>
>  MAINTAINERS                               |   1 +
>  arch/Kconfig                              |  23 +
>  arch/x86/Kconfig                          |   1 +
>  arch/x86/include/asm/mmu.h                |   2 +-
>  arch/x86/include/asm/uaccess.h            |  39 +-
>  arch/x86/include/asm/unwind_user.h        |  69 +-
>  arch/x86/include/asm/unwind_user_sframe.h |  12 +
>  fs/binfmt_elf.c                           |  48 +-
>  include/linux/mm_types.h                  |   3 +
>  include/linux/sframe.h                    |  60 ++
>  include/linux/unwind_user.h               |  18 +
>  include/linux/unwind_user_types.h         |  46 +-
>  include/uapi/linux/elf.h                  |   1 +
>  include/uapi/linux/prctl.h                |   6 +-
>  kernel/fork.c                             |  10 +
>  kernel/sys.c                              |   9 +
>  kernel/unwind/Makefile                    |   3 +-
>  kernel/unwind/sframe.c                    | 840 ++++++++++++++++++++++
>  kernel/unwind/sframe.h                    |  87 +++
>  kernel/unwind/sframe_debug.h              |  68 ++
>  kernel/unwind/user.c                      | 105 ++-
>  mm/init-mm.c                              |   2 +
>  22 files changed, 1414 insertions(+), 39 deletions(-)
>  create mode 100644 arch/x86/include/asm/unwind_user_sframe.h
>  create mode 100644 include/linux/sframe.h
>  create mode 100644 kernel/unwind/sframe.c
>  create mode 100644 kernel/unwind/sframe.h
>  create mode 100644 kernel/unwind/sframe_debug.h
>
> --
> 2.51.0
>
>