linux-kernel - Re: Concerns about SFrame viability for userspace stack walking

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Message-ID: <aQMUGvXv6sy75nKn@tucnak>
Date: Thu, 30 Oct 2025 08:30:34 +0100
From: Jakub Jelinek <jakub@...hat.com>
To: Fangrui Song <maskray@...rceware.org>
Cc: linux-toolchains@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Concerns about SFrame viability for userspace stack walking

On Wed, Oct 29, 2025 at 11:53:32PM -0700, Fangrui Song wrote:
> I've been following the SFrame discussion and wanted to share some concerns about its viability for userspace adoption, based on concrete measurements and comparison with existing compact unwind implementations in LLVM.
> 
> **Size overhead concerns**
> 
> Measurements on a x86-64 clang binary show that .sframe (8.87 MiB) is approximately 10% larger than the combined size of .eh_frame and .eh_frame_hdr (8.06 MiB total).
> This is problematic because .eh_frame cannot be eliminated - it contains essential information for restoring callee-saved registers, LSDA, and personality information needed for debugging (e.g. reading local variables in a coredump) and C++ exception handling.

I believe .sframe only provides a subset of the .eh_frame information, so
can't be used for exception throwing, and you don't want to lose
.eh_frame_hdr either because then dlopen becomes very costly and it will
even slow down exception throwing.

If .eh_frame is considered too large, rather than inventing a new format I'd
suggest to work in the DWARF committee and provide further size
optimizations for .dwarf_frame which can then be used in .eh_frame, or agree
on .eh_frame extensions to make it smaller.

	Jakub