linux-kernel - Re: [tip: objtool/core] objtool: Fix up st

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Message-ID: <CAF1bQ=SBgs6RiOEXakcZ=TaYjwXngMKNrp8gHL9FhfjOodOxiA@mail.gmail.com>
Date: Thu, 8 May 2025 17:45:18 -0700
From: Rong Xu <xur@...gle.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: tip-bot2 for Rong Xu <tip-bot2@...utronix.de>, linux-tip-commits@...r.kernel.org, 
	"Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [tip: objtool/core] objtool: Fix up st_info in COMDAT group section

On Wed, May 7, 2025 at 4:22 PM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> On Wed, Apr 30, 2025 at 12:29:54PM +0000, tip-bot2 for Rong Xu wrote:
> > The following commit has been merged into the objtool/core branch of tip:
> >
> > Commit-ID:     2cb291596e2c1837238bc322ae3545dacb99d584
> > Gitweb:        https://git.kernel.org/tip/2cb291596e2c1837238bc322ae3545dacb99d584
> > Author:        Rong Xu <xur@...gle.com>
> > AuthorDate:    Fri, 25 Apr 2025 13:05:41 -07:00
> > Committer:     Peter Zijlstra <peterz@...radead.org>
> > CommitterDate: Wed, 30 Apr 2025 13:58:34 +02:00
> >
> > objtool: Fix up st_info in COMDAT group section
> >
> > When __elf_create_symbol creates a local symbol, it relocates the first
> > global symbol upwards to make space. Subsequently, elf_update_symbol()
> > is called to refresh the symbol table section. However, this isn't
> > sufficient, as other sections might have the reference to the old
> > symbol index, for instance, the sh_info field of an SHT_GROUP section.
> >
> > This patch updates the `sh_info` field when necessary. This field
> > serves as the key for the COMDAT group. An incorrect key would prevent
> > the linker's from deduplicating COMDAT symbols, leading to duplicate
> > definitions in the final link.
> >
> > Signed-off-by: Rong Xu <xur@...gle.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Link: https://lkml.kernel.org/r/20250425200541.113015-1-xur@google.com
>
> Unfortunately this patch completely destroys performance when adding a
> bunch of symbols.  Which I'm doing in v2 of my klp-build patch set.
The patch won't add symbols. Do you mean the updates take too much time
when adding symbols? It's probably true as we lookup the sections for every
added symbols. I did not notice the compile time issues in my builds.
If this is a problem, it needs to be fixed.
Thanks for working with v2!
>
> What was the use case for this?  I don't remember seeing any COMDAT
> groups in the kernel.

In the PGO or AutoFDO builds, we used many COMDAT variables for counters
and control variables. I think the compiler also puts the functions
defined in header
as COMDAT.
The issue I encountered was objtool moved the variables for
profile_filename and
profile_version, but the comdat keys were not updated.

-Rong

> --
> Josh