linux-kernel - Re: [PATCH] scripts/mksysmap: skip objtool __pfx

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Message-ID: <20250411065054.GM9833@noisy.programming.kicks-ass.net>
Date: Fri, 11 Apr 2025 08:50:54 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Arnd Bergmann <arnd@...nel.org>, Masahiro Yamada <masahiroy@...nel.org>,
	Rae Moar <rmoar@...gle.com>, Shuah Khan <skhan@...uxfoundation.org>,
	Sergio González Collado <sergio.collado@...il.com>,
	David Gow <davidgow@...gle.com>, Arnd Bergmann <arnd@...db.de>,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas.schier@...ux.dev>,
	Mostafa Saleh <smostafa@...gle.com>, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scripts/mksysmap: skip objtool __pfx_ symbols

On Tue, Apr 08, 2025 at 06:58:49PM -0700, Josh Poimboeuf wrote:
> On Fri, Mar 28, 2025 at 11:48:19AM +0100, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> > 
> > The recently added testcase for overly long symbols triggers when
> > CONFIG_FUNCTION_PADDING_CFI is set:
> > 
> > Symbol __pfx_snnnng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nnng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nnnng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nnng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nnnnng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nnng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nnnng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nnng1h2i3j4k5l6m7ng1h2i3j4k5l6m7nng1h2i3j4k5l6m7ng1h2i3j4k5l6m7n too long for kallsyms (517 >= 512).
> > Please increase KSYM_NAME_LEN both in kernel and kallsyms.c
> > 
> > Change the mksymtab table so the prefixed symbols are not included
> > in kallsyms.
> > 
> > Fixes: c104c16073b7 ("Kunit to check the longest symbol length")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> 
> I'm not sure we want to remove the __pfx_ symbols from kallsyms. There
> can be actual code there.
> 
> For example, FineIBT writes code in the __pfx area which can trigger an
> #UD.  And we'd want a sane backtrace for that.

On top of that, clang kcfi builds do a similar thing, they will generate
__cfi_ prefixed symbols.

And yes, those symbols exist for a reason, there is code there under
various circumstances and backtraces look really weird without these
symbols on -- notably the code in the prefix will be attributed to
whatever symbol comes before, most confusing.

So yeah, don't remove these symbols, and fix the kunit test.