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From: Thomas Gleixner <tglx@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>, Dmitry Voytik <voytikd@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>, linux-toolchains@vger.kernel.org,
 Alexander Potapenko <glider@google.com>, Ingo Molnar <mingo@redhat.com>,
 Dave Hansen <dave.hansen@linux.intel.com>, Josh Poimboeuf
 <jpoimboe@kernel.org>, Dmitry Vyukov <dvyukov@google.com>, Andrew Morton
 <akpm@linux-foundation.org>, Ankur Arora <ankur.a.arora@oracle.com>, "H .
 Peter Anvin" <hpa@zytor.com>, Nathan Chancellor <nathan@kernel.org>, Nick
 Desaulniers <nick.desaulniers+lkml@gmail.com>, Bill Wendling
 <morbo@google.com>, Justin Stitt <justinstitt@google.com>, Marco Elver
 <elver@google.com>, x86@kernel.org, linux-kernel@vger.kernel.org,
 kasan-dev@googlegroups.com, linux-mm@kvack.org, llvm@lists.linux.dev
Subject: Re: [PATCH] x86/mm: fix objtool failure with KMSAN enabled
In-Reply-To: <87se61npie.ffs@fw13>
References: <20260701125151.352632-1-voytikd@gmail.com>
 <20260702025342.GJakXStrsCmIRnEwFD@fat_crate.local>
 <CAAX90H0V-=Gy1FyBT9qop2=RvhBxbOPLu6+DYpPH2pTehT=nRw@mail.gmail.com>
 <20260702111550.GH751831@noisy.programming.kicks-ass.net>
 <87se61npie.ffs@fw13>
Date: Thu, 02 Jul 2026 22:05:03 +0200
Message-ID: <87mrw9nonk.ffs@fw13>
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On Thu, Jul 02 2026 at 21:46, Thomas Gleixner wrote:
> On Thu, Jul 02 2026 at 13:15, Peter Zijlstra wrote:
>> A quick test with folio_zero_user() seems to suggest the
>> ASM_CALL_CONSTRAINT movement (your patch) generates the same code for
>> defconfg as does the unmodified code. The change from the thread linked
>> above (input constraint on __builtin_frame_address(0)) generates
>> different code.
>>
>> A third option is to force FRAME_POINTER=y for KMSAN builds -- for now,
>> until clang is taught to be less insane.
>
> With FRAME_POINTER=y I get now with clang-22
>
> vmlinux.o: warning: objtool: folio_zero_user+0xa7b: stack state mismatch: reg1[4]=-1+0 reg2[4]=-2-336
> vmlinux.o: warning: objtool: set_ftrace_ops_ro+0x64: relocation to !ENDBR: machine_kexec_prepare+0x8f0
>
> That's with your objtool patch applied. When I remove that then the
> folio_zero_user one goes away. The relocation to !ENDBR stays.
>
> Oh well...

And none of this boots when KMSAN=y ....