linux-kernel - Re: [tip: x86/asm] x86/asm: Make ASM_CALL_CONSTRAINT conditional on frame pointers

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Message-ID: <28D821BB-96B5-4389-839E-5B7CB4D49F5F@zytor.com>
Date: Mon, 03 Mar 2025 16:35:10 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
CC: linux-kernel@...r.kernel.org,
        tip-bot2 for Josh Poimboeuf <tip-bot2@...utronix.de>,
        linux-tip-commits@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>, x86@...nel.org
Subject: Re: [tip: x86/asm] x86/asm: Make ASM_CALL_CONSTRAINT conditional on frame pointers

On March 3, 2025 2:47:58 PM PST, Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>On Mon, Mar 03, 2025 at 02:45:50PM -0800, Josh Poimboeuf wrote:
>> On Mon, Mar 03, 2025 at 02:31:50PM -0800, H. Peter Anvin wrote:
>> > >+#ifdef CONFIG_UNWINDER_FRAME_POINTER
>> > > #define ASM_CALL_CONSTRAINT "r" (__builtin_frame_address(0))
>> > >+#else
>> > >+#define ASM_CALL_CONSTRAINT
>> > >+#endif
>> > > 
>> > > #endif /* __ASSEMBLY__ */
>> > > 
>> > 
>> > Wait, why was this changed? I actually tested this form at least once
>> > and found that it didn't work under all circumstances...
>> 
>> Do you have any more details about where this didn't work?  I tested
>> with several configs and it seems to work fine.  Objtool will complain
>> if it doesn't work.
>> 
>> See here for the justification (the previous version was producing crap
>> code in Clang):
>
>Gah, that link doesn't work because I forgot to cc lkml.
>
>Here's the tip bot link:
>
>  https://lore.kernel.org/all/174099976253.10177.12542657892256193630.tip-bot2@tip-bot2/
>

One more thing: if we remove ASM_CALL_CONSTRAINTS, we will not be able to use the redzone in future FRED only kernel builds.