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

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Message-ID: <aebc3572-43a9-984b-1c47-1f06b17b2972@gmail.com>
Date: Tue, 4 Mar 2025 16:35:30 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>, 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 4. 03. 25 01:35, H. Peter Anvin wrote:
> 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.

Actually, GCC 15+ will introduce "redzone" clobber, so you will be able 
to write e.g.:

void foo (void) { asm ("" : : : "cc", "memory", "redzone"); }

Please see [1] and:

+@...m "redzone"
+The @code{"redzone"} clobber tells the compiler that the assembly code
+may write to the stack red zone, area below the stack pointer which on
+some architectures in some calling conventions is guaranteed not to be
+changed by signal handlers, interrupts or exceptions and so the compiler
+can store there temporaries in leaf functions.  On targets which have
+no concept of the stack red zone, the clobber is ignored.
+It should be used e.g.@: in case the assembly code uses call instructions
+or pushes something to the stack without taking the red zone into account
+by subtracting red zone size from the stack pointer first and restoring
+it afterwards.
+

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117312

Uros.