Re: [PATCH 9/9] s390/bug: Prevent tail-call optimization

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Dec 09, 2025 at 01:17:01PM +0100, Heiko Carstens wrote:
> For the exception based __WARN_trap() implementation it is technically not
> necessary to prevent tail-call optimization, however it may be confusing to
> see warning messages like:
> 
> WARNING: arch/s390/kernel/setup.c:1017 at foobar+0x2c/0x50, CPU#0: swapper/0/0
> 
> together with a disassembly of a different function caused by tail-call
> optimaziation for the __WARN_trap() call. Prevent that by adding an empty
> asm statement. This generates slightly worse code, but should hopefully
> avoid confusion.

Aah, because:

	bar()
	  foo()
	    __WARN_trap()


when foo() does a tail-call, your link reg points to bar() and not the
expected foo().

And at this point you don't have enough clues to conditionally do that
psw/r14 fixup either.

Oh well.

> With this the output looks like:
> 
> WARNING: arch/s390/kernel/setup.c:1017 at foobar+0x2c/0x50, CPU#0: swapper/0/0
> ...
> Krnl PSW : 0704c00180000000 000003ffe0119788 (foobar+0x38/0x50)
> ...
> Krnl Code: 000003ffe0119776: e3e0f0980024        stg     %r14,152(%r15)
>            000003ffe011977c: c02000b8992a        larl    %r2,000003ffe182c9d0
>           *000003ffe0119782: c0e5007270b7        brasl   %r14,000003ffe0f678f0
>           >000003ffe0119788: ebeff0a00004        lmg     %r14,%r15,160(%r15)
>            000003ffe011978e: 07fe                bcr     15,%r14
>            000003ffe0119790: 47000700            bc      0,1792
>            000003ffe0119794: 0707                bcr     0,%r7
>            000003ffe0119796: 0707                bcr     0,%r7
> Call Trace:
>  [<000003ffe0119788>] foobar+0x38/0x50
>  [<000003ffe185bc2e>] arch_cpu_finalize_init+0x26/0x60
>  [<000003ffe185654c>] start_kernel+0x53c/0x5d8
>  [<000003ffe010002e>] startup_continue+0x2e/0x40
> 
> A better solution would be to replace or patch the branch instruction to
> __WARN_trap() with the monitor call instruction, similar to what is done
> for x86 [1]. However s390 does not support static_cond_calls(). Therefore
> use the simple approach for the time being.

Right, and no objtool for you either :/ Because all you need is
something to find all the __WARN_trap() callsites and stick them in a
section.

> [1] commit 860238af7a33 ("x86_64/bug: Inline the UD1")
> 
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
> ---
>  arch/s390/include/asm/bug.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/s390/include/asm/bug.h b/arch/s390/include/asm/bug.h
> index e6e8b492c0e7..89187ec6f6b0 100644
> --- a/arch/s390/include/asm/bug.h
> +++ b/arch/s390/include/asm/bug.h
> @@ -99,6 +99,8 @@ do {									\
>  	int __flags = (flags) | BUGFLAG_WARNING | BUGFLAG_ARGS;		\
>  									\
>  	__WARN_trap(__WARN_bug_entry(__flags, format), ## arg);		\
> +	/* prevent tail-call optimization */				\
> +	asm("");							\
>  } while (0)
>  
>  #define __WARN_printf(taint, fmt, arg...) \
> -- 
> 2.51.0
>