Re: [PATCH] x86: Implement _THIS_IP_ using inline asm for 32-bit

[Peter Zijlstra <peterz@infradead.org>]

On Thu, May 21, 2026 at 02:00:09AM +0200, Marco Elver wrote:
> Both GCC [1] and Clang [2] consider the generic version of _THIS_IP_ to
> be broken:
> 
>         #define _THIS_IP_  ({ __label__ __here; __here: (unsigned long)&&__here; })
> 
> In particular, the address of a label is only expected to be used with a
> computed goto.
> 
> While the generic version more or less works today, it is known to be
> brittle and may break with current and future optimizations. For
> example, Clang -O2 always returns 1 when this function is inlined:
> 
>         static inline unsigned long get_ip(void)
>         { return ({ __label__ __here; __here: (unsigned long)&&__here; }); }
> 

Oh gawd :/

> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120071 [1]
> Link: https://github.com/llvm/llvm-project/issues/138272 [2]
> Signed-off-by: Marco Elver <elver@google.com>
> ---
>  arch/x86/include/asm/linkage.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/linkage.h b/arch/x86/include/asm/linkage.h
> index a7294656ad90..bce3c6f4b94f 100644
> --- a/arch/x86/include/asm/linkage.h
> +++ b/arch/x86/include/asm/linkage.h
> @@ -13,11 +13,12 @@
>   * The generic version tends to create spurious ENDBR instructions under
>   * certain conditions.
>   */
> -#define _THIS_IP_ ({ unsigned long __here; asm ("lea 0(%%rip), %0" : "=r" (__here)); __here; })
> +#define _THIS_IP_ ({ unsigned long __here; asm volatile("lea 0(%%rip), %0" : "=r" (__here)); __here; })
>  #endif
>  
>  #ifdef CONFIG_X86_32
>  #define asmlinkage CPP_ASMLINKAGE __attribute__((regparm(0)))
> +#define _THIS_IP_ ({ unsigned long __ip; asm volatile("call 1f\n1: pop %0" : "=r" (__ip)); __ip; })

This will mess up the RSB and cause bad performance ripple effects for a
bit each use. Now, I don't think anybody still cares about performance
on 32bit (I certainly don't), so perhaps this is fine. But urgh.