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

[Peter Zijlstra <peterz@infradead.org>]

On Thu, May 21, 2026 at 10:34:38AM +0200, Marco Elver wrote:
> On Thu, 21 May 2026 at 09:09, Peter Zijlstra <peterz@infradead.org> wrote:
> > 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.
> 
> Yeah - up to you. GCC appears to do the right thing still even for
> 32-bit: https://godbolt.org/z/3PWPK8E4f
> 
> Here's "Clang returns 1": https://godbolt.org/z/KjMvEWeM5

asm volatile ("1: mov $1b, %0" : "=r" (__here))

seems to work?

And then there is this form:

asm volatile ("1: lea 1b, %0" : "=r" (__here))

which should also work but be one more byte. Both are shorter than
CALL+POP and neither mess up the RSB. Am I missing something?