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

[Marco Elver]

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; }); }

Like the 64-bit version (which was missing 'volatile'), fix it by
overriding _THIS_IP_ in <asm/linkage.h> using an inline asm version for
32-bit x86.

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@xxxxxxxxxx>
---
 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; })
 #endif /* CONFIG_X86_32 */
 
 #define __ALIGN		.balign CONFIG_FUNCTION_ALIGNMENT, 0x90;
-- 
2.54.0.746.g67dd491aae-goog