arm64: Implement _THIS_IP_ using inline asm

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; }); }

Fix it by overriding _THIS_IP_ in <asm/linkage.h> (which is included by
<linux/instruction_pointer.h>) using an architecture-specific inline asm
version. Additionally, avoiding taking the address of a label prevents
compilers from emitting spurious indirect branch targets (e.g.  ENDBR or
BTI) under control-flow integrity schemes.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120071
Link: https://github.com/llvm/llvm-project/issues/138272
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/include/asm/linkage.h b/arch/arm64/include/asm/linkage.h
index 40bd17add53973410ed1979c84ff1c0c22aeecde..73eabc82a6bb2344cc2ba1842bb6981e64ebbbf5 100644 (file)
--- a/arch/arm64/include/asm/linkage.h
+++ b/arch/arm64/include/asm/linkage.h
@@ -43,4 +43,6 @@
        SYM_TYPED_START(name, SYM_L_GLOBAL, SYM_A_ALIGN)        \
        bti c ;
 
+#define _THIS_IP_ ({ unsigned long __ip; asm volatile("adr %0, ." : "=r" (__ip)); __ip; })
+
 #endif