locking/rwsem/x86: Add stack frame dependency for ____down_write()

Arnd reported the following objtool warning:

  kernel/locking/rwsem.o: warning: objtool: down_write_killable()+0x16: call without frame pointer save/setup

The warning means gcc placed the ____down_write() inline asm (and its
call instruction) before the frame pointer setup in
down_write_killable(), which breaks frame pointer convention and can
result in incorrect stack traces.

Force the stack frame to be created before the call instruction by
listing the stack pointer as an output operand in the inline asm
statement.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1188b7015f04baf361e59de499ee2d7272c59dce.1476393828.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
index 3d33a719f5c1d8652be749c43ba4b7dc5560c4d5..a34e0d4b957d639afb5978863e57fefb74a173f2 100644 (file)
--- a/arch/x86/include/asm/rwsem.h
+++ b/arch/x86/include/asm/rwsem.h
@@ -103,8 +103,10 @@ static inline bool __down_read_trylock(struct rw_semaphore *sem)
 ({                                                     \
        long tmp;                                       \
        struct rw_semaphore* ret;                       \
+       register void *__sp asm(_ASM_SP);               \
+                                                       \
        asm volatile("# beginning down_write\n\t"       \
-                    LOCK_PREFIX "  xadd      %1,(%3)\n\t"      \
+                    LOCK_PREFIX "  xadd      %1,(%4)\n\t"      \
                     /* adds 0xffff0001, returns the old value */ \
                     "  test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t" \
                     /* was the active mask 0 before? */\
@@ -112,7 +114,7 @@ static inline bool __down_read_trylock(struct rw_semaphore *sem)
                     "  call " slow_path "\n"           \
                     "1:\n"                             \
                     "# ending down_write"              \
-                    : "+m" (sem->count), "=d" (tmp), "=a" (ret)        \
+                    : "+m" (sem->count), "=d" (tmp), "=a" (ret), "+r" (__sp) \
                     : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS) \
                     : "memory", "cc");                 \
        ret;                                            \