Re: [PATCH 3/3] locking/rwsem/x86: Add stack frame dependency for __downgrade_write()

[Josh Poimboeuf]

On Wed, Sep 06, 2017 at 12:26:13AM -0500, Miguel Bernal Marin wrote:
> kernel/locking/rwsem.o: warning: objtool: downgrade_write()+0x22: call without frame pointer save/setup
> 
> The warning means gcc 7.2.0 placed the __downgrade_write() inline asm (and
> its call instruction) before the frame pointer setup in downgrade_write(),
> which breaks frame pointer convention and can result in incorrect
> stack traces.
> 
> Force a stack frame to be created before the call instruction by listing
> the stack pointer as an output operand in the inline asm statement.
> 
> Signed-off-by: Miguel Bernal Marin <miguel.bernal.marin@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/rwsem.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
> index d26b6916b935..a749dc6a3103 100644
> --- a/arch/x86/include/asm/rwsem.h
> +++ b/arch/x86/include/asm/rwsem.h
> @@ -205,8 +205,10 @@ static inline void __up_write(struct rw_semaphore *sem)
>   */
>  static inline void __downgrade_write(struct rw_semaphore *sem)
>  {
> +	register void *__sp asm(_ASM_SP);
> +
>  	asm volatile("# beginning __downgrade_write\n\t"
> -		     LOCK_PREFIX _ASM_ADD "%2,(%1)\n\t"
> +		     LOCK_PREFIX _ASM_ADD "%2,(%2)\n\t"

The '%2' should be changed to '%3'

		     LOCK_PREFIX _ASM_ADD "%3,(%2)\n\t"

because both inputs' indices are shifted by the new output constraint.

>  		     /*
>  		      * transitions 0xZZZZ0001 -> 0xYYYY0001 (i386)
>  		      *     0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64)
> @@ -215,7 +217,7 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
>  		     "  call call_rwsem_downgrade_wake\n"
>  		     "1:\n\t"
>  		     "# ending __downgrade_write\n"
> -		     : "+m" (sem->count)
> +		     : "+m" (sem->count), "+r" (__sp)
>  		     : "a" (sem), "er" (-RWSEM_WAITING_BIAS)
>  		     : "memory", "cc");

-- 
Josh