ACK: [PATCH] lib: fwts_safe_mem: really force reads

[Alex Hung]

On 2017-08-12 03:47 AM, Colin King wrote:
> From: Colin Ian King <colin.king at canonical.com>
> 
> Being totally paranoid here, but I'd like to really ensure
> data is always being read by forcing a prefetch on each data item
> being read and force the compiler to never optimize this out by
> stashing the read data into a circular buffer.  Smarter compilers
> on some arches were optimizing out the original reads; I think
> this change will really force reads.
> 
> Signed-off-by: Colin Ian King <colin.king at canonical.com>
> ---
>   src/lib/src/fwts_safe_mem.c | 15 +++++++++++----
>   1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/src/lib/src/fwts_safe_mem.c b/src/lib/src/fwts_safe_mem.c
> index 1feb1ff2..f3f639b1 100644
> --- a/src/lib/src/fwts_safe_mem.c
> +++ b/src/lib/src/fwts_safe_mem.c
> @@ -68,8 +68,10 @@ int OPTIMIZE0 fwts_safe_memcpy(void *dst, const void *src, const size_t n)
>    */
>   int OPTIMIZE0 fwts_safe_memread(const void *src, const size_t n)
>   {
> -	const volatile uint8_t *ptr = src;
> -	const volatile uint8_t *end = ptr + n;
> +	static uint8_t buffer[256];
> +	const uint8_t *ptr, *end = src + n;
> +	uint8_t *bufptr;
> +	const uint8_t *bufend = buffer + sizeof(buffer);
>   
>   	if (sigsetjmp(jmpbuf, 1) != 0)
>   		return FWTS_ERROR;
> @@ -77,8 +79,13 @@ int OPTIMIZE0 fwts_safe_memread(const void *src, const size_t n)
>   	fwts_sig_handler_set(SIGSEGV, sig_handler, &old_segv_action);
>   	fwts_sig_handler_set(SIGBUS, sig_handler, &old_bus_action);
>   
> -	while (ptr < end)
> -		(void)*(ptr++);
> +	for (bufptr = buffer, ptr = src; ptr < end; ptr++) {
> +		/* Force data to be read */
> +		__builtin_prefetch(ptr, 0, 3);
> +		*bufptr = *ptr;
> +		bufptr++;
> +		bufptr = (bufptr >= bufend) ? buffer : bufptr;
> +	}
>   
>   	fwts_sig_handler_restore(SIGSEGV, &old_segv_action);
>   	fwts_sig_handler_restore(SIGBUS, &old_bus_action);
> 


Acked-by: Alex Hung <alex.hung at canonical.com>