[PATCH] lib: fwts_safe_mem: really force reads

[Naresh Bhat]

On 12 August 2017 at 16:17, Colin King <colin.king at canonical.com> 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);
> --
> 2.11.0
>

Thanks for the patch.  Just back from long week-end.

Acked-By/Tested-By: Naresh Bhat <naresh.bhat at linaro.org>
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