Re: [Xen-devel] [PATCH] xenpmd: prevent format-truncation warning with gcc 8.2 + ARM 32-bit

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Aug 17, 2018 at 06:22:16PM -0700, Christopher Clark wrote:
> xenpmd writes battery information to xenstore, including a string with a
> formatted hex value calculated from summing the lengths of four strings,
> plus some constants.
> 
> Each of the four strings has a maximum length of 31 bytes, excluding the
> terminating zero byte. The strings are stored in 32-byte arrays in a
> struct that is zeroed before it is populated, and logic that writes to
> the strings uses strncpy and explicit zero termination.
> 
> The maximum value to be supplied to the xenstore string is:
>   (9 * 4) + (31 * 4) + 4 , which is 164, ie. 0xa4.
> 
> When used with this value, '%02x' will always fit within 3 bytes, but
> gcc 8.2 is apparently not able to deduce this (observed when building
> for a 32-bit ARM platform).
> 
> This commit assists the compiler by applying a mask (0xff) to the value,
> enabling it to observe a lower maximum value and so pass the truncation
> length check.
> 
> Prior to this change, building fails with the compiler warning:
> 
> | xenpmd.c: In function 'write_battery_info_to_xenstore':
> | xenpmd.c:354:23: error: '%02x' directive output may be truncated
> writing between 2 and 8 bytes into a region of size 3
> [-Werror=format-truncation=]
> |      snprintf(val, 3, "%02x",
> |                        ^~~~
> | xenpmd.c:354:22: note: directive argument in the range [40, 2147483778]
> |      snprintf(val, 3, "%02x",
> |                       ^~~~~~
> | xenpmd.c:354:5: note: 'snprintf' output between 3 and 9 bytes into a
> destination of size 3
> |      snprintf(val, 3, "%02x",
> |      ^~~~~~~~~~~~~~~~~~~~~~~~
> |               (unsigned int)(9*4 +
> |               ~~~~~~~~~~~~~~~~~~~~
> |                              strlen(info->model_number) +
> |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> |                              strlen(info->serial_number) +
> |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> |                              strlen(info->battery_type) +
> |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> |                              strlen(info->oem_info) + 4));
> |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> | cc1: all warnings being treated as errors
> 
> Signed-off-by: Christopher Clark <christopher.clark6@xxxxxxxxxxxxxx>

Hmm... I just pushed a different fix for this to staging. Can you try if
that works for you?

Wei.

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