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

[Christopher Clark <christopher.w.clark@gmail.com>]

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@baesystems.com>
---
 tools/xenpmd/xenpmd.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/tools/xenpmd/xenpmd.c b/tools/xenpmd/xenpmd.c
index 56412a9..0c0787e 100644
--- a/tools/xenpmd/xenpmd.c
+++ b/tools/xenpmd/xenpmd.c
@@ -350,8 +350,10 @@ void write_battery_info_to_xenstore(struct battery_info *info)
    
     memset(val, 0, 1024);
     memset(string_info, 0, 256);
-    /* write 9 dwords (so 9*4) + length of 4 strings + 4 null terminators */
-    snprintf(val, 3, "%02x", 
+    /* write 9 dwords (so 9*4) + length of 4 strings + 4 null terminators.
+     * mask informs the compiler that format truncation will not occur.
+     */
+    snprintf(val, 3, "%02x", 0xff &
              (unsigned int)(9*4 +
                             strlen(info->model_number) +
                             strlen(info->serial_number) +
-- 
2.7.4


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