From e4d78a67ffbacf30b66464080898227f18f6bf49 Mon Sep 17 00:00:00 2001 From: Christopher Clark Date: Fri, 17 Aug 2018 17:46:10 -0700 Subject: [PATCH] xenpmd: prevent format-truncation warning with gcc 8.2 + ARM 32-bit To: xen-devel@lists.xenproject.org Cc: ian.jackson@eu.citrix.com, wei.liu2@citrix.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 --- 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) +