[Bug sanitizer/84732] false-positive -Wstringop-truncation warning with -fsanitize-coverage=trace-pc

[arnd at linaro dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84732

--- Comment #5 from Arnd Bergmann <arnd at linaro dot org> ---
Created attachment 43586
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43586&action=edit
drivers/gpu/drm/drm_property.c, preprocessed

I found another case that appears to be related but not the same, attaching
another (non-reduced) file for reference. The code that triggered a warning
this time is:

        strncpy(property->name, name, DRM_PROP_NAME_LEN);
        property->name[DRM_PROP_NAME_LEN-1] = '\0';

but unlike the first one, this only happens with -fsanitize=kernel-address but
not with -fsanitize-coverage=trace-pc:

$ x86_64-linux-gcc-8.0.1 -fno-strict-aliasing -O2 -Wall -S drm_property.i  
-fsanitize=kernel-address

/git/arm-soc/drivers/gpu/drm/drm_property.c: In function 'drm_property_create':
/git/arm-soc/include/linux/string.h:254:9: warning: '__builtin_strncpy'
specified bound 32 equals destination size [-Wstringop-truncation]
  return __builtin_strncpy(p, q, size);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/git/arm-soc/drivers/gpu/drm/drm_property.c: In function
'drm_property_add_enum':
/git/arm-soc/include/linux/string.h:254:9: warning: '__builtin_strncpy'
specified bound 32 equals destination size [-Wstringop-truncation]
  return __builtin_strncpy(p, q, size);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/git/arm-soc/include/linux/string.h:254:9: warning: '__builtin_strncpy'
specified bound 32 equals destination size [-Wstringop-truncation]
  return __builtin_strncpy(p, q, size);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~