[Qemu-devel] [PATCH v5 1/5] qemu/compiler: Define QEMU_NONSTRING

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

GCC 8 introduced the -Wstringop-truncation checker to detect truncation by
the strncat and strncpy functions (closely related to -Wstringop-overflow,
which detect buffer overflow by string-modifying functions declared in
<string.h>).

In tandem of -Wstringop-truncation, the "nonstring" attribute was added:

  The nonstring variable attribute specifies that an object or member
  declaration with type array of char, signed char, or unsigned char,
  or pointer to such a type is intended to store character arrays that
  do not necessarily contain a terminating NUL. This is useful in detecting
  uses of such arrays or pointers with functions that expect NUL-terminated
  strings, and to avoid warnings when such an array or pointer is used as
  an argument to a bounded string manipulation function such as strncpy.

  From the GCC manual: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-nonstring-variable-attribute

Add the QEMU_NONSTRING macro which checks if the compiler supports this
attribute.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
v4: reordered the commit description to make sens (eblake)

Note this trigger the following checkpatch warning (patchew)
which I'm ignoring because this is consistent with how the
rest of this file uses:

  WARNING: architecture specific defines should be avoided
  #50: FILE: include/qemu/compiler.h:163:
  +#if __has_attribute(nonstring)
---
 include/qemu/compiler.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
index 261842beae..2d8f507c73 100644
--- a/include/qemu/compiler.h
+++ b/include/qemu/compiler.h
@@ -151,6 +151,21 @@
 # define QEMU_ERROR(X)
 #endif
 
+/*
+ * The nonstring variable attribute specifies that an object or member
+ * declaration with type array of char or pointer to char is intended
+ * to store character arrays that do not necessarily contain a terminating
+ * NUL character. This is useful in detecting uses of such arrays or pointers
+ * with functions that expect NUL-terminated strings, and to avoid warnings
+ * when such an array or pointer is used as an argument to a bounded string
+ * manipulation function such as strncpy.
+ */
+#if __has_attribute(nonstring)
+# define QEMU_NONSTRING __attribute__((nonstring))
+#else
+# define QEMU_NONSTRING
+#endif
+
 /* Implement C11 _Generic via GCC builtins.  Example:
  *
  *    QEMU_GENERIC(x, (float, sinf), (long double, sinl), sin) (x)
-- 
2.17.2