Re: [libvirt] [PATCH] build: make ATTRIBUTE_NONNULL() a NOP unless STATIC_ANALYSIS is on

The ATTRIBUTE_NONNULL(m) macro normally resolves to the gcc builtin __attribute__((__nonnull__(m))). The effect of this in gcc is unfortunately only to make gcc believe that "m" can never possibly be NULL, *not* to add in any checks to guarantee that it isn't ever NULL (i.e. it is an optimization aid, *not* something to verify code correctness.) - see the following gcc bug report for more details: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17308 Static source analyzers such as clang and coverity apparently can use ATTRIBUTE_NONNULL(), though, to detect dead code (in the case that the arg really is guaranteed non-NULL), as well as situations where an obviously NULL arg is given to the function. https://bugzilla.redhat.com/show_bug.cgi?id=815270 is a good example of a bug caused by erroneous application of ATTRIBUTE_NONNULL(). Several people spent a long time staring at this code and not finding the problem, because the problem wasn't in the function itself, but in the prototype that specified ATTRIBUTE_NONNULL() for an arg that actually *wasn't* always non-NULL, and caused a segv when dereferenced (even though the code that dereferenced the pointer was inside an if() that checked for a NULL pointer, that code was optimized out by gcc). There may be some very small gain to be had from the optimizations that can be inferred from ATTRIBUTE_NONNULL(), but it seems safer to err on the side of generating code that behaves as expected, while turning on the attribute for static analyzers. (dissenting opinions welcome :-)

--- src/internal.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/src/internal.h b/src/internal.h index ef81cda..83f468d 100644 --- a/src/internal.h +++ b/src/internal.h @@ -182,7 +182,7 @@ # endif # ifndef ATTRIBUTE_NONNULL -# if __GNUC_PREREQ (3, 3) +# if __GNUC_PREREQ (3, 3) && STATIC_ANALYSIS # define ATTRIBUTE_NONNULL(m) __attribute__((__nonnull__(m)))