On Wed, 11 Oct 2023 16:33:48 -0600
Ross Zwisler <zwisler@xxxxxxxxxx> wrote:
> On Thu, Oct 05, 2023 at 09:22:33PM -0400, Steven Rostedt wrote:
> > From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
> >
> > In the new addition to make sure that pointers passed to traceeval_init()
> > and other functions that require a static array and not a dynamic one will
> > cause the build to fail, this causes NULL pointers to fail the build too.
> >
> > Although keys must be filled, vals are allowed to be NULL. It was assumed
> > that:
> >
> > (void *)vals == NULL ? TRACEEVAL_ARRAY_SIZE(vals))
> >
> > Would solve this, but it gcc was actually still giving a warning about
> >
> > warning: division 'sizeof (void *) / sizeof (void)' does not compute the number of array elements
> >
> > But now it actually fails to build with the magic check.
> >
> > Change TRACEEVAL_ARRAY_SIZE() to handle NULL for both keys and vals, by
> > not only having:
> >
> > #define TRACEEVAL_ARRAY_SIZE(data) \
> > ((void *)(data) == NULL ? 0 : __TRACEEVAL_ARRAY_SIZE(data))
> >
> > But that is not enough, as gcc still evaluates the second part, and it
> > will fail to build. To handle this, change that to:
> >
> > #define __TRACEEVAL_ARRAY_SIZE(data) \
> > ((sizeof(data) / (sizeof((data)[0])) + 0) + \
> >
> > The above adds " + 0" to the "sizeof((data)[0])" which quiets the warning
> > mentioned above (the addition of zero breaks the normal pattern of finding
> > an array size).
> >
> > (int)(sizeof(struct { \
> > int:(-!!(__builtin_types_compatible_p(typeof(data), \
> > typeof(&((data)[0]))) && \
> > (void *)(data) != NULL)); \
> >
> > Added "&& (void *)(data) != NULL" that makes the above return false (zero)
> > for a static array and NULL, which is exactly what we want.
>
> Don't we already know it's not NULL because of the check in
> TRACEEVAL_ARRAY_SIZE()? Or do we really need to check for NULL in both
> macros?
Unfortunately what happens is that the compiler still checks the above. So
if we have just:
(int)(sizeof(struct { \
int:(-!!(__builtin_types_compatible_p(typeof(data), \
typeof(&((data)[0])))));
Then the with NULL turns into:
struct { int: -1; }
and fails the compile because:
__builtin_types_compatible_p(typeof(NULL), typeof(&((NULL)[0])))
Returns true.
So if we pair that with (void *)(data) != NULL, it will then return false
and turns into:
struct { int: 0; }
Which is valid.
>
> >
> > })))
>
> A few random parens in the changelog. :)
>
Yeah, the code had it too and I had to fix it. But didn't update the change log. :-p
-- Steve
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