[PATCH v1] Remove 'restrict' from 'nptr' in strtol(3)-like functions

[Martin Uecker]

Am Freitag, dem 05.07.2024 um 17:23 +0200 schrieb Alejandro Colomar:
> Hi Martin,
> 
> On Fri, Jul 05, 2024 at 05:02:15PM GMT, Martin Uecker wrote:
> > > But when the thing gets non-trivial, as in strtol(3), GCC misses the
> > > -Wrestrict diagnostic, as reported in
> > > <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112833>.
> > > 
> > > Let's write a reproducer by altering the dumb.c program from above, with
> > > just another reference:
> > > 
> > > 	int
> > > 	dumb2(int *restrict a, int *restrict *restrict ap)
> > > 	{
> > > 		// We don't access the objects
> > > 		return a == *ap;
> > > 	}
> > > 
> > > 	int
> > > 	main(void)
> > > 	{
> > > 		int x = 3;
> > > 		int *xp = &x;
> > > 
> > > 		return dumb2(&x, &xp);
> > > 	}
> > > 
> > > GCC doesn't report anything bad here, even though it's basically the
> > > same as the program from above:
> > > 
> > > 	$ cc -Wall -Wextra dumb2.c
> > > 	$
> > 
> > strtol does have  a "char * restrict * restrict" though, so the
> > situation is different.   A "char **" and a "const char *"
> > shouldn't alias anyway. 
> 
> Pedantically, it is actually declared as 'char **restrict' (the inner
> one is not declared as restrict, even though it will be restricted,
> since there are no other unrestricted pointers).
> 
> I've written functions that more closely resemble strtol(3), to show
> that in the end they all share the same issue regarding const-ness:
> 
> 	$ cat d.c 
> 	int d(const char *restrict ca, char *restrict a)
> 	{
> 		return ca > a;
> 	}
> 
> 	int main(void)
> 	{
> 		char x = 3;
> 		char *xp = &x;
> 		d(xp, xp);
> 	}
> 	$ cc -Wall -Wextra d.c 
> 	d.c: In function ‘main’:
> 	d.c:10:9: warning: passing argument 2 to ‘restrict’-qualified parameter aliases with argument 1 [-Wrestrict]
> 	   10 |         d(xp, xp);
> 	      |         ^
> 
> This trivial program causes a diagnostic.  (Although I think the '>'
> should also cause a diagnostic!!)
> 
> Let's add a reference, to resemble strtol(3):
> 
> 	$ cat d2.c 
> 	int d2(const char *restrict ca, char *restrict *restrict ap)
> 	{
> 		return ca > *ap;
> 	}
> 
> 	int main(void)
> 	{
> 		char x = 3;
> 		char *xp = &x;
> 		d2(xp, &xp);
> 	}
> 	$ cc -Wall -Wextra d2.c 
> 	$ 
> 
> Why does this not cause a -Wrestrict diagnostic, while d.c does?  How
> are these programs any different regarding pointer restrict-ness?

It would require data flow anaylsis to produce the diagnostic while
the first can simply be diagnosed by comparing arguments.

Martin

> 
> > > Well, I don't know how to report that defect to WG14.  If you help me,
> > > I'll be pleased to do so.  Do they have a public mailing list or
> > > anything like that?
> > 
> > One can submit clarification or change requests:
> > 
> > https://www.open-std.org/jtc1/sc22/wg14/www/contributing.html
> 
> Thanks!  Will do.  Anyway, I think this should be discussed in glibc/gcc
> in parallel, since it's clearly a missed diagnostic, and possibly a
> dangerous use of restrict if the compiler does any assumptions that
> shouldn't be done.
> 
> Have a lovely day!
> Alex
>