clang: Reject %n for __attribute__((format(__freebsd_kprintf__)))

[cem]

Summary

A follow-up to r360849.

Reported by: imp

Test Plan

Seems to work:

/home/conrad/a.c:94:21: warning: invalid conversion specifier 'n' [-Wformat-invalid-specifier]
        my_kprintflike("2 %n %s\n", &loops, "not ok");
                          ~^

a.c:

extern void
my_kprintflike(const char *s, ...) __attribute__((__format__(__freebsd_kprintf__, 1, 2)));

int main(int argc, char **argv)
{
…
        my_kprintflike("1 %s\n", "ok");
        my_kprintflike("2 %n %s\n", &loops, "not ok");
}

[emaste]

Sounds good to me, we'll want to submit this upstream with a test case.

[cem]

In D24786#545386, @emaste wrote:

Sounds good to me, we'll want to submit this upstream with a test case.

Hm, unfortunately our contrib version of LLVM excludes the test subdirectories, so I can't easily add a test case here. I expect it would be easy for someone with an existing upstream relationship with LLVM to write one.

[imp]

Love it. Easy to upstream.