[PATCH] D159138: [clang][Sema] Fix format size estimator's handling of %o, %x, %X with alternative form

[Nick Desaulniers via Phabricator via cfe-commits]

nickdesaulniers accepted this revision.
nickdesaulniers added a comment.
This revision is now accepted and ready to land.

Thanks for the patch!



================
Comment at: clang/test/Sema/warn-fortify-source.c:100-102
+  __builtin_snprintf(buf, 2, "%#x", n);
+  __builtin_snprintf(buf, 2, "%#X", n);
+  __builtin_snprintf(buf, 2, "%#o", n);
----------------
hazohelet wrote:
> nickdesaulniers wrote:
> > Note that GCC -Wformat-truncation can warn on some of these.
> > 
> > https://godbolt.org/z/jE3axWe1W
> > 
> > Looks like the diagnostic keeps an up and lower bounds on the estimated format string expansion.
> > 
> > Trunk for Clang also warns for these, so is this change a regression? Or are both GCC and Clang (trunk) incorrect?
> Clang trunk is saying something factually incorrect because it says the output `will always be truncated`, when in fact `__builtin_snprintf(buf, 2, "%#x", n);` doesn't trigger truncation if `n` is zero.
> 
> GCC is correct but is more conservative than clang's `ALWAYS be truncated` diagnostics.
> GCC's warning message is `... directive output MAY BE truncated` .
> GCC doesn't warn on it when `n` is known to be zero. (https://godbolt.org/z/E51a3Pfhr)
> 
> GCC's behavior makes sense here because the truncation does happen whenever `n` is not zero. If the user knows `n` is zero then they have no reason to use `%#x` specifier.
> So, I think it makes sense to assume `n` is not zero and emit diagnostics, but it definitely needs diagnostics rewording like `is likely to be truncated`.
I see; thanks for the explanation. Sorry for the code review delay; I missed this comment in my inbox.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D159138/new/

https://reviews.llvm.org/D159138