This is the mail archive of the
mailing list for the glibc project.
Re: [patch] For BZ #17328, mark __errno_location with __attribute__((returns_nonnull)) for gcc >=4.9.0
- From: Rich Felker <dalias at libc dot org>
- To: Zack Weinberg <zackw at panix dot com>
- Cc: Paul Pluzhnikov <ppluzhnikov at gmail dot com>, GLIBC Devel <libc-alpha at sourceware dot org>
- Date: Mon, 2 Mar 2015 17:39:54 -0500
- Subject: Re: [patch] For BZ #17328, mark __errno_location with __attribute__((returns_nonnull)) for gcc >=4.9.0
- Authentication-results: sourceware.org; auth=none
- References: <CALoOobOuAEpw+zxRrrDyHB7UVbAZMzreXqpujzZOWNLS7+aRUA at mail dot gmail dot com> <20150301011753 dot GV23507 at brightrain dot aerifal dot cx> <CAPC3xapQBMH+DJdup2Y8_tt6xdcFAnQLB_K8VpT3ouCavvzXXA at mail dot gmail dot com> <CAKCAbMgyAEueVsYB=avfUg0pK49YiY-Qyi8w1qXvfTCzfPbUiQ at mail dot gmail dot com>
On Mon, Mar 02, 2015 at 05:23:58PM -0500, Zack Weinberg wrote:
> On Sat, Feb 28, 2015 at 8:54 PM, Paul Pluzhnikov <firstname.lastname@example.org> wrote:
> > On Sat, Feb 28, 2015 at 5:17 PM, Rich Felker <email@example.com> wrote:
> >> No objection, but I'm curious if there's any practical benefit of
> >> this. I can't think of many situations where knowledge that
> >> &errno!=NULL would assist the compiler in optimizing or diagnostics.
> > Hmm, maybe you are right.
> > I was reading https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58689 and
> > threads linked from it, and it really looks like not much is gained
> > inside (or outside) GLIBC -- it's not like anyone ever checks whether
> > __errno_location() returned NULL.
> > Zack, do you have any additional motivation for filing
> > https://sourceware.org/bugzilla/show_bug.cgi?id=17328
> My original motivation was to improve code generation with
> -fsanitize=undefined, which, among other things, instruments *every
> use of errno* with a check to ensure that the pointer returned by
> __errno_location is non-null. For instance, the admittedly silly code
This is indeed ugly, but I'm not necessarily convinced that improving
code optimization with ubsan is a worthwhile use of human effort or
code complexity. Anyone else have opinions on this?
> clang 3.5 does essentially the same thing. However, as is, adding
> __attribute__((returns_nonnull)) doesn't improve matters, because
> neither gcc nor clang will optimize out the checks based on that. I
> did file a GCC bug for that, but people were reluctant to make it
> smarter for fear of losing necessary checks:
This seems reasonable if ubsan is actually intended only for debugging
and/or hardcode expensive hardening, which is my impression.
> .... so I gotta wonder if it mightn't be a better use of people's time
> to make errno a proper thread-local variable instead. What's blocking
I seem to recall there was some issue with buggy programs having
conflicting symbol definitions of errno as a non-TLS symbol. But I
don't see why something like the following wouldn't work:
extern __thread int __errno_tls;
#define errno __errno_tls
Presumably IE-model could even be used for references since there
would always be a definition. I don't know if there are any cases
where this would pessimize code generation or not (versus the current