[Bug nptl/13347] Threaded setuid() can wrongly report success when failing to drop privileges

["bugdal at aerifal dot cx" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=13347

--- Comment #5 from Rich Felker <bugdal at aerifal dot cx> 2012-07-24 23:04:20 UTC ---
By the user, I assume you mean the calling application. The problem is that
there's no established way to report "something beyond your control went
horribly wrong and you're in an inconsistent state" to the application. You can
report failure, but applications are likely to assume this means the call had
no effect, not that it had a partial effect.

Ultimately, Linux should add a proper setuid syscall that affects all threads
of the current process (thread group) atomically, and glibc should use that if
it's available and only fallback to the ugly method currently in use on older
kernels that don't provide the new syscall.

Anyway I'll start out with a patch to make it report failure, and see where
things go from there...

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