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Message-ID: <20160112162243.GC13558@port70.net>
Date: Tue, 12 Jan 2016 17:22:44 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: Possible infinite loop in qsort()
* Alexander Cherepanov <ch3root@...nwall.com> [2016-01-12 17:31:31 +0300]:
> On 2016-01-12 15:48, Szabolcs Nagy wrote:
> >in musl things are documented in the git log for now, e.g.:
> >http://git.musl-libc.org/cgit/musl/commit/?id=3cd6f5229f079f892411e82fce3fe15c78eef4d8
>
> IMHO such things should be documented in user-facing documentation, not in
> source code comments, git log or email posts.
>
yes, that would be a useful project
> >i think if an implementation does not give this guarantee
> >that should be considered a bug.
>
> Some consider it a bug, others -- a feature.
>
> But if you want to provide this guarantee it's not that easy. Compilers are
> not under your control. Even with gcc (which tries to provide this
> guarantee) you can create VLA 2.5GB in size and run it with `ulimit -s
> unlimited` (at least as a 32-bit binary on a 64-bit host).
>
large vla sounds like a problem, the libc can guard against that
by placing a guard page in the way on the main thread.
but stack allocations are kind of outside the c language:
stack limits are not admitted in the standard causing technical
issues around correctness proofs.
> Then, a user can create an object of any size via mmap with MAP_FIXED flag,
> right?
creating a single object by two mmaps that happen to be
adjacent sounds like a grey area (not sure if that's strictly
conforming in posix/c).
the user can get a large object behind the libc (e.g. by using
raw syscalls) but the portable ways are controlled by the libc.
> >(glibc does not guarantee this and indeed it is full of invalid
> >pointer arithmetics,
>
> Care to provide examples?
>
this is what i did: went to the glibc string directory
looked for pointer - operations, there are already several
in various argz_* functions so i didnt have to go further
than the letter 'a' to find
char *match = strstr (arg, str);
...
size_t to_len = match - arg;
in argz_replace, if arg is a large object and str only
matches near the end, then match - arg is ub.
i think the argz buffer is internally allocated by
the libc so it could protect against large objects but
i don't see such protection.
the first obvious example i see is in memccpy
void *p = memchr (src, c, n);
...
return __mempcpy (dest, src, p - src + 1);
again p - src can be ub.
> >but more importantly a huge number of
> >existing libraries depend on this)
> >
> >>BTW the support in compilers for working with objects larger than half the
> >>address space is buggy -- see
> >>https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67999 . The same situation --
> >>it neither works nor documented. Somewhat puzzling...
> >
> >yes, but it's not possible to support reasonably
>
> Why is that?
i guess it can be supported but you lose some useful
transformations
e.g. p < q can be transformed to p-q < 0
in the compiler if the diff cannot overflow
i don't know how much this matters for optimization
and how much harder it is to implement, but i do
think there are easy mistakes to be made if p-q can
overflow.
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