Date: Thu, 14 Jan 2016 17:21:51 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Possible infinite loop in qsort() On Tue, Jan 12, 2016 at 05:22:44PM +0100, Szabolcs Nagy wrote: > > >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. While the C standard fails to specify it as such, overflowing the stack has to be treated as undefined behavior. One such case of overflow is an object >SIZE_MAX/2 bytes. > > 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). POSIX is not clear on how the memory obtained by mmap becomes C "objects", but it's not important anyway. You cannot use MAP_FIXED to create such objects because passing an address to mmap/MAP_FIXED that you don't already own/control produces UB. You could use opportunistic address requests to attempt to produce such a large contiguous region, but you still would not be justified in interpreting them as a single large object. > 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. These are not formal objects; if you do stupid stuff by calling syscalls directly, you get what you deserve. Rich
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