Date: Sat, 29 Mar 2014 13:22:12 -0400 From: Rich Felker <dalias@...ifal.cx> To: musl@...ts.openwall.com Cc: u-igbb@...ey.se Subject: Re: malloc not behaving well when brk space is limited? On Sat, Mar 29, 2014 at 07:15:02PM +0200, Timo Teras wrote: > On Sat, 29 Mar 2014 17:00:32 +0000 > u-igbb@...ey.se wrote: > > > Background: > > Compiling a native musl-based toolchain for ia32 on Linux 2.6+. > > Using the standalone dynamic loader mode. > > (The latter seems to lead to a quite limited heap space, by kernel > > behaviour/design) > > > > I encounter out of memory errors. A look at the malloc source does not > > find any fallback to mmap when heap is exhausted. What would you > > suggest as a suitable approach to make it work? > > > > Somebody has possibly already encountered and solved this with musl? > > Yes, been there done that. I patched kernel. > > The thread that follows on sending the patch upstream is e.g. at: > https://groups.google.com/forum/#!msg/linux.kernel/mOf1EWrrhZc/bl96BAE4fyQJ > > Also using grsec kernel would fix the issue mostly, since grsec creates > "better" memory layout for PIE binaries. > > > I see also reports about a related out of memory problem with > > pae-executables which means a solution might help many musl users. > > > > The other standard libraries I am using (glibc, uclibc) seem to > > happily switch to allocation from mmap() when the heap is full. I > > understand that this costs some code and performance but a breakup is > > no good either. > > > > Any ideas? Maintaining and using an external libmalloc or substituting > > malloc in musl? This feels like quite a burden... > > (Would musl internal calls to malloc notice the external library > > and resolve to its entry points instead of the internal malloc?) > > musl does not support external malloc. musl internal calls to > malloc() are not overridable. > > I think you need to fix kernel. Rewrite allocator in musl. Or add the > fallback code to mmap - but dalias said it's "hard". Perhaps still > should be still reconsidered. Unfortunately the approach I want to use with mmap seems to be what glibc uses for its thread-local arenas, and it performs something like 2 to 10 times worse than brk... So unless we can solve that, I don't think it's a good option. It could be a fallback, but I still don't want PIE binaries running that much slower just because the kernel is doing something wacky and wrong. We need a good solution for this problem but I don't have one yet. Rich
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