jemalloc() assumes DSS is aligned
John Baldwin
jhb at freebsd.org
Wed Jun 13 17:32:14 UTC 2012
On Wednesday, June 13, 2012 12:29:26 pm Jason Evans wrote:
> On Jun 13, 2012, at 8:31 AM, John Baldwin wrote:
> > I tracked down a weird bug at work on the older jemalloc in FreeBSD 8/9 that a
> > co-worker tripped over. Specifically, if you build the program below and link
> > it with gold, the program will have an _end symbol that is on an odd address
> > (std::nothrow results in some single-byte symbol being added to the end of the
> > BSS). This causes the first arena allocated by jemalloc to use an odd
> > address, and the rbt_nil structures for that arena's embedded trees (like
> > runs_avail) to be allocated on odd addresses. This interferes with the RB
> > trees using the low bit to distinguish red vs black. Specifically, the
> > program ends up setting the right node of rbt_nil to an incorrect pointer
> > value (the low bit gets cleared) resulting in an eventual segfault. Looking
> > at phkmalloc, it always applied round_page() to the results from sbrk(). I
> > believe that for jemalloc only the very first allocation from the DSS needs to
> > check for misalignment, and the patch below does fix the segfault on FreeBSD
> > 8. I have a stab at porting the change to jemalloc 3.0.0 in HEAD, but I'm not
> > sure if it is quite correct. Also, I only made the DSS align on the quantum
> > boundary rather than a page boundary. BTW, I filed a bug with the binutils
> > folks as I initially thought this was a gold bug. However, POSIX doesn't make
> > any guarantees about the return value of sbrk(), so I think gold is not
> > broken.
>
> Hi John,
>
> Your fix for FreeBSD 7/8/9 looks correct to me. I don't currently have any development machines running anything but 10-CURRENT, so I'd be
grateful if you could commit the fix, assuming it isn't much trouble for you. (I'll set up additional development installations if needed.)
Sure, I'm fine with doing that.
> I don't think this is an issue for HEAD's chunk_alloc_dss(), because there is logic to always insert enough padding to allocate on chunk alignment
boundaries, and also base_alloc() no longer makes any attempt to use a partial dss 'chunk'.
Ok, this was my main concern was to ensure it was fixed going forward.
> Thanks,
> Jason
>
> P.S. Sorry about putting off responding to your original email for too long.
No problem, I figured the original got lost. :-P
--
John Baldwin
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