jemalloc() assumes DSS is aligned

[Jason Evans]

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.)

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'.

Thanks,
Jason

P.S. Sorry about putting off responding to your original email for too long.