On 03/06/2019 12:50 AM, Jonathan Wakely wrote: > On 06/03/19 02:43 +0000, Pádraig Brady wrote: >> >> >> On 02/26/2019 04:23 PM, Padraig Brady wrote: >>> >>>> Note jemalloc >= 5.1 is required to fix a bug with 0 sizes. >>>> >>>> How serious is the bug? What are the symptoms? >>>> >>> I've updated the commit summary to say it's a crash. >>> Arguably that's better than mem corruption. >>> >>>> It looks like 5.1.0 is less than a year old, so older versions are >>>> presumably still in wide use. >>>> >>>> We could potentially workaround it by making >>>> new_allocator::allocate(0) call ::operator new(1) when >>>> __cpp_sized_deallocation is defined, and deallocate(p, 0) call >>>> ::operator delete(p, 1). Obviously I'd prefer not to do that, because >>>> the default operator new already has an equivalent check, and only >>>> programs linking to jemalloc require the workaround. >>>> >>> Right the jemalloc fix was released May 2018. >>> It would be great to avoid the extra workarounds. >>> Given this would be released about a year after the jemalloc fix was >>> released, >>> and that this would only manifest upon program rebuild, >>> I would be inclined to not add any workarounds. >>> For reference tcmalloc and glibc malloc were seen to work fine with >>> this. >> >> Actually the jemalloc issue will only be fixed with the release of 5.2 >> (a few weeks away). >> I've updated the commit message in the attached accordingly. > > Hmm, I'm a bit nervous about making a change that will cause crashes > unless using an unreleased version (I know it will be released by the > time GCC 9.1 is released, but some people might upgrade GCC without > upgrading jemalloc). Yes it's not ideal. It does make it a lot less risky that one has to rebuild programs to get the new functionality, so existing programs will be unaffected. Also -fsized-deallocation is only enabled by default on gcc with -std >= c++14. > > On the other hand, zero sized allocations should be rare in practice. Yes they were rare in testing here
So programs _rebuilt_ against the following would need to update to jemalloc 5.2: zero sized allocs, jemalloc<5.2, c++>=14, GCC>=9.1 Hopefully that's a small enough set. thanks, Pádraig
