On Sat, Apr 15, 2017 at 02:30:18PM -0700, Andres Freund wrote: > On 2017-04-15 17:24:54 -0400, Tom Lane wrote: > > Andres Freund <and...@anarazel.de> writes: > > > On 2017-04-15 17:09:38 -0400, Tom Lane wrote: > > >> Why doesn't Windows' ability to map the segment into the new process > > >> before it executes take care of that? > > > > > Because of ASLR of the main executable (i.e. something like PIE). > > > > Not following. Are you saying that the main executable gets mapped into > > the process address space immediately, but shared libraries are not?
At the time of the pgwin32_ReserveSharedMemoryRegion() call, the child process contains only ntdll.dll and the executable. > Without PIE/ASLR we can somewhat rely on pgwin32_ReserveSharedMemoryRegion > to find the space that PGSharedMemoryCreate allocated still unoccupied. I've never had access to a Windows system that can reproduce the fork failures. My best theory is that antivirus or similar software injects an additional DLL at that early stage. > > I wonder whether we could work around that by just destroying the created > > process and trying again if we get a collision. It'd be a tad > > inefficient, but hopefully collisions wouldn't happen often enough to be a > > big problem. > > That might work, although it's obviously not pretty. I didn't like that idea when Michael proposed it in 2015. Since disabling ASLR on the exe proved insufficient, I do like it now. It degrades nicely; if something raises the collision rate from 1% to 10%, that just looks like fork latency degrading. > We could also just > default to some out-of-the-way address for MapViewOfFileEx, that might > also work. Dave Vitek proposed that: https://www.postgresql.org/message-id/flat/5046CAEB.4010600%40grammatech.com I estimate more risk than retrying forks. There's no guarantee that a fixed address helpful today won't make the collisions worse after the next Windows security patch. -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers