On Mon, Jun 5, 2017 at 4:58 PM, Magnus Hagander <mag...@hagander.net> wrote: > On Mon, Jun 5, 2017 at 1:16 PM, Amit Kapila <amit.kapil...@gmail.com> wrote: >> >> On Mon, Jun 5, 2017 at 9:15 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: >> > Amit Kapila <amit.kapil...@gmail.com> writes: >> > >> >> I think the same problem can happen during reattach as well. >> >> Basically, MapViewOfFileEx can fail to load image at predefined >> >> address (UsedShmemSegAddr). >> > >> > Once we've successfully done the VirtualAllocEx call, that should hold >> > until the VirtualFree call in PGSharedMemoryReAttach, allowing the >> > immediately-following MapViewOfFileEx call to succeed. Were that not >> > the >> > case, we'd have had problems even without ASLR. We did have problems >> > exactly like that before the pgwin32_ReserveSharedMemoryRegion code was >> > added. >> > >> >> I could not find anything directly in specs which could prove the >> theory either way. However, in one of the StackOverflow discussions, >> it has been indicated that MapViewOfFile can opt to load the image at >> an address different than the predefined address due to ASLR. >> >> > So my feeling about this is that retrying the process creation as >> > in my prototype patch ought to be sufficient; if you think it isn't, the >> > burden of proof is on you. >> > >> >> Sure. I think it is slightly tricky because specs don't say clearly >> how ASLR can impact the behavior of any API and in my last attempt I >> could not reproduce the issue. >> >> I can try to do basic verification with the patch you have proposed, >> but I fear, to do the actual tests as suggested by you is difficult as >> it is not reproducible on my machine, but I can still try. >> >> >> [1] - >> https://stackoverflow.com/questions/9718616/what-does-mapviewoffile-return/11233456 >> Refer below text: >> >> "Yes, MapViewOfFile returns the virtual memory base address where the >> image has been loaded. The value (content) of this address depends on >> whether the image has been successfully loaded at its predefined >> address (which has been setup by the linker) or whether the image has >> been relocated (because the desired, predefined address is already >> occupied or because the image has opt-in to support ASLR)." >> > > That statements refers to mapping executables though, like DLL and EXE. Not > mapping of data segments. > > It does randomize the entire location of the heap, in which case it might > also change. But not for the individual block. > > But in neither of those cases does it help to retry without restarting the > process, >
Okay, the question here is do we need some handling during reattach operation where we do MapViewOfFileEx at the predefined location? -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
