This is an user visible API so make sure you CC linux-api (added) On Sun 06-08-17 10:04:23, Rik van Riel wrote: > v2: fix MAP_SHARED case and kbuild warnings > > Introduce MADV_WIPEONFORK semantics, which result in a VMA being > empty in the child process after fork. This differs from MADV_DONTFORK > in one important way. > > If a child process accesses memory that was MADV_WIPEONFORK, it > will get zeroes. The address ranges are still valid, they are just empty. > > If a child process accesses memory that was MADV_DONTFORK, it will > get a segmentation fault, since those address ranges are no longer > valid in the child after fork. > > Since MADV_DONTFORK also seems to be used to allow very large > programs to fork in systems with strict memory overcommit restrictions, > changing the semantics of MADV_DONTFORK might break existing programs. > > The use case is libraries that store or cache information, and > want to know that they need to regenerate it in the child process > after fork. > > Examples of this would be: > - systemd/pulseaudio API checks (fail after fork) > (replacing a getpid check, which is too slow without a PID cache) > - PKCS#11 API reinitialization check (mandated by specification) > - glibc's upcoming PRNG (reseed after fork) > - OpenSSL PRNG (reseed after fork) > > The security benefits of a forking server having a re-inialized > PRNG in every child process are pretty obvious. However, due to > libraries having all kinds of internal state, and programs getting > compiled with many different versions of each library, it is > unreasonable to expect calling programs to re-initialize everything > manually after fork. > > A further complication is the proliferation of clone flags, > programs bypassing glibc's functions to call clone directly, > and programs calling unshare, causing the glibc pthread_atfork > hook to not get called. > > It would be better to have the kernel take care of this automatically. > > This is similar to the OpenBSD minherit syscall with MAP_INHERIT_ZERO: > > https://man.openbsd.org/minherit.2 > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majord...@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"d...@kvack.org";> em...@kvack.org </a>
-- Michal Hocko SUSE Labs
