Date: Mon, 27 Jun 2022 19:33:14 +0200 From: Szabolcs Nagy <nsz@...t70.net> To: Florian Weimer <fweimer@...hat.com> Cc: Nick Peng <pymumu@...il.com>, musl@...ts.openwall.com Subject: Re: BUG: Calling readdir/dirfd after vfork will cause deadlock. * Florian Weimer <fweimer@...hat.com> [2022-06-27 11:23:48 +0200]: > * Szabolcs Nagy: > > > * Florian Weimer <fweimer@...hat.com> [2022-06-27 09:42:57 +0200]: > >> * Szabolcs Nagy: > >> > >> > * Nick Peng <pymumu@...il.com> [2022-06-25 11:40:17 +0800]: > >> >> Description: After vfork, calling functions such as readdir/dirfd may > >> >> cause deadlock. GNU C is OK. > >> > > >> > why do you think "GNU C is OK"? is this from some real software? > >> > >> glibc supports opendir/readdir/closedir after vfork as an extension. > >> The JVM depends on it. > > > > how does that work? i think glibc just calls vfork syscall (or > > clone(CLONE_VM|CLONE_VFORK)) from asm and opendir allocates. > > so i'd expect a deadlock where the parent waits for the child > > to exec while holding the malloc lock. > > vfork stops the thread in the parent and uses its resources. It is the > same userspace thread (with the same TCB), only the kernel TID is wrong. > glibc's malloc-internal locks do not rely on the TID, so there is no > incrased risk of deadlock. The malloc locks are internal, so user code > cannot call vfork while they are locked. If another thread has locked > them at the point of vfork, that thread will eventually unlock them, > unblocking the vfork'ed subprocess. This relies on the shared address > space of vfork. > > Without the shared address space, none of this would work, and for fork, > we have complicated code to manage glibc-internal locks (including the > malloc locks). ah i thought vfork stops the entire parent process (based on linux man). ok then i think musl should work too in practice.. however on some targets musl just uses a fork syscall instead of vfork. i think that's the reason why this fails on aarch64 and mips. so i think if musl adds src/process/ARCH/vfork.s then it would work.
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.