On Mon, Nov 04, 2024 at 01:36:43PM +0100, Florian Weimer wrote: > * Peter Zijlstra: > > > On Sat, Nov 02, 2024 at 10:58:42PM +0100, Florian Weimer wrote: > > > >> QEMU hints towards further problems (in linux-user/syscall.c): > >> > >> case TARGET_NR_set_robust_list: > >> case TARGET_NR_get_robust_list: > >> /* The ABI for supporting robust futexes has userspace pass > >> * the kernel a pointer to a linked list which is updated by > >> * userspace after the syscall; the list is walked by the kernel > >> * when the thread exits. Since the linked list in QEMU guest > >> * memory isn't a valid linked list for the host and we have > >> * no way to reliably intercept the thread-death event, we can't > >> * support these. Silently return ENOSYS so that guest userspace > >> * falls back to a non-robust futex implementation (which should > >> * be OK except in the corner case of the guest crashing while > >> * holding a mutex that is shared with another process via > >> * shared memory). > >> */ > >> return -TARGET_ENOSYS; > > > > I don't think we can sanely fix that. Can't QEMU track the robust thing > > itself and use waitpid() to discover the thread is gone and fudge things > > from there? > > There are race conditions with munmap, I think, and they probably get a > lot of worse if QEMU does that. > > See Rich Felker's bug report: > > | The corruption is performed by the kernel when it walks the robust > | list. The basic situation is the same as in PR #13690, except that > | here there's actually a potential write to the memory rather than just > | a read. > | > | The sequence of events leading to corruption goes like this: > | > | 1. Thread A unlocks the process-shared, robust mutex and is preempted > | after the mutex is removed from the robust list and atomically > | unlocked, but before it's removed from the list_op_pending field of > | the robust list header. > | > | 2. Thread B locks the mutex, and, knowing by program logic that it's > | the last user of the mutex, unlocks and unmaps it, allocates/maps > | something else that gets assigned the same address as the shared mutex > | mapping, and then exits. > | > | 3. The kernel destroys the process, which involves walking each > | thread's robust list and processing each thread's list_op_pending > | field of the robust list header. Since thread A has a list_op_pending > | pointing at the address previously occupied by the mutex, the kernel > | obliviously "unlocks the mutex" by writing a 0 to the address and > | futex-waking it. However, the kernel has instead overwritten part of > | whatever mapping thread A created. If this is private memory it > | (probably) doesn't matter since the process is ending anyway (but are > | there race conditions where this can be seen?). If this is shared > | memory or a shared file mapping, however, the kernel corrupts it. > | > | I suspect the race is difficult to hit since thread A has to get > | preempted at exactly the wrong time AND thread B has to do a fair > | amount of work without thread A getting scheduled again. So I'm not > | sure how much luck we'd have getting a test case. > > > <https://sourceware.org/bugzilla/show_bug.cgi?id=14485#c3> So I've only managed to conjure up two horrible solutions for this: - put the robust futex operations under user-space RCU, and mandate a matching synchronize_rcu() before any munmap() calls. - add a robust-barrier syscall that waits until all list_op_pending are either NULL or changed since invocation. And mandate this call before munmap(). Neither are particularly pretty I admit, but at least they should work. But doing this and mandating the alignment thing should at least make this qemu thing workable, no? > We also have a silent unlocking failure because userspace does not know > about ROBUST_LIST_LIMIT: > > Bug 19089 - Robust mutexes do not take ROBUST_LIST_LIMIT into account > <https://sourceware.org/bugzilla/show_bug.cgi?id=19089> > > (I think we may have discussed this one before, and you may have > suggested to just hard-code 2048 in userspace because the constant is > not expected to change.) > > So the in-mutex linked list has quite a few problems even outside of > emulation. 8-( It's futex, ofcourse its a pain in the arse :-) And yeah, no better ideas on that limit for now...
