On Mon, Mar 02, 2020 at 02:13:33AM +1100, Aleksa Sarai wrote: > On 2020-03-01, Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> wrote: > > This fixes a deadlock in the tracer when tracing a multi-threaded > > application that calls execve while more than one thread are running. > > > > I observed that when running strace on the gcc test suite, it always > > blocks after a while, when expect calls execve, because other threads > > have to be terminated. They send ptrace events, but the strace is no > > longer able to respond, since it is blocked in vm_access. > > > > The deadlock is always happening when strace needs to access the > > tracees process mmap, while another thread in the tracee starts to > > execve a child process, but that cannot continue until the > > PTRACE_EVENT_EXIT is handled and the WIFEXITED event is received: > > > > strace D 0 30614 30584 0x00000000 > > Call Trace: > > __schedule+0x3ce/0x6e0 > > schedule+0x5c/0xd0 > > schedule_preempt_disabled+0x15/0x20 > > __mutex_lock.isra.13+0x1ec/0x520 > > __mutex_lock_killable_slowpath+0x13/0x20 > > mutex_lock_killable+0x28/0x30 > > mm_access+0x27/0xa0 > > process_vm_rw_core.isra.3+0xff/0x550 > > process_vm_rw+0xdd/0xf0 > > __x64_sys_process_vm_readv+0x31/0x40 > > do_syscall_64+0x64/0x220 > > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > expect D 0 31933 30876 0x80004003 > > Call Trace: > > __schedule+0x3ce/0x6e0 > > schedule+0x5c/0xd0 > > flush_old_exec+0xc4/0x770 > > load_elf_binary+0x35a/0x16c0 > > search_binary_handler+0x97/0x1d0 > > __do_execve_file.isra.40+0x5d4/0x8a0 > > __x64_sys_execve+0x49/0x60 > > do_syscall_64+0x64/0x220 > > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > The proposed solution is to have a second mutex that is > > used in mm_access, so it is allowed to continue while the > > dying threads are not yet terminated. > > > > I also took the opportunity to improve the documentation > > of prepare_creds, which is obviously out of sync. > > > > Signed-off-by: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> > > I can't comment on the validity of the patch, but I also found and > reported this issue in 2016[1] and the discussion quickly veered into > the problem being more complicated (and uglier) than it seems at first > glance. > > You should probably also Cc stable, given this has been a long-standing > issue and your patch doesn't look (too) invasive. > > [1]: https://lore.kernel.org/lkml/20160921152946.GA24210@xxxxxxxxxxxxxx/ Yeah, I remember you mentioning this a while back. Bernd, we really want a reproducer for this sent alongside with this patch added to: tools/testing/selftests/ptrace/ Having a test for this bug irrespective of whether or not we go with this as fix seems really worth it. Oleg seems to have suggested that a potential alternative fix is to wait in de_thread() until all other threads in the thread-group have passed exit_notiy(). Right now we only kill them but don't wait. Currently de_thread() only waits for the thread-group leader to pass exit_notify() whenever a non-thread-group leader thread execs (because the exec'ing thread becomes the new thread-group leader with the same pid as the former thread-group leader). Christian
