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/ -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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