On 4/30/20 1:59 AM, Jann Horn wrote: > On Thu, Apr 30, 2020 at 1:22 AM Linus Torvalds > <torva...@linux-foundation.org> wrote: >> On Wed, Apr 29, 2020 at 3:38 PM Linus Torvalds >> <torva...@linux-foundation.org> wrote: >>> >>> If you do it properly, with a helper function instead of repeating >>> that fragile nasty thing, maybe it will look better to me.
I added the BIG FAT WARNNIG comments as a mitigation for that. Did you like those comments? >> >> Side note: if it has a special helper function for the "get lock, >> repeat if it was invalid", you can do a better job than return >> -EAGAIN. >> >> In particular, you can do this >> >> set_thread_flag(TIF_SIGPENDING); >> return -RESTARTNOINTR; >> >> which will actually restart the system call. So a ptrace() user (or >> somebody doing a "write()" to /proc/<pid>/attr/xyz, wouldn't even see >> the impossible EAGAIN error. > > Wouldn't you end up livelocked in the scenario that currently deadlocks? Like: > > - tracer attaches to thread A > - thread B goes into execve, blocks on waiting for A's death > - tracer tries to attach to B and hits the -EAGAIN > > If we make the PTRACE_ATTACH call restart, the tracer will just end up > looping without ever resolving the deadlock. If we want to get through > this cleanly with this approach, userspace needs to either > deprioritize the "I want to attach to pid X" and go back into its > eventloop, or to just treat -EAGAIN as a fatal error and give up > trying to attach to that task. > Yes, exactly, the point is the caller is expected to call wait in that scenario, otherwise the -EAGAIN just repeats forever, that is an API change, yes, but something unavoidable, and the patch tries hard to limit it to cases where the live-lock or pseudo-dead-lock is unavoidable anyway. Bernd.
