On Tue, Mar 03, 2020 at 09:18:44AM -0600, Eric W. Biederman wrote: > Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> writes: > > > 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: > > A couple of things. > > Why do we think it is safe to change the behavior exposed to userspace? > Not the deadlock but all of the times the current code would not > deadlock? > > Especially given that this is a small window it might be hard for people > to track down and report so we need a strong argument that this won't > break existing userspace before we just change things. > > Usually surveying all of the users of a system call that we can find > and checking to see if they might be affected by the change in behavior > is difficult enough that we usually opt for not being lazy and > preserving the behavior. > > This patch is up to two changes in behavior now, that could potentially > affect a whole array of programs. Adding linux-api so that this change > in behavior can be documented if/when this change goes through. > > If you can split the documentation and test fixes out into separate > patches that would help reviewing this code, or please make it explicit > that the your are changing documentation about behavior that is changing > with this patch. Agreed. I think it'd be good to do it in three patches: 1. unrelated documentation update 2. fix + documentation changes specific to the fix 3. test(s) Christian
