On Tue, Mar 03, 2020 at 04:48:01PM +0000, Bernd Edlinger wrote: > On 3/3/20 4:18 PM, 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. > > > > Hmm, I tend to agree. > > > 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. > > > > One is PTRACE_ACCESS possibly returning EAGAIN, yes. > > We could try to restrict that behavior change to when any > thread is ptraced when execve starts, can't be too complicated. > > > But the other is only SYS_seccomp returning EAGAIN, when a different > thread of the current process is calling execve at the same time. > > I would consider it completely impossible to have any user-visual effect, > since de_thread is just terminating all threads, including the thread > where the -EAGAIN was returned, so we will never know what happened. I think if we risk a user-space facing change we should try the simple thing first before making the fix more convoluted? But it's a tough call... > > > > 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. > > > > I am not sure if I have touched the right user documentation. > > I only saw a document referring to a non-existent "current->cred_replace_mutex" > I haven't digged the git history, but that must be pre-historic IMHO. > It appears to me that is some developer documentation, but it's nevertheless > worth to keep up to date when the code changes. > > So where would I add the possibility for PTRACE_ATTACH to return -EAGAIN ? Since that would be a potentially user-visible change it would make the most sense to add it to man ptrace(2) if/when we land this change. For developers, placing a comment in kernel/ptrace.c:ptrace_attach() would make the most sense? We already have something about exec protection in there. Christian
