On Thu, Nov 30, 2023 at 02:00:01PM -0500, Mathieu Desnoyers wrote: > On 2023-11-30 13:54, Tycho Andersen wrote: > > On Thu, Nov 30, 2023 at 07:37:02PM +0100, Florian Weimer wrote: > > > * Tycho Andersen: > > > > > > > From: Tycho Andersen <tandersen@xxxxxxxxxxx> > > > > > > > > We are using the pidfd family of syscalls with the seccomp userspace > > > > notifier. When some thread triggers a seccomp notification, we want to do > > > > some things to its context (munge fd tables via pidfd_getfd(), maybe write > > > > to its memory, etc.). However, threads created with ~CLONE_FILES or > > > > ~CLONE_VM mean that we can't use the pidfd family of syscalls for this > > > > purpose, since their fd table or mm are distinct from the thread group > > > > leader's. In this patch, we relax this restriction for pidfd_open(). > > > > > > Does this mean that pidfd_getfd cannot currently be used to get > > > descriptors for a TID if that TID doesn't happen to share its descriptor > > > set with the thread group leader? > > > > Correct, that's what I'm trying to solve. > > > > > I'd like to offer a userspace API which allows safe stashing of > > > unreachable file descriptors on a service thread. > > > > By "safe" here do you mean not accessible via pidfd_getfd()? > > For the LTTng-UST use-case, we need to be able to create and > use a file descriptor from an agent thread injected within the target > process in a way that is safe against patterns where the application > blindly close all file descriptors (for-loop doing close(2), > closefrom(2) or closeall(2)). > > The main issue here is that even though we could handle errors > (-1, errno=EBADF) in the sendmsg/recvmsg calls, re-use of a file > descriptor by the application can lead to data corruption, which > is certainly an unwanted consequence. > > AFAIU glibc has similar requirements with respect to io_uring > file descriptors. I see, thanks. And this introduces another problem: what if one of these things is a memfd, then that memory needs to be invisible to the process as well presumably? This "invisible to the process" mapping would solve another longstanding problem with seccomp: handlers could copy syscall arguments to this safe memory area and then _CONTINUE things safely... Tycho
