On Wed, Jun 22, 2011 at 11:00, Greg Kurz <gkurz@xxxxxxxxxx> wrote: > On Mon, 2011-06-20 at 13:37 -0400, Bryan Donlan wrote: >> On Mon, Jun 20, 2011 at 07:45, Greg Kurz <gkurz@xxxxxxxxxx> wrote: >> > On Thu, 2011-06-16 at 13:54 -0400, Bryan Donlan wrote: >> >> >> Although getting the in-namespace PID is a useful thing, wouldn't a >> >> truly race-free API be preferable? Any access by PID has the race >> >> condition in which the target process could die, and its PID get >> >> recycled between retrieving the PID and doing something with it. >> > >> > Well the PID is a racy construct when used by another task than the >> > parent... fortunately, most userland code can cope with it ! :) >> >> That doesn't mean we shouldn't try to fix the race! :) >> >> >> Perhaps a file-descriptor API would be better, such as something like >> >> this: >> >> >> >> int openpid(int id, int flags); >> >> int rt_sigqueueinfo_fd(int process_fd, int sig, siginfo_t *info); >> >> int sigqueue_fd(int process_fd, int sig, const union sigval value); // >> >> glibc wrapper >> >> >> > >> > The race still exists: openpid() is being passed a PID... Only the >> > parent can legitimately know that this PID identifies a specific >> > unwaited child. >> >> Yes, the idea would be either the parent process, or the target >> process itself would open the PID, then pass the resulting file >> descriptor to whatever process is actually doing the killing. > > Agreed. Such an API would be useful in a scenario where the task to be > killed and the killing task can share a file descriptor: same thread > group or inherited with clone() or connected with an AF_UNIX socket. > My point was just that the racy pid based API will still be needed to > handle all the other scenarios. But maybe it's fine to have two sets of > process handling calls. Actually, with a /proc/self/sigqueue file as Eric suggested, , it would be possible to mitigate some races even if you're not passed a fd from the target or its parent. Consider: 1) Read somedaemon.pid 2) Open the /proc/$pid directory 3) Use openat to open /proc/$pid/status; check that Name: matches the daemon's name; if not go to 1 4) Use openat to open /proc/$pid/sigqueue; send a signal It's not foolproof, in that you might hit a different process with the same name+uid+gid+groups+cmdline+environment+etc, but any such cross-process operation is racy to the extent that, at some level, you need to have criteria for selecting your target, and such criteria might allow for false positives. _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers
