On Sat, 2018-03-17 at 15:52 +0000, Al Viro wrote: > On Sat, Mar 17, 2018 at 11:43:28AM -0400, Jeff Layton wrote: > > On Sat, 2018-03-17 at 15:05 +0000, Al Viro wrote: > > > On Sat, Mar 17, 2018 at 10:25:20AM -0400, Jeff Layton wrote: > > > > From: Jeff Layton <jlayton@xxxxxxxxxx> > > > > > > > > POSIX mandates that open fds and their associated file locks should be > > > > preserved across an execve. This works, unless the process is > > > > multithreaded at the time that execve is called. > > > > > > > > In that case, we'll end up unsharing the files_struct but the locks will > > > > still have their fl_owner set to the address of the old one. Eventually, > > > > when the other threads die and the last reference to the old > > > > files_struct is put, any POSIX locks get torn down since it looks like > > > > a close occurred on them. > > > > > > > > The result is that all of your open files will be intact with none of > > > > the locks you held before execve. The simple answer to this is "use OFD > > > > locks", but this is a nasty surprise and it violates the spec. > > > > > > > > On a successful execve, change ownership of any POSIX file_locks > > > > associated with the old files_struct to the new one, if we ended up > > > > swapping it out. > > > > > > TBH, I don't like the way you implement that. Why not simply use > > > iterate_fd()? > > > > Ahh, I wasn't aware of it. I copied the loop in change_lock_owners from > > close_files. I'll have a look at iterate_fd(). > > BTW, if iterate_fd() turns out to be slower, it might make sense to have it > look at the bitmap to skip unpopulated parts of descriptor table faster - > other users might also benefit from that. Thanks, I'll keep that in mind. Full disclosure: I haven't done any performance testing with this. My assumption is that threaded programs that execve without forking first are rather rare. I don't know of a great way to confirm that though. I made a small change to the v2 patch as well to use struct files_struct * instead of fl_owner_t here. That gives us more type safety and should prevent any problems if Bruce's patch to remove fl_owner_t gets merged. Thanks, -- Jeff Layton <jlayton@xxxxxxxxxx>
