On Fri, Jan 09, 2015 at 03:24:12PM -0800, Andy Lutomirski wrote: > On Fri, Jan 9, 2015 at 3:12 PM, Rich Felker <dal...@aerifal.cx> wrote: > > On Fri, Jan 09, 2015 at 10:57:43PM +0000, Al Viro wrote: > >> On Fri, Jan 09, 2015 at 05:42:52PM -0500, Rich Felker wrote: > >> > >> > Here's a very simple way it could work -- it could put the O_PATH fd > >> > on a previously-unused fd number, and put a special flag on the fd, > >> > like FD_CLOEXEC, but that causes the kernel to close it whenever it's > >> > opened. The pathname passed could then simply be /dev/fd/%d or > >> > /proc/self/fd/%d, and although this is presently dependent on /proc > >> > being mounted, virtual /dev/fd/* could someday be something completely > >> > independent of procfs. The kernel keeps all the freedom to choose how > >> > to pass the name to the interpreter. I'm not proposing any kernel > >> > API/ABI lock-in and I'm with you in opposing such lock-in. > >> > >> Huh? open() on procfs symlinks does *NOT* work the way - the symlink is > >> traversed and after that point there is no information whatsoever how we > >> got to that vfsmount/dentry pair. I can imagine several kludges that would > >> work, but they are unspeakably ugly, and do_last() is already far too > >> convoluted as it is. > > > > I'm not sure where you're disagreeing with me. open of procfs symlinks > > does not resolve the symlink and open the resulting pathname. They are > > "magic symlinks" which are bound to the inode of the open file. I > > don't see why this action, which is already special for magic > > symlinks, can't check a flag on the magic symlink and possibly close > > the corresponding file descriptor as part of its action. > > > > In any case, whether/how fexecve works with interpreters is something > > the kernel can change without breaking userspace expectations. My goal > > is to avoid creating any new API/ABI requirement here. > > I think that, if we really want to support clean fexecve on O_CLOEXEC > scripts some day, the right way to do it is to fix the script > interface for real. Have a special flag in the headers of script > interpreters that support a new interface that says "when I'm a script > interpreter, I expect an auxv entry AT_SCRIPT_FD with an open fd with > CLOEXEC set". Then we can directly exec scripts by fd, even with > O_CLOEXEC set, without any races.
This is also acceptable, but I don't think you'd really need a special header flag. Just pass it, and also pass /dev/fd/%d or /proc/self/fd/%d in argv[]. If the interpreter supports it, everything works fine. If not, it still works as long as /proc is mounted, but with a partial fd leak. (Note: the leak is not so bad since the interpreter would inherit a close-on-exec fd and thus would not leak it further.) Aside from setting up the new auxv entry, the main trick the kernel would have to do is bypassing FD_CLOEXEC at exec time while keeping the FD_CLOEXEC flag present on the fd after exec. Rich -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
