On Sat, Oct 18, 2014 at 08:40:05AM -0700, Linus Torvalds wrote: > On Sat, Oct 18, 2014 at 8:35 AM, Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > Look around for AIO. Look around for the loop driver. Look around for > > a number of things that do "fget()" and that you completely ignored. > > .. actually, there are more instances of "get_file()" than of > "fget()", the aio one just happened to be the latter form. Lots and > lots of ways to get ahold of a file descriptor that keeps it open past > the "last close". FWIW, procfs patch touches a very annoying issue: ->show_fdinfo() being blocking. I would really like to get rid of that particular get_file() and even more so - of get_files_struct() in there. I certainly agree that anyone who expects that close() means the end of IO is completely misguided. Mappings don't disappear on close(), neither does a descriptor returned by dup(), or one that child got over fork(), or something sent over in SCM_RIGHTS datagram, or, as you suggested, made backing store for /dev/loop, etc. What's more, in the example given upthread, somebody might've spotted that file in /proc/<pid>/fd/* and *opened* it. At which point umount would have to fail with EBUSY. And the same lsof(8) might've done just that. It's not a matter of correctness or security, especially since somebody who could do that, could've stopped your process, PTRACE_POKEd a fairly short series of syscalls that would connect to AF_UNIX socket, send the file over to them and clean after itself, then single-stepped through all of that, restored the original state and resumed your process. It is a QoI matter, though. And get_files_struct() in there is a lot more annoying than get_file()/fput(). Suppose you catch the process during exit(). All of a sudden, read from /proc/<pid>/fdinfo/<n> ends up doing shitloads of filp_close(). It would be nice to avoid that. Folks, how much pain would it be to make ->show_fdinfo() non-blocking? -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
