Re: [RFC] freeing unliked file indefinitely delayed

[Ben Myers]

Hey Al,

On Wed, Jul 08, 2015 at 02:42:38AM +0100, Al Viro wrote:
> 	Normally opening a file, unlinking it and then closing will have
> the inode freed upon close() (provided that it's not otherwise busy and
> has no remaining links, of course).  However, there's one case where that
> does *not* happen.  Namely, if you open it by fhandle with cold dcache,
> then unlink() and close().
> 
> 	In normal case you get d_delete() in unlink(2) notice that dentry
> is busy and unhash it; on the final dput() it will be forcibly evicted from
> dcache, triggering iput() and inode removal.  In this case, though, we end
> up with *two* dentries - disconnected (created by open-by-fhandle) and
> regular one (used by unlink()).  The latter will have its reference to inode
> dropped just fine, but the former will not - it's considered hashed (it
> is on the ->s_anon list), so it will stay around until the memory pressure
> will finally do it in.  As the result, we have the final iput() delayed
> indefinitely.  It's trivial to reproduce -
> 
> #define _GNU_SOURCE
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> 
> void flush_dcache(void)
> {
>         system("mount -o remount,rw /");
> }
> 
> static char buf[20 * 1024 * 1024];
> 
> main()
> {
>         int fd;
>         union { 
>                 struct file_handle f;
>                 char buf[MAX_HANDLE_SZ];
>         } x;
>         int m;
> 
>         x.f.handle_bytes = sizeof(x);
>         chdir("/root");
>         mkdir("foo", 0700);
>         fd = open("foo/bar", O_CREAT | O_RDWR, 0600);
>         close(fd);
>         name_to_handle_at(AT_FDCWD, "foo/bar", &x.f, &m, 0);
>         flush_dcache();
>         fd = open_by_handle_at(AT_FDCWD, &x.f, O_RDWR);
>         unlink("foo/bar");
>         write(fd, buf, sizeof(buf));
>         system("df .");			/* 20Mb eaten */
>         close(fd);
>         system("df .");			/* should've freed those 20Mb */
>         flush_dcache();
>         system("df .");			/* should be the same as #2 */
> }
> 
> will spit out something like
> Filesystem     1K-blocks   Used Available Use% Mounted on
> /dev/root         322023 303843      1131 100% /
> Filesystem     1K-blocks   Used Available Use% Mounted on
> /dev/root         322023 303843      1131 100% /
> Filesystem     1K-blocks   Used Available Use% Mounted on
> /dev/root         322023 283282     21692  93% /
> - inode gets freed only when dentry is finally evicted (here we trigger
> than by remount; normally it would've happened in response to memory
> pressure hell knows when).
> 
> IMO it's a bug.  Between the close() and final flush_dcache() the file has
> no surviving links, is *not* busy, won't show up in fuser/lsof/whatnot
> output, and yet it's still not freed.  I'm not saying that it's realistically
> exploitable (albeit with nfsd it might be), but it's a very unpleasant
> self-LART.
> 
> FWIW, my prefered fix would be simply to have the final dput() treat
> disconnected dentries as "kill on sight"; checking for i_nlink of the
> inode, as Bruce suggested several years ago, will *not* work, simply
> because having another link to that file and unlinking it after close
> will reproduce the situation - disconnected dentry sticks around in
> dcache past its final dput() and past the last unlink() of our file.
> Theoretically it might cause an overhead for nfsd (no_subtree_check v3
> export might see more d_alloc()/d_free(); icache retention will still
> prevent constant rereading the inode from disk).  _IF_ that proves to
> be noticable, we might need to come up with something more elaborate
> (e.g. have unlink() and rename() kick disconnected aliases out if the link
> count has reached 0), but it's more complex and needs careful ananlysis
> of correctness - we need to prove that there's no way to miss the link
> count reaching 0.

The bug rings a bell for me so I will stick my neck out instead of
lurking.  Don't you need to sample that link count under the filesystems
internal lock in order to avoid an unlink/iget race?  I suggest creating
a helper to prune disconnected dentries which a filesystem could call in
.unlink.  That would avoid the risk of unintended side effects with the
d_alloc/d_free/icache approach and have provable link count correctness.

Thanks,
Ben
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