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Message-ID: <862e36a2-2a6f-4e26-3228-8cab4b4cf230@gmail.com>
Date: Sun, 7 Oct 2018 11:48:37 +0100
From: Alan Jenkins <alan.christopher.jenkins@...il.com>
To: David Howells <dhowells@...hat.com>, viro@...iv.linux.org.uk
Cc: torvalds@...ux-foundation.org, ebiederm@...ssion.com,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
mszeredi@...hat.com
Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE
[ver #12]
On 05/10/2018 19:24, Alan Jenkins wrote:
> On 21/09/2018 17:30, David Howells wrote:
>> From: Al Viro <viro@...iv.linux.org.uk>
>>
>> Allow a detached tree created by open_tree(..., OPEN_TREE_CLONE) to be
>> attached by move_mount(2).
>>
>> If by the time of final fput() of OPEN_TREE_CLONE-opened file its
>> tree is
>> not detached anymore, it won't be dissolved. move_mount(2) is adjusted
>> to handle detached source.
>>
>> That gives us equivalents of mount --bind and mount --rbind.
>>
>> Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
>> Signed-off-by: David Howells <dhowells@...hat.com>
>> ---
>>
>> fs/namespace.c | 26 ++++++++++++++++++++------
>> 1 file changed, 20 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/namespace.c b/fs/namespace.c
>> index dd38141b1723..caf5c55ef555 100644
>> --- a/fs/namespace.c
>> +++ b/fs/namespace.c
>> @@ -1785,8 +1785,10 @@ void dissolve_on_fput(struct vfsmount *mnt)
>> {
>> namespace_lock();
>> lock_mount_hash();
>> - mntget(mnt);
>> - umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
>> + if (!real_mount(mnt)->mnt_ns) {
>> + mntget(mnt);
>> + umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
>> + }
>> unlock_mount_hash();
>> namespace_unlock();
>> }
>> @@ -2393,6 +2395,7 @@ static int do_move_mount(struct path *old_path,
>> struct path *new_path)
>> struct mount *old;
>> struct mountpoint *mp;
>> int err;
>> + bool attached;
>> mp = lock_mount(new_path);
>> err = PTR_ERR(mp);
>> @@ -2403,10 +2406,19 @@ static int do_move_mount(struct path
>> *old_path, struct path *new_path)
>> p = real_mount(new_path->mnt);
>> err = -EINVAL;
>> - if (!check_mnt(p) || !check_mnt(old))
>> + /* The mountpoint must be in our namespace. */
>> + if (!check_mnt(p))
>> + goto out1;
>> + /* The thing moved should be either ours or completely
>> unattached. */
>> + if (old->mnt_ns && !check_mnt(old))
>> goto out1;
>> - if (!mnt_has_parent(old))
>> + attached = mnt_has_parent(old);
>> + /*
>> + * We need to allow open_tree(OPEN_TREE_CLONE) followed by
>> + * move_mount(), but mustn't allow "/" to be moved.
>> + */
>> + if (old->mnt_ns && !attached)
>> goto out1;
>> if (old->mnt.mnt_flags & MNT_LOCKED)
>
> Hi
>
> I replied last time to wonder about the MNT_UMOUNT mnt_flag. So I've
> tested it now :-), on David's current tree (commit 5581f4935add).
>
> The modified do_move_mount() allows re-attaching something that was
> lazy-unmounted. But the lazy unmount sets MNT_UMOUNT. And this flag is
> not cleared when the mount is re-attached.
>
> I wasn't sure what effect this would have. Luckily it showed up
> straight away, when I tried to unmount again. It causes a soft lockup.
>
> Debug printk:
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 4dfe7e23b7ee..ac8de9191cfe 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2472,6 +2472,10 @@ static int do_move_mount(struct path *old_path,
> struct path *new_path)
> if (old->mnt.mnt_flags & MNT_LOCKED)
> goto out1;
>
> + pr_info("mnt_flags=%x umount=%x\n",
> + (unsigned) old->mnt.mnt_flags,
> + (unsigned) !!(old->mnt.mnt_flags & MNT_UMOUNT);
> +
> if (old_path->dentry != old_path->mnt->mnt_root)
> goto out1;
The lockup seems to be a general problem with the cleanup code. Even if
I use this as advertised, i.e. for a simple bind mount.
(I was suspicious that being able to pass around detached trees as an
FD, and re-attach them in any namespace, allows leaking memory by
creating a namespace loop. I.e. maybe it gives you enough rope to skip
the test in mnt_ns_loop(). But I didn't get that far).
I converted test-fsmount.c for my own purposes:
diff --git a/samples/vfs/test-fsmount.c b/samples/vfs/test-fsmount.c
index 74124025ade0..da6e3fbf0513 100644
--- a/samples/vfs/test-fsmount.c
+++ b/samples/vfs/test-fsmount.c
@@ -83,6 +83,11 @@ static inline int move_mount(int from_dfd, const char *from_pathname,
to_dfd, to_pathname, flags);
}
+static inline int open_tree(int dfd, const char *pathname, unsigned flags)
+{
+ return syscall(__NR_open_tree, dfd, pathname, flags);
+}
+
#define E_fsconfig(fd, cmd, key, val, aux) \
do { \
if (fsconfig(fd, cmd, key, val, aux) == -1) \
@@ -93,6 +98,7 @@ int main(int argc, char *argv[])
{
int fsfd, mfd;
+#if 0
/* Mount a publically available AFS filesystem */
fsfd = fsopen("afs", 0);
if (fsfd == -1) {
@@ -115,4 +121,9 @@ int main(int argc, char *argv[])
E(close(mfd));
exit(0);
+#endif
+
+ E( mfd = open_tree(-1, "/mnt", OPEN_TREE_CLONE) );
+ E( fchdir(mfd) );
+ E( execl("/bin/bash", "/bin/bash", NULL) );
}
If I close() the mount FD "mfd", and then do "mount --move . /mnt", my
printk() shows MNT_UMOUNT has been set. ( I guess fchdir() works more
like openat(... , O_PATH) than dup() ). Then unmounting /mnt hangs, as I
would expect from my previous test.
If I instead do the mount+unmount first, and close the FD as a second
step, I think there's a lockup in the close(). The lockup happens in
the same place as the unmount lockup from before. (Except there's a line
"Code: Bad RIP value", I don't know why that happens).
# unshare --mount
# test-fsmount
# mount --move . /mnt
[ 270.859542] umount=0 mnt_flags=20
Check the flags are still the same:
# mount --move /mnt /mnt
[ 305./mnt: mount(2) system call failed: Too many levels of symbolic links.
[ 313.737030] umount=0 mnt_flags=20
Clean up the bind mount, and then the inherited mount FD.
# cd
# umount /mnt
# exit
[ 351.898629] watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [bash:1483]
[ 351.899841] Modules linked in: xt_CHECKSUM(E) ipt_MASQUERADE(E) tun(E) bridge(E) stp(E) llc(E) ip6t_rpfilter(E) ip6t_REJECT(E) nf_reject_ipv6(E) xt_conntrack(E) ip6table_nat(E) nf_nat_ipv6(E) devlink(E) ip6table_mangle(E) ip6table_raw(E) ip6table_security(E) iptable_nat(E) nf_nat_ipv4(E) nf_nat(E) nf_conntrack(E) nf_defrag_ipv6(E) libcrc32c(E) nf_defrag_ipv4(E) iptable_mangle(E) iptable_raw(E) iptable_security(E) ip6table_filter(E) ip6_tables(E) snd_hda_codec_generic(E) snd_hda_intel(E) snd_hda_codec(E) snd_hwdep(E) snd_hda_core(E) snd_seq(E) snd_seq_device(E) snd_pcm(E) joydev(E) crc32_pclmul(E) snd_timer(E) ghash_clmulni_intel(E) snd(E) crct10dif_pclmul(E) virtio_balloon(E) serio_raw(E) soundcore(E) crc32c_intel(E) qxl(E) drm_kms_helper(E) virtio_console(E) ttm(E) virtio_net(E) net_failover(E)
[ 351.912077] failover(E) drm(E) qemu_fw_cfg(E) pata_acpi(E) ata_generic(E)
[ 351.912888] CPU: 0 PID: 1483 Comm: bash Tainted: G E 4.19.0-rc3+ #7
[ 351.914221] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20180531_142017-buildhw-08.phx2.fedoraproject.org-1.fc28 04/01/2014
[ 351.916582] RIP: 0010:pin_kill+0x128/0x140
[ 351.917369] Code: f2 5a 00 48 8b 44 24 20 48 39 c5 0f 84 6f ff ff ff 48 89 df e8 e9 4a 5b 00 8b 43 18 85 c0 7e b3 c6 03 00 fb 66 0f 1f 44 00 00 <e9> 51 ff ff ff e8 be 11 dd ff 0f 1f 40 00 66 2e 0f 1f 84 00 00 00
[ 351.920729] RSP: 0018:ffffa1b381be3d88 EFLAGS: 00000202 ORIG_RAX: ffffffffffffff13
[ 351.921801] RAX: 0000000000000000 RBX: ffff909cf2ea68b0 RCX: dead000000000200
[ 351.922807] RDX: 0000000000000001 RSI: ffffa1b381be3d28 RDI: ffff909cf2ea68b0
[ 351.923811] RBP: ffffa1b381be3da8 R08: ffff909d59621760 R09: 0000000000000000
[ 351.924813] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000010000000
[ 351.925818] R13: ffff909cf5db9a38 R14: ffff909cf2ea67a0 R15: ffff909cedc07300
[ 351.926824] FS: 00007f1eb90ac740(0000) GS:ffff909d59600000(0000) knlGS:0000000000000000
[ 351.927957] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 351.928772] CR2: 00007f1eabedb180 CR3: 000000000f20a003 CR4: 00000000003606f0
[ 351.929779] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 351.930785] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 351.931791] Call Trace:
[ 351.932160] ? finish_wait+0x80/0x80
[ 351.932684] group_pin_kill+0x1a/0x30
[ 351.933207] namespace_unlock+0x6f/0x80
[ 351.933766] __fput+0x239/0x240
[ 351.934217] task_work_run+0x84/0xa0
[ 351.934743] do_exit+0x2d3/0xae0
[ 351.935206] ? __do_page_fault+0x263/0x4e0
[ 351.935799] do_group_exit+0x3a/0xa0
[ 351.936307] __x64_sys_exit_group+0x14/0x20
[ 351.936911] do_syscall_64+0x5b/0x160
[ 351.937436] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 351.938164] RIP: 0033:0x7f1eb877adb6
[ 351.938688] Code: Bad RIP value.
[ 351.939149] RSP: 002b:00007ffd56e019d8 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
[ 351.940216] RAX: ffffffffffffffda RBX: 00007f1eb8a69740 RCX: 00007f1eb877adb6
[ 351.941222] RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000
[ 351.942229] RBP: 0000000000000000 R08: 00000000000000e7 R09: ffffffffffffff80
[ 351.943236] R10: 00007ffd56e0188a R11: 0000000000000246 R12: 00007f1eb8a69740
[ 351.944242] R13: 0000000000000001 R14: 00007f1eb8a72708 R15: 0000000000000000
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