On Wed, Oct 12, 2016 at 03:33:26PM +0200, Miklos Szeredi wrote:
> This is a proof of concept patch to fix the following.
>
> /ovl is in overlay mount and /ovl/foo exists on the lower layer only.
>
> rofd = open("/ovl/foo", O_RDONLY);
> rwfd = open("/ovl/foo", O_WRONLY); /* this causes copy up */
> write(rwfd, "bar", 3);
> read(rofd, buf, 3);
> assert(memcmp(buf, "bar", 3) == 0);
>
> Similar problem exists with an MAP_SHARED mmap created from rofd.
>
> While this has only caused few problems (yum/dnf failure is the only one I know
> of) and easily worked around in userspace, many see it as a proof that overlayfs
> can never be a proper "POSIX" filesystem.
>
> To quell those worries, here's a simple patch that should address the above.
>
> The only VFS change is that f_op is initialized from f_path.dentry->d_inode
> instead of file_inode(filp) in open. The effect of this is that overlayfs can
> intercept open and other file operations, while the file still effectively
> belongs to the underlying fs.
>
> The patch does not give up on the nice properties of overlayfs, like sharing the
> page cache with the underlying files. It does cause copy up in one case where
> previously there wasn't one and that's the O_RDONLY/MAP_SHARED case. I haven't
> done much research into this, but running some tests in chroot didn't trigger
> this.
>
> Comments, testing are welcome.
Hi Miklos,
This looks like a very interesting idea. In fact once file has been copied
up and writen to, and if I do fstat(rofd), it shows the size of copied up
file but one can read the contents. So fixing that anomaly would be nice.
Hopefully O_RDONLY/MAP_SHARED is not a common case and we get away with
this forced copy up penalty.
[..]
> +static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *to)
> +{
> + struct file *file = iocb->ki_filp;
> + bool isupper = OVL_TYPE_UPPER(ovl_path_type(file->f_path.dentry));
> + ssize_t ret = -EINVAL;
> +
> + if (likely(!isupper)) {
> + const struct file_operations *fop = ovl_real_fop(file);
> +
> + if (likely(fop->read_iter))
> + ret = fop->read_iter(iocb, to);
> + } else {
> + struct file *upperfile = filp_clone_open(file);
> +
IIUC, every read of lower file will call filp_clone_open(). Looking at the
code of filp_clone_open(), I am concerned about the overhead of this call.
Is it significant? Don't want to be paying too much of penalty for read
operation on lower files. That would be a common case for containers.
BTW, I did a quick testing. Using docker launched a fedora container and
called "dnf update" inside that. And later I noticed following on serial
console.
Thanks
Vivek
[ 309.075885] ======================================================
[ 309.076841] [ INFO: possible circular locking dependency detected ]
[ 309.077818] 4.9.0-rc1+ #197 Not tainted
[ 309.078411] -------------------------------------------------------
[ 309.079377] dnf/2468 is trying to acquire lock:
[ 309.080082] ([ 309.080324] &type->s_vfs_rename_key
#2[ 309.080942] ){+.+.+.}
, at: [ 309.081435] [<ffffffff8129f652>] lock_rename+0x32/0x100
[ 309.082261]
[ 309.082261] but task is already holding lock:
[ 309.083158] ([ 309.083399] &mm->mmap_sem
){++++++}[ 309.083974] , at:
[ 309.084316] [<ffffffff8121df0c>] vm_mmap_pgoff+0x8c/0x100
[ 309.085150]
[ 309.085150] which lock already depends on the new lock.
[ 309.085150]
[ 309.086393]
[ 309.086393] the existing dependency chain (in reverse order) is:
[ 309.088279]
-> #3[ 309.088612] (
&mm->mmap_sem[ 309.089091] ){++++++}
[ 309.089470] :
[ 309.089735] [ 309.090046] [<ffffffff81112786>] lock_acquire+0xf6/0x1f0
[ 309.090884] [ 309.091197] [<ffffffff812316c0>] __might_fault+0x70/0xa0
[ 309.092047] [ 309.092357] [<ffffffff812aa585>] filldir+0xb5/0x140
[ 309.093128] [ 309.093434] [<ffffffff8132f235>] call_filldir+0x65/0x130
[ 309.094273] [ 309.094590] [<ffffffff8132fc0f>] ext4_readdir+0x6cf/0x8a0
[ 309.095425] [ 309.095742] [<ffffffff812aa24b>] iterate_dir+0x17b/0x1b0
[ 309.096572] [ 309.096878] [<ffffffff812aa76c>] SyS_getdents+0x9c/0x130
[ 309.097716] [ 309.098026] [<ffffffff818a4bc1>] entry_SYSCALL_64_fastpath+0x1f/0xc2
[ 309.099005]
-> #2[ 309.099304] (
&type->i_mutex_dir_key[ 309.099888] #3
){++++++}[ 309.100301] :
[ 309.100576] [ 309.100881] [<ffffffff81112786>] lock_acquire+0xf6/0x1f0
[ 309.101711] [ 309.102017] [<ffffffff818a1279>] down_write+0x49/0x80
[ 309.102798] [ 309.103098] [<ffffffff8129fea5>] vfs_rmdir+0x55/0x140
[ 309.103878] [ 309.104179] [<ffffffff812a5bdd>] do_rmdir+0x1bd/0x230
[ 309.104958] [ 309.105256] [<ffffffff812a6a12>] SyS_unlinkat+0x22/0x30
[ 309.106063] [ 309.106364] [<ffffffff818a4bc1>] entry_SYSCALL_64_fastpath+0x1f/0xc2
[ 309.107345]
-> #1[ 309.107661] (
&type->i_mutex_dir_key[ 309.108256] #3
/1[ 309.108597] ){+.+.+.}
[ 309.108971] :
[ 309.109225] [ 309.109532] [<ffffffff81112786>] lock_acquire+0xf6/0x1f0
[ 309.110370] [ 309.110686] [<ffffffff8110c8df>] down_write_nested+0x4f/0x80
[ 309.111606] [ 309.111916] [<ffffffff8129f701>] lock_rename+0xe1/0x100
[ 309.112752] [ 309.113062] [<ffffffff812a7842>] SyS_renameat+0x212/0x3f0
[ 309.113918] [ 309.114224] [<ffffffff818a4bc1>] entry_SYSCALL_64_fastpath+0x1f/0xc2
[ 309.115218]
-> #0[ 309.115525] (
&type->s_vfs_rename_key[ 309.116138] #2
){+.+.+.}[ 309.116564] :
[ 309.116837] [ 309.117146] [<ffffffff811121a0>] __lock_acquire+0x1110/0x12a0
[ 309.118047] [ 309.118354] [<ffffffff81112786>] lock_acquire+0xf6/0x1f0
[ 309.119193] [ 309.119500] [<ffffffff818a0159>] mutex_lock_nested+0x79/0x3c0
[ 309.120406] [ 309.120723] [<ffffffff8129f652>] lock_rename+0x32/0x100
[ 309.121564] [ 309.121875] [<ffffffffa02d1617>] ovl_copy_up_one+0xf7/0x6a0 [overlay]
[ 309.122866] [ 309.123170] [<ffffffffa02d1ccb>] ovl_copy_up+0x10b/0x13d [overlay]
[ 309.124136] [ 309.124442] [<ffffffffa02cd7a5>] ovl_mmap+0x55/0x90 [overlay]
[ 309.125348] [ 309.125677] [<ffffffff8123f3a4>] mmap_region+0x394/0x630
[ 309.126510] [ 309.126829] [<ffffffff8123fa86>] do_mmap+0x446/0x530
[ 309.127616] [ 309.127928] [<ffffffff8121df3d>] vm_mmap_pgoff+0xbd/0x100
[ 309.128783] [ 309.129092] [<ffffffff8123d5c1>] SyS_mmap_pgoff+0x1c1/0x290
[ 309.129973] [ 309.130284] [<ffffffff8103b71b>] SyS_mmap+0x1b/0x30
[ 309.131058] [ 309.131368] [<ffffffff818a4bc1>] entry_SYSCALL_64_fastpath+0x1f/0xc2
[ 309.132377]
[ 309.132377] other info that might help us debug this:
[ 309.132377]
[ 309.133608] Chain exists of:
[ 309.134084] &type->s_vfs_rename_key
#2[ 309.134694] -->
&type->i_mutex_dir_key[ 309.135324] #3
--> [ 309.135705] &mm->mmap_sem
[ 309.136131]
[ 309.136131]
[ 309.136599] Possible unsafe locking scenario:
[ 309.136599]
[ 309.137499] CPU0 CPU1
[ 309.138202] ---- ----
[ 309.138905] lock([ 309.139211] &mm->mmap_sem
[ 309.139646] );
[ 309.139914] lock([ 309.140602] &type->i_mutex_dir_key
#3[ 309.141190] );
[ 309.141472] lock([ 309.142175] &mm->mmap_sem
[ 309.142610] );
[ 309.142877] lock([ 309.143186] &type->s_vfs_rename_key
#2[ 309.143796] );
[ 309.144084]
[ 309.144084] *** DEADLOCK ***
[ 309.144084]
[ 309.144991] 1 lock held by dnf/2468:
[ 309.145543] #0: [ 309.145827] (
&mm->mmap_sem[ 309.146299] ){++++++}
, at: [ 309.146782] [<ffffffff8121df0c>] vm_mmap_pgoff+0x8c/0x100
[ 309.147634]
[ 309.147634] stack backtrace:
[ 309.148310] CPU: 4 PID: 2468 Comm: dnf Not tainted 4.9.0-rc1+ #197
[ 309.149257] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.2-20150714_191134- 04/01/2014
[ 309.150717] ffffc900033cf900 ffffffff8144b253 ffffffff828b29f0 ffffffff828e8d50
[ 309.151942] ffffc900033cf940 ffffffff8110f83e 00000000f7ac0000 ffff8801f7ac0860
[ 309.153660] ffff8801f7ac0000 ffff8801f7ac0838 0000000000000001 0000000000000000
[ 309.154877] Call Trace:
[ 309.155266] [<ffffffff8144b253>] dump_stack+0x86/0xc3
[ 309.156062] [<ffffffff8110f83e>] print_circular_bug+0x1be/0x210
[ 309.156991] [<ffffffff811121a0>] __lock_acquire+0x1110/0x12a0
[ 309.157896] [<ffffffff8112ee3d>] ? debug_lockdep_rcu_enabled+0x1d/0x20
[ 309.158912] [<ffffffff813c3ed4>] ? avc_has_perm+0x34/0x290
[ 309.159768] [<ffffffff81112786>] lock_acquire+0xf6/0x1f0
[ 309.160597] [<ffffffff8129f652>] ? lock_rename+0x32/0x100
[ 309.161438] [<ffffffff818a0159>] mutex_lock_nested+0x79/0x3c0
[ 309.162351] [<ffffffff8129f652>] ? lock_rename+0x32/0x100
[ 309.163202] [<ffffffff813c92fb>] ? selinux_inode_getattr+0x8b/0xb0
[ 309.164162] [<ffffffff8129f652>] lock_rename+0x32/0x100
[ 309.164981] [<ffffffffa02d1617>] ovl_copy_up_one+0xf7/0x6a0 [overlay]
[ 309.165987] [<ffffffff813c3fd3>] ? avc_has_perm+0x133/0x290
[ 309.166864] [<ffffffff813c3ed4>] ? avc_has_perm+0x34/0x290
[ 309.167723] [<ffffffff810e348a>] ? __might_sleep+0x4a/0x80
[ 309.168580] [<ffffffffa02d1ccb>] ovl_copy_up+0x10b/0x13d [overlay]
[ 309.169539] [<ffffffffa02cd7a5>] ovl_mmap+0x55/0x90 [overlay]
[ 309.170436] [<ffffffff8123f3a4>] mmap_region+0x394/0x630
[ 309.171269] [<ffffffff8123fa86>] do_mmap+0x446/0x530
[ 309.172064] [<ffffffff8121df3d>] vm_mmap_pgoff+0xbd/0x100
[ 309.172908] [<ffffffff8123d5c1>] SyS_mmap_pgoff+0x1c1/0x290
[ 309.173777] [<ffffffff8103b71b>] SyS_mmap+0x1b/0x30
[ 309.174539] [<ffffffff818a4bc1>] entry_SYSCALL_64_fastpath+0x1f/0xc2
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