On Tue, Sep 02, 2014 at 09:17:34PM +0400, Pavel Emelyanov wrote:
> Hi,
>
> There's a problem with getting information about who has a flock on
> a specific file. The thing is that the "owner" field, that is shown in
> /proc/locks is the pid of the task who created the flock, not the one
> who _may_ hold it.
>
> If the flock creator shared the file with some other task (by forking
> or via scm_rights) and then died or closed the file, the information
> shown in proc no longer corresponds to the reality.
>
> This is critical for CRIU project, that tries to dump (and restore)
> the state of running tasks. For example, let's take two tasks A and B
> both opened a file "/foo", one of tasks places a LOCK_SH lock on the
> file and then "obfuscated" the owner field in /proc/locks. After this
> we have no ways to find out who is the lock holder.
>
> I'd like to note, that for LOCK_EX this problem is not critical -- we
> may go to both tasks and "ask" them to LOCK_EX the file again (we can
> do it in CRIU, I can tell more if required). The one who succeeds is
> the lock holder.
It could be both, actually, right?
> With LOCK_SH this doesn't work. Trying to drop the
> lock doesn't work either, as flock(LOCK_UN) reports 0 in both cases:
> if the file is locked and if it is not.
>
> That said, I'd like to propose the LOCK_TEST flag to the flock call,
> that would check whether the lock of the given type (LOCK_SH or LOCK_EX)
> exists on the file we test. It's not the same as the existing in-kernel
> FL_ACCESS flag, which checks whether the new lock is possible, but
> it's a new FL_TEST flag, that checks whether the existing lock is there.
>
> What do you think?
I guess I can't see anything really wrong with it.
It ignores the (poorly documented) LOCK_MAND case, but maybe that's OK.
Would it make sense to return the lock type held instead, so you could
do one flock(fd, LOCK_TEST) instead of flock(fd, LOCK_TEST|LOCK_SH) and
flock(fd, LOCK_TEST|LOCK_EX) ?
It'd be nice if we could fix /proc/locks. (You'd think I'd know better,
but I've certainly been confused before when /proc/locks told me a lock
was owned by a nonexistant PID.) But as long as multiple PIDs can "own"
a flock and as long as there's no simple ID we can use to refer to a
given struct file, I don't see an easy solution.
--b.
>
> Signed-off-by: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
>
> ---
>
> diff --git a/fs/locks.c b/fs/locks.c
> index bb08857..50842bf 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -830,7 +830,7 @@ static int flock_lock_file(struct file *filp, struct file_lock *request)
> int found = 0;
> LIST_HEAD(dispose);
>
> - if (!(request->fl_flags & FL_ACCESS) && (request->fl_type != F_UNLCK)) {
> + if (!(request->fl_flags & (FL_ACCESS|FL_TEST)) && (request->fl_type != F_UNLCK)) {
> new_fl = locks_alloc_lock();
> if (!new_fl)
> return -ENOMEM;
> @@ -850,11 +850,18 @@ static int flock_lock_file(struct file *filp, struct file_lock *request)
> continue;
> if (request->fl_type == fl->fl_type)
> goto out;
> + if (request->fl_flags & FL_TEST)
> + break;
> found = 1;
> locks_delete_lock(before, &dispose);
> break;
> }
>
> + if (request->fl_flags & FL_TEST) {
> + error = -ENOENT;
> + goto out;
> + }
> +
> if (request->fl_type == F_UNLCK) {
> if ((request->fl_flags & FL_EXISTS) && !found)
> error = -ENOENT;
> @@ -1852,15 +1859,16 @@ SYSCALL_DEFINE2(flock, unsigned int, fd, unsigned int, cmd)
> {
> struct fd f = fdget(fd);
> struct file_lock *lock;
> - int can_sleep, unlock;
> + int can_sleep, unlock, test;
> int error;
>
> error = -EBADF;
> if (!f.file)
> goto out;
>
> + test = (cmd & LOCK_TEST);
> can_sleep = !(cmd & LOCK_NB);
> - cmd &= ~LOCK_NB;
> + cmd &= ~(LOCK_NB|LOCK_TEST);
> unlock = (cmd == LOCK_UN);
>
> if (!unlock && !(cmd & LOCK_MAND) &&
> @@ -1872,6 +1880,8 @@ SYSCALL_DEFINE2(flock, unsigned int, fd, unsigned int, cmd)
> goto out_putf;
> if (can_sleep)
> lock->fl_flags |= FL_SLEEP;
> + if (test)
> + lock->fl_flags |= FL_TEST;
>
> error = security_file_lock(f.file, lock->fl_type);
> if (error)
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 9418772..9230e1d 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -844,6 +844,7 @@ static inline struct file *get_file(struct file *f)
> #define FL_DOWNGRADE_PENDING 256 /* Lease is being downgraded */
> #define FL_UNLOCK_PENDING 512 /* Lease is being broken */
> #define FL_OFDLCK 1024 /* lock is "owned" by struct file */
> +#define FL_TEST 2048
>
> /*
> * Special return value from posix_lock_file() and vfs_lock_file() for
> diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
> index 7543b3e..7302e36 100644
> --- a/include/uapi/asm-generic/fcntl.h
> +++ b/include/uapi/asm-generic/fcntl.h
> @@ -184,6 +184,7 @@ struct f_owner_ex {
> #define LOCK_READ 64 /* which allows concurrent read operations */
> #define LOCK_WRITE 128 /* which allows concurrent write operations */
> #define LOCK_RW 192 /* which allows concurrent read & write ops */
> +#define LOCK_TEST 256 /* check for my SH|EX locks present */
>
> #define F_LINUX_SPECIFIC_BASE 1024
>
>
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