[PATCH] locks: Ability to test for flock presence on fd

[Pavel Emelyanov]

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. 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?

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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