From 84d535ade62b6f8ce852745731ad6200c46b977c Mon Sep 17 00:00:00 2001 From: Pavel Emelyanov Date: Tue, 11 Sep 2007 16:38:13 +0400 Subject: [PATCH] Memory shortage can result in inconsistent flocks state When the flock_lock_file() is called to change the flock from F_RDLCK to F_WRLCK or vice versa the existing flock can be removed without appropriate warning. Look: for_each_lock(inode, before) { struct file_lock *fl = *before; if (IS_POSIX(fl)) break; if (IS_LEASE(fl)) continue; if (filp != fl->fl_file) continue; if (request->fl_type == fl->fl_type) goto out; found = 1; locks_delete_lock(before); <<<<<< ! break; } if after this point the subsequent locks_alloc_lock() will fail the return code will be -ENOMEM, but the existing lock is already removed. This is a known feature that such "re-locking" is not atomic, but in the racy case the file should stay locked (although by some other process), but in this case the file will be unlocked. The proposal is to prepare the lock in advance keeping no chance to fail in the future code. Found during making the flocks pid-namespaces aware. (Note: Thanks to Reuben Farrelly for finding a bug in an earlier version of this patch.) Signed-off-by: Pavel Emelyanov Signed-off-by: J. Bruce Fields Cc: Reuben Farrelly --- fs/locks.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/fs/locks.c b/fs/locks.c index efe1affe6bed..6e22c8129a80 100644 --- a/fs/locks.c +++ b/fs/locks.c @@ -733,6 +733,15 @@ static int flock_lock_file(struct file *filp, struct file_lock *request) lock_kernel(); if (request->fl_flags & FL_ACCESS) goto find_conflict; + + if (request->fl_type != F_UNLCK) { + error = -ENOMEM; + new_fl = locks_alloc_lock(); + if (new_fl == NULL) + goto out; + error = 0; + } + for_each_lock(inode, before) { struct file_lock *fl = *before; if (IS_POSIX(fl)) @@ -754,10 +763,6 @@ static int flock_lock_file(struct file *filp, struct file_lock *request) goto out; } - error = -ENOMEM; - new_fl = locks_alloc_lock(); - if (new_fl == NULL) - goto out; /* * If a higher-priority process was blocked on the old file lock, * give it the opportunity to lock the file.