Messages in this thread | | | Date | Fri, 28 Nov 2008 20:25:08 +0100 | From | Oleg Nesterov <> | Subject | BUG? "Call fasync() functions without the BKL" is racy |
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Hi.
Let's suppose we have the tasks T1, T2, T3 which share the same file, all do sys_fcntl(file, F_SETFL) in parallel. file->f_flags == 0.
setfl(arg) does:
if ((arg ^ filp->f_flags) & FASYNC) // --- WINDOW_1 --- filp->f_op->fasync(fd, filp, (arg & FASYNC) != 0) // --- WINDOW_2 --- filp->f_flags = arg;
T1 calls setfl(FASYNC), preempted in WINDOW_1.
T2 calls setfl(FASYNC), does all job and returns.
T3 calls setfl(0), sees ->f_flags & FASYNC, does ->fasync(on => 0), preempted in WINDOW_2.
T1 resumes, does ->fasync(on => 1) again, update ->f_flags (it already has FASYNC) and returns.
T3 resumes, and clears FASYNC from ->f_flags.
Now, this file was added into some "struct fasync_struct", but ->f_flags doesn't have FASYNC. This means __fput() will skip ->fasync(on => 0) and the next kill_fasync() can crash because fa_file points to the freed/reused memory.
I think a238b790d5f99c7832f9b73ac8847025815b85f7 should be reverted. Or do you see the better fix?
Unless I missed something of course.
Oleg.
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