Messages in this thread Patch in this message | | | Date | Sat, 3 Jan 2009 17:45:44 +0100 | From | Oleg Nesterov <> | Subject | Re: RFC: Fix f_flags races without the BKL |
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On 01/02, Al Viro wrote: > > FWIW, it's still bloody tempting to try. How about hlist from struct file > through fasync_struct? Possibly with reference from fasync_struct back > to the queue it's on, while we are at it - would make fasync_helper simpler...
Well, don't ask me ;) I don't know whether this change is good or bad...
Let's forget about files with several fasync_struct's, suppose we have a pointer ->f_xxx to fasync_struct in struct file. Now __fput() can check f_xxx != NULL and call ->fasync() if true. But how can we ensure that (->f_flags & FASYNC) matches (->f_xxx != NULL) ? Looks like we should kill FASYNC and uglify the code which sets/gets ->f_flags, for example do_fcntl(F_GETFL) should do
case F_GETFL: err = filp->f_flags | (filp->f_xxx ? FASYNC : 0)
And what if some strange driver doesn't use the "standard" fasync_helper/ kill_fasync helpers?
Offtopic, but while we are here...
pipe_rdwr_fasync:
retval = fasync_helper(fd, filp, on, &pipe->fasync_readers);
if (retval >= 0) retval = fasync_helper(fd, filp, on, &pipe->fasync_writers);
Suppose that on == 1 and the first fasync_helper(fasync_readers) succeeds, but the second fasync_helper(fasync_writers) fails. We return the error and this file doesn't get FASYNC, but still it is placed on ->fasync_readers. This looks just wrong, we return the error to the user-space, but the file owner can receive the notifications from ->fasync_readers.
And. Don't we leak fasync_struct in this case? With the recent changes, pipe_rdwr_release() does not call pipe_rdwr_fasync(on => 0) unconditionally. Instead, __fput() does this, but depending on ->f_flags & FASYNC.
IOW, perhaps we need something like the patch below?
--- a/fs/pipe.c +++ b/fs/pipe.c @@ -702,9 +702,11 @@ pipe_rdwr_fasync(int fd, struct file *fi retval = fasync_helper(fd, filp, on, &pipe->fasync_readers); - if (retval >= 0) + if (retval >= 0) { retval = fasync_helper(fd, filp, on, &pipe->fasync_writers); - + if (retval < 0) /* can only happen if on == true */ + fasync_helper(-1, filp, 0, &pipe->fasync_readers); + } mutex_unlock(&inode->i_mutex); if (retval < 0)
And why pipe_xxx_fasync() take ->i_mutex around fasync_helper() ? Afaics, nothing bad can happen if pipe_xxx_fasync() races with, say, pipe_read().
Oleg.
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