[PATCH] fcntl F_SETFL and read-only IS_APPEND files

There is code in setfl() which attempts to preserve the O_APPEND flag on
IS_APPEND files...  however IS_APPEND files could also be opened O_RDONLY
and in that case setfl() should not require O_APPEND...

coreutils 5.93 tail -f attempts to set O_NONBLOCK even on regular files...
unfortunately if you try this on an append-only log file the result is
this:

fcntl64(3, F_GETFL)                     = 0x8000 (flags O_RDONLY|O_LARGEFILE)
fcntl64(3, F_SETFL, O_RDONLY|O_NONBLOCK|O_LARGEFILE) = -1 EPERM (Operation not permitted)

I offer up the patch below as one way of fixing the problem...  i've tested
it fixes the problem with tail -f but haven't really tested beyond that.

(I also reported the coreutils bug upstream... it shouldn't fail imho...
<https://savannah.gnu.org/bugs/index.php?func=detailitem&item_id=15473>)
Signed-off-by: dean gaudet <dean@arctic.org>
Cc: Al Viro <viro@ftp.linux.org.uk>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/fcntl.c

@@ -208,8 +208,11 @@ static int setfl(int fd, struct file * filp, unsigned long arg)
 	struct inode * inode = filp->f_dentry->d_inode;
 	int error = 0;
 
-	/* O_APPEND cannot be cleared if the file is marked as append-only */
-	if (!(arg & O_APPEND) && IS_APPEND(inode))
+	/*
+	 * O_APPEND cannot be cleared if the file is marked as append-only
+	 * and the file is open for write.
+	 */
+	if (((arg ^ filp->f_flags) & O_APPEND) && IS_APPEND(inode))
 		return -EPERM;
 
 	/* O_NOATIME can only be set by the owner or superuser */