iomap: fix short copy in iomap_write_iter()
authorJan Stancek <jstancek@redhat.com>
Thu, 19 Oct 2023 16:41:36 +0000 (09:41 -0700)
committerDarrick J. Wong <djwong@kernel.org>
Thu, 19 Oct 2023 16:41:36 +0000 (09:41 -0700)
commit3ac974796e5d94509b85a403449132ea660127c2
treec6679f031385948bf8281f34e2ac5b2b3bf56681
parent684f7e6d28e8087502fc8efdb6c9fe82400479dd
iomap: fix short copy in iomap_write_iter()

Starting with commit 5d8edfb900d5 ("iomap: Copy larger chunks from
userspace"), iomap_write_iter() can get into endless loop. This can
be reproduced with LTP writev07 which uses partially valid iovecs:
        struct iovec wr_iovec[] = {
                { buffer, 64 },
                { bad_addr, 64 },
                { buffer + 64, 64 },
                { buffer + 64 * 2, 64 },
        };

commit bc1bb416bbb9 ("generic_perform_write()/iomap_write_actor():
saner logics for short copy") previously introduced the logic, which
made short copy retry in next iteration with amount of "bytes" it
managed to copy:

                if (unlikely(status == 0)) {
                        /*
                         * A short copy made iomap_write_end() reject the
                         * thing entirely.  Might be memory poisoning
                         * halfway through, might be a race with munmap,
                         * might be severe memory pressure.
                         */
                        if (copied)
                                bytes = copied;

However, since 5d8edfb900d5 "bytes" is no longer carried into next
iteration, because it is now always initialized at the beginning of
the loop. And for iov_iter_count < PAGE_SIZE, "bytes" ends up with
same value as previous iteration, making the loop retry same copy
over and over, which leads to writev07 testcase hanging.

Make next iteration retry with amount of bytes we managed to copy.

Fixes: 5d8edfb900d5 ("iomap: Copy larger chunks from userspace")
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
fs/iomap/buffered-io.c