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charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 15, 2018 at 11:00 PM Eric Biggers wrote: > On Mon, Oct 15, 2018 at 05:04:18PM +0200, Jann Horn wrote: > > Before this patch, it was possible for two pipes to affect each other after > > data had been transferred between them with tee(): > > > > ============ > > $ cat tee_test.c > > > > int main(void) { > > int pipe_a[2]; > > if (pipe(pipe_a)) err(1, "pipe"); > > int pipe_b[2]; > > if (pipe(pipe_b)) err(1, "pipe"); > > if (write(pipe_a[1], "abcd", 4) != 4) err(1, "write"); > > if (tee(pipe_a[0], pipe_b[1], 2, 0) != 2) err(1, "tee"); > > if (write(pipe_b[1], "xx", 2) != 2) err(1, "write"); > > > > char buf[5]; > > if (read(pipe_a[0], buf, 4) != 4) err(1, "read"); > > buf[4] = 0; > > printf("got back: '%s'\n", buf); > > } > > $ gcc -o tee_test tee_test.c > > $ ./tee_test > > got back: 'abxx' > > $ > > ============ > > > > Fix it by explicitly marking buffers as mergeable and clearing that flag in > > splice_pipe_to_pipe() and link_pipe(). > > > > Cc: > > Fixes: 7c77f0b3f920 ("splice: implement pipe to pipe splicing") > > Fixes: 70524490ee2e ("[PATCH] splice: add support for sys_tee()") > > Signed-off-by: Jann Horn > > --- > > Cleanup in the next patch, to simplify backporting. > > > > fs/pipe.c | 5 +++-- > > fs/splice.c | 6 ++++++ > > include/linux/pipe_fs_i.h | 8 ++++++++ > > 3 files changed, 17 insertions(+), 2 deletions(-) > > > > diff --git a/fs/pipe.c b/fs/pipe.c > > index bdc5d3c0977d..4e2eee77f855 100644 > > --- a/fs/pipe.c > > +++ b/fs/pipe.c > > @@ -379,7 +379,8 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from) > > struct pipe_buffer *buf = pipe->bufs + lastbuf; > > int offset = buf->offset + buf->len; > > > > - if (buf->ops->can_merge && offset + chars <= PAGE_SIZE) { > > + if (buf->ops->can_merge && offset + chars <= PAGE_SIZE && > > + (buf->flags & PIPE_BUF_FLAG_MAYMERGE)) { > > ret = pipe_buf_confirm(pipe, buf); > > if (ret) > > goto out; > > @@ -439,7 +440,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from) > > buf->ops = &anon_pipe_buf_ops; > > buf->offset = 0; > > buf->len = copied; > > - buf->flags = 0; > > + buf->flags = PIPE_BUF_FLAG_MAYMERGE; > > if (is_packetized(filp)) { > > buf->ops = &packet_pipe_buf_ops; > > buf->flags = PIPE_BUF_FLAG_PACKET; > > diff --git a/fs/splice.c b/fs/splice.c > > index b3daa971f597..111977c80dfd 100644 > > --- a/fs/splice.c > > +++ b/fs/splice.c > > @@ -1593,6 +1593,9 @@ static int splice_pipe_to_pipe(struct pipe_inode_info *ipipe, > > */ > > obuf->flags &= ~PIPE_BUF_FLAG_GIFT; > > > > + /* We can't merge data into a buffer we don't own. */ > > + obuf->flags &= ~PIPE_BUF_FLAG_MAYMERGE; > > + > > obuf->len = len; > > opipe->nrbufs++; > > ibuf->offset += obuf->len; > > @@ -1667,6 +1670,9 @@ static int link_pipe(struct pipe_inode_info *ipipe, > > */ > > obuf->flags &= ~PIPE_BUF_FLAG_GIFT; > > > > + /* We can't merge data into a buffer we don't own. */ > > + obuf->flags &= ~PIPE_BUF_FLAG_MAYMERGE; > > + > > if (obuf->len > len) > > obuf->len = len; > > > > diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h > > index 5a3bb3b7c9ad..8893711f9171 100644 > > --- a/include/linux/pipe_fs_i.h > > +++ b/include/linux/pipe_fs_i.h > > @@ -8,6 +8,14 @@ > > #define PIPE_BUF_FLAG_ATOMIC 0x02 /* was atomically mapped */ > > #define PIPE_BUF_FLAG_GIFT 0x04 /* page is a gift */ > > #define PIPE_BUF_FLAG_PACKET 0x08 /* read() as a packet */ > > +/* > > + * Set this flag if the generic pipe read/write may coalesce data into an > > + * existing buffer. If this is not set, a new pipe page segment is always used > > + * for new data. > > + * When pipe data is copied by reference (as in the tee() syscall), this flag > > + * must be cleared on the copy. > > + */ > > +#define PIPE_BUF_FLAG_MAYMERGE 0x10 > > > > /** > > * struct pipe_buffer - a linux kernel pipe buffer > > -- > > 2.19.0.605.g01d371f741-goog > > > > Deja vu... https://marc.info/?l=linux-fsdevel&m=149003133809192 Oh, hah, even the reproducer looks almost the same. I hadn't seen that... > Thanks for fixing this; I think this is the right fix. > I verified it works for my reproducer too. > > Did you check whether fuse_dev_splice_write() needs to clear > PIPE_BUF_FLAG_MAYMERGE? No, I hadn't looked at that. But from what I can tell, fuse_dev_splice_write() shouldn't write into the provided buffers, only read from them.