On Thu, 4 Apr 2019 23:59:25 +0200 Jann Horn <[email protected]> wrote:
> This fixes multiple issues in buffer_pipe_buf_ops: > > - The ->steal() handler must not return zero unless the pipe buffer has > the only reference to the page. But generic_pipe_buf_steal() assumes > that every reference to the pipe is tracked by the page's refcount, > which isn't true for these buffers - buffer_pipe_buf_get(), which > duplicates a buffer, doesn't touch the page's refcount. > Fix it by using generic_pipe_buf_nosteal(), which refuses every > attempted theft. It should be easy to actually support ->steal, but the > only current users of pipe_buf_steal() are the virtio console and FUSE, > and they also only use it as an optimization. So it's probably not worth > the effort. > - The ->get() and ->release() handlers can be invoked concurrently on pipe > buffers backed by the same struct buffer_ref. Make them safe against > concurrency by using refcount_t. > - The pointers stored in ->private were only zeroed out when the last > reference to the buffer_ref was dropped. As far as I know, this > shouldn't be necessary anyway, but if we do it, let's always do it. Honestly, the entire implementation of the splice code for me was very confusing. Not much documentation on it, so I just looked at what others did and tried my best to copy them, bugs and all ;-) > > Cc: [email protected] # v4.11 > Signed-off-by: Jann Horn <[email protected]> > --- > Completely untested (apart from compiling it). I don't really know > anything about how the tracing subsystem works. I applied this and ran it through my suite of trace-cmd tests which uses the splice code quite heavily, and it didn't trigger any bugs. I don't see anything wrong with this patch, so I'll pull it and run it through the rest of my tests. Thanks! -- Steve > > fs/splice.c | 4 ++-- > include/linux/pipe_fs_i.h | 1 + > kernel/trace/trace.c | 28 ++++++++++++++-------------- > 3 files changed, 17 insertions(+), 16 deletions(-) > > diff --git a/fs/splice.c b/fs/splice.c > index 3ee7e82df48f..e75807380caa 100644 > --- a/fs/splice.c > +++ b/fs/splice.c > @@ -330,8 +330,8 @@ const struct pipe_buf_operations default_pipe_buf_ops = { > .get = generic_pipe_buf_get, > }; > > -static int generic_pipe_buf_nosteal(struct pipe_inode_info *pipe, > - struct pipe_buffer *buf) > +int generic_pipe_buf_nosteal(struct pipe_inode_info *pipe, > + struct pipe_buffer *buf) > { > return 1; > } > diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h > index 787d224ff43e..a830e9a00eb9 100644 > --- a/include/linux/pipe_fs_i.h > +++ b/include/linux/pipe_fs_i.h > @@ -174,6 +174,7 @@ void free_pipe_info(struct pipe_inode_info *); > void generic_pipe_buf_get(struct pipe_inode_info *, struct pipe_buffer *); > int generic_pipe_buf_confirm(struct pipe_inode_info *, struct pipe_buffer *); > int generic_pipe_buf_steal(struct pipe_inode_info *, struct pipe_buffer *); > +int generic_pipe_buf_nosteal(struct pipe_inode_info *, struct pipe_buffer *); > void generic_pipe_buf_release(struct pipe_inode_info *, struct pipe_buffer > *); > void pipe_buf_mark_unmergeable(struct pipe_buffer *buf); > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > index 21153e64bf1c..0cfa13a60086 100644 > --- a/kernel/trace/trace.c > +++ b/kernel/trace/trace.c > @@ -7025,19 +7025,23 @@ struct buffer_ref { > struct ring_buffer *buffer; > void *page; > int cpu; > - int ref; > + refcount_t refcount; > }; > > +static void buffer_ref_release(struct buffer_ref *ref) > +{ > + if (!refcount_dec_and_test(&ref->refcount)) > + return; > + ring_buffer_free_read_page(ref->buffer, ref->cpu, ref->page); > + kfree(ref); > +} > + > static void buffer_pipe_buf_release(struct pipe_inode_info *pipe, > struct pipe_buffer *buf) > { > struct buffer_ref *ref = (struct buffer_ref *)buf->private; > > - if (--ref->ref) > - return; > - > - ring_buffer_free_read_page(ref->buffer, ref->cpu, ref->page); > - kfree(ref); > + buffer_ref_release(ref); > buf->private = 0; > } > > @@ -7046,14 +7050,14 @@ static void buffer_pipe_buf_get(struct > pipe_inode_info *pipe, > { > struct buffer_ref *ref = (struct buffer_ref *)buf->private; > > - ref->ref++; > + refcount_inc(&ref->refcount); > } > > /* Pipe buffer operations for a buffer. */ > static const struct pipe_buf_operations buffer_pipe_buf_ops = { > .confirm = generic_pipe_buf_confirm, > .release = buffer_pipe_buf_release, > - .steal = generic_pipe_buf_steal, > + .steal = generic_pipe_buf_nosteal, > .get = buffer_pipe_buf_get, > }; > > @@ -7066,11 +7070,7 @@ static void buffer_spd_release(struct splice_pipe_desc > *spd, unsigned int i) > struct buffer_ref *ref = > (struct buffer_ref *)spd->partial[i].private; > > - if (--ref->ref) > - return; > - > - ring_buffer_free_read_page(ref->buffer, ref->cpu, ref->page); > - kfree(ref); > + buffer_ref_release(ref); > spd->partial[i].private = 0; > } > > @@ -7125,7 +7125,7 @@ tracing_buffers_splice_read(struct file *file, loff_t > *ppos, > break; > } > > - ref->ref = 1; > + refcount_set(&ref->refcount, 1); > ref->buffer = iter->trace_buffer->buffer; > ref->page = ring_buffer_alloc_read_page(ref->buffer, > iter->cpu_file); > if (IS_ERR(ref->page)) {
