tracing: Fix buffer_ref pipe ops

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.

Link: http://lkml.kernel.org/r/20190404215925.253531-1-jannh@google.com
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@vger.kernel.org
Fixes: 73a757e63114d ("ring-buffer: Return reader page back into existing ring buffer")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/fs/splice.c b/fs/splice.c
index 3ee7e82..e758073 100644 (file)
--- 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 787d224..a830e9a 100644 (file)
--- 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 21153e6..0cfa13a 100644 (file)
--- 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)) {