[PATCH] dma-buf: document fd flags and O_CLOEXEC requirement

[Rob Clark]

From: Rob Clark <rob at ti.com>

Otherwise subsystems will get this wrong and end up with a second
export ioctl with the flag and O_CLOEXEC support added.

Signed-off-by: Rob Clark <rob at ti.com>
Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
Updated version of Daniel's original documentation patch with (hopefully)
improved wording, and a better description of the motivation.

 Documentation/dma-buf-sharing.txt |   18 ++++++++++++++++++
 1 files changed, 18 insertions(+), 0 deletions(-)

diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index 225f96d..3b51134 100644
--- a/Documentation/dma-buf-sharing.txt
+++ b/Documentation/dma-buf-sharing.txt
@@ -223,6 +223,24 @@ Miscellaneous notes:
 - Any exporters or users of the dma-buf buffer sharing framework must have
   a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
 
+- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
+  on the file descriptor.  This is not just a resource leak, but a
+  potential security hole.  It could give the newly exec'd application
+  access to buffers, via the leaked fd, to which it should otherwise
+  not be permitted access.
+
+  The problem with doing this via a separate fcntl() call, versus doing it
+  atomically when the fd is created, is that this is inherently racy in a
+  multi-threaded app[3].  The issue is made worse when it is library code
+  opening/creating the file descriptor, as the application may not even be
+  aware of the fd's.
+
+  To avoid this problem, userspace must have a way to request O_CLOEXEC
+  flag be set when the dma-buf fd is created.  So any API provided by
+  the exporting driver to create a dmabuf fd must provide a way to let
+  userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
+
 References:
 [1] struct dma_buf_ops in include/linux/dma-buf.h
 [2] All interfaces mentioned above defined in include/linux/dma-buf.h
+[3] https://lwn.net/Articles/236486/
-- 
1.7.5.4