[libcamera-devel] [PATCH 8/8] [RFC-Only] libcamera: request: A request canary

[Laurent Pinchart]

Hi Kieran,

Thank you for the patch.

On Fri, Mar 12, 2021 at 06:11:31AM +0000, Kieran Bingham wrote:
> Request objects are created and owned by the application, but are of
> course utilised widely throughout the internals of libcamera.
> 
> If the application free's the requests while they are still active
> within libcamera a use after free will occur. While this can be trapped
> by tools such as valgrind, given the importance of this object and the
> relationship of external ownership, it may have some value to provide
> Debug build (disabled at Release build) assertions on the condition of
> these objects.
> 
> Make sure the request fails an assertion immediately if used after free.
> 
> Signed-off-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> 
> ---
> RFC only, as I was going to dispose of this patch. We added it while
> debugging the IPU3 stability, and it may prove useful to catch errors if
> requests are used after they are released.
> 
> However this may be redundant as pipeline handlers should guarantee that
> requests are fully completed when they stop(). Of course the IPU3 wasn't
> meeting that requirement, and we do not have a specific assertion that
> validates that requirement on all pipeline handlers. So perhaps this
> canary might serve as a beneficial guard?

I think we should move that check from the IPU3 pipeline handler to the
core, as commented on the corresponding patch, so this patch wouldn't
really be needed.

> If not, at least posting it will mean it can be used in the future if it
> comes up again, or the concept could be applied to other objects if
> appropriate.
> 
>  include/libcamera/request.h |  2 ++
>  src/libcamera/request.cpp   | 15 +++++++++++++++
>  2 files changed, 17 insertions(+)
> 
> diff --git a/include/libcamera/request.h b/include/libcamera/request.h
> index 59d7f4bac0d2..fc56d63c8c67 100644
> --- a/include/libcamera/request.h
> +++ b/include/libcamera/request.h
> @@ -78,6 +78,8 @@ private:
>  	const uint64_t cookie_;
>  	Status status_;
>  	bool cancelled_;
> +
> +	int canary_;

If we want to do this, couldn't we add a RequestFreed status to avoid
adding a separate field ?

>  };
>  
>  } /* namespace libcamera */
> diff --git a/src/libcamera/request.cpp b/src/libcamera/request.cpp
> index 12c2e7d425f9..83169a11e1e5 100644
> --- a/src/libcamera/request.cpp
> +++ b/src/libcamera/request.cpp
> @@ -18,6 +18,8 @@
>  #include "libcamera/internal/log.h"
>  #include "libcamera/internal/tracepoints.h"
>  
> +#define REQUEST_CANARY 0x1F2E3D4C
> +
>  /**
>   * \file request.h
>   * \brief Describes a frame capture request to be processed by a camera
> @@ -89,6 +91,8 @@ Request::Request(Camera *camera, uint64_t cookie)
>  
>  	LIBCAMERA_TRACEPOINT(request_construct, this);
>  
> +	canary_ = REQUEST_CANARY;
> +
>  	LOG(Request, Debug) << "Created request - cookie: " << cookie_;
>  }
>  
> @@ -99,6 +103,8 @@ Request::~Request()
>  	delete metadata_;
>  	delete controls_;
>  	delete validator_;
> +
> +	canary_ = 0;
>  }
>  
>  /**
> @@ -113,6 +119,8 @@ Request::~Request()
>   */
>  void Request::reuse(ReuseFlag flags)
>  {
> +	ASSERT(canary_ == REQUEST_CANARY);
> +
>  	LIBCAMERA_TRACEPOINT(request_reuse, this);
>  
>  	pending_.clear();
> @@ -176,6 +184,8 @@ void Request::reuse(ReuseFlag flags)
>   */
>  int Request::addBuffer(const Stream *stream, FrameBuffer *buffer)
>  {
> +	ASSERT(canary_ == REQUEST_CANARY);
> +
>  	if (!stream) {
>  		LOG(Request, Error) << "Invalid stream reference";
>  		return -EINVAL;
> @@ -211,6 +221,8 @@ int Request::addBuffer(const Stream *stream, FrameBuffer *buffer)
>   */
>  FrameBuffer *Request::findBuffer(const Stream *stream) const
>  {
> +	ASSERT(canary_ == REQUEST_CANARY);
> +
>  	const auto it = bufferMap_.find(stream);
>  	if (it == bufferMap_.end())
>  		return nullptr;
> @@ -262,6 +274,7 @@ FrameBuffer *Request::findBuffer(const Stream *stream) const
>   */
>  void Request::complete()
>  {
> +	ASSERT(canary_ == REQUEST_CANARY);
>  	ASSERT(status_ == RequestPending);
>  	ASSERT(!hasPendingBuffers());
>  
> @@ -289,6 +302,8 @@ void Request::complete()
>   */
>  bool Request::completeBuffer(FrameBuffer *buffer)
>  {
> +	ASSERT(canary_ == REQUEST_CANARY);
> +
>  	LIBCAMERA_TRACEPOINT(request_complete_buffer, this, buffer);
>  
>  	int ret = pending_.erase(buffer);

-- 
Regards,

Laurent Pinchart