Re: [PATCH v2 3/4] binder: Add flags to relinquish ownership of fds

[Carlos Llamas]

On Mon, Jan 23, 2023 at 07:17:25PM +0000, T.J. Mercier wrote:
> From: Hridya Valsaraju <hridya@xxxxxxxxxx>
> 
> This patch introduces flag BINDER_FD_FLAG_XFER_CHARGE that a process
> sending an individual fd or fd array to another process over binder IPC
> can set to relinquish ownership of the fd(s) being sent for memory
> accounting purposes. If the flag is found to be set during the fd or fd
> array translation and the fd is for a DMA-BUF, the buffer is uncharged
> from the sender's cgroup and charged to the receiving process's cgroup
> instead.
> 
> It is up to the sending process to ensure that it closes the fds
> regardless of whether the transfer failed or succeeded.
> 
> Most graphics shared memory allocations in Android are done by the
> graphics allocator HAL process. On requests from clients, the HAL
> process allocates memory and sends the fds to the clients over binder
> IPC. The graphics allocator HAL will not retain any references to the
> buffers. When the HAL sets BINDER_FD_FLAG_XFER_CHARGE, binder will
> transfer the charge for the buffer from the allocator process cgroup to
> the client process cgroup.
> 
> The pad [1] and pad_flags [2] fields of binder_fd_object and
> binder_fda_array_object come from alignment with flat_binder_object and
> have never been exposed for use from userspace. This new flags use
> follows the pattern set by binder_buffer_object.
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/include/uapi/linux/android/binder.h?id=feba3900cabb8e7c87368faa28e7a6936809ba22
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/include/uapi/linux/android/binder.h?id=5cdcf4c6a638591ec0e98c57404a19e7f9997567
> 
> Signed-off-by: Hridya Valsaraju <hridya@xxxxxxxxxx>
> Signed-off-by: T.J. Mercier <tjmercier@xxxxxxxxxx>
> ---
>  Documentation/admin-guide/cgroup-v2.rst |  3 ++-
>  drivers/android/binder.c                | 25 +++++++++++++++++++++----
>  include/uapi/linux/android/binder.h     | 19 +++++++++++++++----
>  3 files changed, 38 insertions(+), 9 deletions(-)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 538ae22bc514..d225295932c0 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1457,7 +1457,8 @@ PAGE_SIZE multiple when read back.
>  
>  	  dmabuf (npn)
>  		Amount of memory used for exported DMA buffers allocated by the cgroup.
> -		Stays with the allocating cgroup regardless of how the buffer is shared.
> +		Stays with the allocating cgroup regardless of how the buffer is shared
> +		unless explicitly transferred.
>  
>  	  workingset_refault_anon
>  		Number of refaults of previously evicted anonymous pages.
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index 880224ec6abb..5e707974793f 100644
> --- a/drivers/android/binder.c
> +++ b/drivers/android/binder.c
> @@ -42,6 +42,7 @@
>  
>  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
>  
> +#include <linux/dma-buf.h>
>  #include <linux/fdtable.h>
>  #include <linux/file.h>
>  #include <linux/freezer.h>
> @@ -2237,7 +2238,7 @@ static int binder_translate_handle(struct flat_binder_object *fp,
>  	return ret;
>  }
>  
> -static int binder_translate_fd(u32 fd, binder_size_t fd_offset,
> +static int binder_translate_fd(u32 fd, binder_size_t fd_offset, __u32 flags,
>  			       struct binder_transaction *t,
>  			       struct binder_thread *thread,
>  			       struct binder_transaction *in_reply_to)
> @@ -2275,6 +2276,20 @@ static int binder_translate_fd(u32 fd, binder_size_t fd_offset,
>  		goto err_security;
>  	}
>  
> +	if (IS_ENABLED(CONFIG_MEMCG) && (flags & BINDER_FD_FLAG_XFER_CHARGE)) {

Do we need to test for MEMCG here? it seems this has been offloaded to
dma_buf_transfer_charge()?

> +		ret = dma_buf_transfer_charge(file, target_proc->tsk);
> +		if (unlikely(ret == -EBADF)) {
> +			binder_user_error(
> +				"%d:%d got transaction with XFER_CHARGE for non-DMA-BUF fd, %d\n",
> +				proc->pid, thread->pid, fd);
> +			goto err_dmabuf;
> +		} else if (ret) {
> +			pr_warn("%d:%d Unable to transfer DMA-BUF fd charge to %d\n",
> +				proc->pid, thread->pid, target_proc->pid);
> +			goto err_xfer;
> +		}
> +	}
> +
>  	/*
>  	 * Add fixup record for this transaction. The allocation
>  	 * of the fd in the target needs to be done from a
> @@ -2294,6 +2309,8 @@ static int binder_translate_fd(u32 fd, binder_size_t fd_offset,
>  	return ret;
>  
>  err_alloc:
> +err_xfer:
> +err_dmabuf:
>  err_security:
>  	fput(file);
>  err_fget:
> @@ -2604,7 +2621,7 @@ static int binder_translate_fd_array(struct list_head *pf_head,
>  
>  		ret = copy_from_user(&fd, sender_ufda_base + sender_uoffset, sizeof(fd));
>  		if (!ret)
> -			ret = binder_translate_fd(fd, offset, t, thread,
> +			ret = binder_translate_fd(fd, offset, fda->flags, t, thread,
>  						  in_reply_to);
>  		if (ret)
>  			return ret > 0 ? -EINVAL : ret;
> @@ -3383,8 +3400,8 @@ static void binder_transaction(struct binder_proc *proc,
>  			struct binder_fd_object *fp = to_binder_fd_object(hdr);
>  			binder_size_t fd_offset = object_offset +
>  				(uintptr_t)&fp->fd - (uintptr_t)fp;
> -			int ret = binder_translate_fd(fp->fd, fd_offset, t,
> -						      thread, in_reply_to);
> +			int ret = binder_translate_fd(fp->fd, fd_offset, fp->flags,
> +						      t, thread, in_reply_to);
>  
>  			fp->pad_binder = 0;
>  			if (ret < 0 ||
> diff --git a/include/uapi/linux/android/binder.h b/include/uapi/linux/android/binder.h
> index e72e4de8f452..4b20dd1dccb1 100644
> --- a/include/uapi/linux/android/binder.h
> +++ b/include/uapi/linux/android/binder.h
> @@ -91,14 +91,14 @@ struct flat_binder_object {
>  /**
>   * struct binder_fd_object - describes a filedescriptor to be fixed up.
>   * @hdr:	common header structure
> - * @pad_flags:	padding to remain compatible with old userspace code
> + * @flags:	One or more BINDER_FD_FLAG_* flags
>   * @pad_binder:	padding to remain compatible with old userspace code
>   * @fd:		file descriptor
>   * @cookie:	opaque data, used by user-space
>   */
>  struct binder_fd_object {
>  	struct binder_object_header	hdr;
> -	__u32				pad_flags;
> +	__u32				flags;
>  	union {
>  		binder_uintptr_t	pad_binder;
>  		__u32			fd;
> @@ -107,6 +107,17 @@ struct binder_fd_object {
>  	binder_uintptr_t		cookie;
>  };
>  
> +enum {
> +	/**
> +	 * @BINDER_FD_FLAG_XFER_CHARGE
> +	 *
> +	 * When set, the sender of a binder_fd_object wishes to relinquish ownership of the fd for
> +	 * memory accounting purposes. If the fd is for a DMA-BUF, the buffer is uncharged from the
> +	 * sender's cgroup and charged to the receiving process's cgroup instead.
> +	 */
> +	BINDER_FD_FLAG_XFER_CHARGE = 0x01,
> +};
> +
>  /* struct binder_buffer_object - object describing a userspace buffer
>   * @hdr:		common header structure
>   * @flags:		one or more BINDER_BUFFER_* flags
> @@ -141,7 +152,7 @@ enum {
>  
>  /* struct binder_fd_array_object - object describing an array of fds in a buffer
>   * @hdr:		common header structure
> - * @pad:		padding to ensure correct alignment
> + * @flags:		One or more BINDER_FD_FLAG_* flags
>   * @num_fds:		number of file descriptors in the buffer
>   * @parent:		index in offset array to buffer holding the fd array
>   * @parent_offset:	start offset of fd array in the buffer
> @@ -162,7 +173,7 @@ enum {
>   */
>  struct binder_fd_array_object {
>  	struct binder_object_header	hdr;
> -	__u32				pad;
> +	__u32				flags;
>  	binder_size_t			num_fds;
>  	binder_size_t			parent;
>  	binder_size_t			parent_offset;
> -- 
> 2.39.0.246.g2a6d74b583-goog
> 

Other than the previous question this looks good to me. Also, the error
from the test robot seems to indicate a missing stub for
dma_buf_transfer_charg() when !CONFIG_DMA_SHARED_BUFFER. However, this
is likely to be fixed outside of this patch. Feel free to add this tag
to the following round:

Acked-by: Carlos Llamas <cmllamas@xxxxxxxxxx>

Thanks,