binder: Add flags to relinquish ownership of fds

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@google.com>
Signed-off-by: T.J. Mercier <tjmercier@google.com>

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.

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)) {
+		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 ||

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;