dma-buf updates for 3.12

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Merge tag 'for-3.12' of git://git.linaro.org/people/sumitsemwal/linux-dma-buf

Pull dma-buf updates from Sumit Semwal:
 "Yet another small one - dma-buf framework now supports size discovery
  of the buffer via llseek"

* tag 'for-3.12' of git://git.linaro.org/people/sumitsemwal/linux-dma-buf:
  dma-buf: Expose buffer size to userspace (v2)
  dma-buf: Check return value of anon_inode_getfile

Documentation/dma-buf-sharing.txt

@@ -407,6 +407,18 @@ Being able to mmap an export dma-buf buffer object has 2 main use-cases:
    interesting ways depending upong the exporter (if userspace starts depending
    upon this implicit synchronization).
 
+Other Interfaces Exposed to Userspace on the dma-buf FD
+------------------------------------------------------
+
+- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
+  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
+  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
+  llseek operation will report -EINVAL.
+
+  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
+  cases. Userspace can use this to detect support for discovering the dma-buf
+  size using llseek.
+
 Miscellaneous notes
 -------------------
 

drivers/base/dma-buf.c

@@ -77,9 +77,36 @@ static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
 	return dmabuf->ops->mmap(dmabuf, vma);
 }
 
+static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
+{
+	struct dma_buf *dmabuf;
+	loff_t base;
+
+	if (!is_dma_buf_file(file))
+		return -EBADF;
+
+	dmabuf = file->private_data;
+
+	/* only support discovering the end of the buffer,
+	   but also allow SEEK_SET to maintain the idiomatic
+	   SEEK_END(0), SEEK_CUR(0) pattern */
+	if (whence == SEEK_END)
+		base = dmabuf->size;
+	else if (whence == SEEK_SET)
+		base = 0;
+	else
+		return -EINVAL;
+
+	if (offset != 0)
+		return -EINVAL;
+
+	return base + offset;
+}
+
 static const struct file_operations dma_buf_fops = {
 	.release	= dma_buf_release,
 	.mmap		= dma_buf_mmap_internal,
+	.llseek		= dma_buf_llseek,
 };
 
 /*
@@ -133,7 +160,12 @@ struct dma_buf *dma_buf_export_named(void *priv, const struct dma_buf_ops *ops,
 	dmabuf->exp_name = exp_name;
 
 	file = anon_inode_getfile("dmabuf", &dma_buf_fops, dmabuf, flags);
+	if (IS_ERR(file)) {
+		kfree(dmabuf);
+		return ERR_CAST(file);
+	}
 
+	file->f_mode |= FMODE_LSEEK;
 	dmabuf->file = file;
 
 	mutex_init(&dmabuf->lock);