btrfs: new function read_extent_buffer_to_user

This new function reads the content of an extent directly to user memory.

Signed-off-by: Gerhard Heift <Gerhard@Heift.Name>
Signed-off-by: Chris Mason <clm@fb.com>
Acked-by: David Sterba <dsterba@suse.cz>

2 files changed, 40 insertions, 0 deletions

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 0b5fa91d9a88..930f23dfaa2b 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -5067,6 +5067,43 @@ void read_extent_buffer(struct extent_buffer *eb, void *dstv,
         }
 }
 
+int read_extent_buffer_to_user(struct extent_buffer *eb, void __user *dstv,
+                        unsigned long start,
+                        unsigned long len)
+{
+        size_t cur;
+        size_t offset;
+        struct page *page;
+        char *kaddr;
+        char __user *dst = (char __user *)dstv;
+        size_t start_offset = eb->start & ((u64)PAGE_CACHE_SIZE - 1);
+        unsigned long i = (start_offset + start) >> PAGE_CACHE_SHIFT;
+        int ret = 0;
+
+        WARN_ON(start > eb->len);
+        WARN_ON(start + len > eb->start + eb->len);
+
+        offset = (start_offset + start) & (PAGE_CACHE_SIZE - 1);
+
+        while (len > 0) {
+                page = extent_buffer_page(eb, i);
+
+                cur = min(len, (PAGE_CACHE_SIZE - offset));
+                kaddr = page_address(page);
+                if (copy_to_user(dst, kaddr + offset, cur)) {
+                        ret = -EFAULT;
+                        break;
+                }
+
+                dst += cur;
+                len -= cur;
+                offset = 0;
+                i++;
+        }
+
+        return ret;
+}
+
 int map_private_extent_buffer(struct extent_buffer *eb, unsigned long start,
                                unsigned long min_len, char **map,
                                unsigned long *map_start,
diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
index 8b63f2d46518..15ce5f2a2b62 100644
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h
@@ -304,6 +304,9 @@ int memcmp_extent_buffer(struct extent_buffer *eb, const void *ptrv,
 void read_extent_buffer(struct extent_buffer *eb, void *dst,
                         unsigned long start,
                         unsigned long len);
+int read_extent_buffer_to_user(struct extent_buffer *eb, void __user *dst,
+                               unsigned long start,
+                               unsigned long len);
 void write_extent_buffer(struct extent_buffer *eb, const void *src,
                          unsigned long start, unsigned long len);
 void copy_extent_buffer(struct extent_buffer *dst, struct extent_buffer *src,