[PATCH] btrfs: Fix csum_tree_block to avoid tripping on -Werror=array-bounds

[pengfuyuan]

When compiling on a mips 64-bit machine we get these warnings:

    In file included from ./arch/mips/include/asm/cacheflush.h:13,
	             from ./include/linux/cacheflush.h:5,
	             from ./include/linux/highmem.h:8,
		     from ./include/linux/bvec.h:10,
		     from ./include/linux/blk_types.h:10,
                     from ./include/linux/blkdev.h:9,
	             from fs/btrfs/disk-io.c:7:
    fs/btrfs/disk-io.c: In function â??csum_tree_blockâ??:
    fs/btrfs/disk-io.c:100:34: error: array subscript 1 is above array bounds of â??struct page *[1]â?? [-Werror=array-bounds]
      100 |   kaddr = page_address(buf->pages[i]);
          |                        ~~~~~~~~~~^~~
    ./include/linux/mm.h:2135:48: note: in definition of macro â??page_addressâ??
     2135 | #define page_address(page) lowmem_page_address(page)
          |                                                ^~~~
    cc1: all warnings being treated as errors

We can check if i overflows to solve the problem. However, this doesn't make
much sense, since i == 1 and num_pages == 1 doesn't execute the body of the loop.
In addition, i < num_pages can also ensure that buf->pages[i] will not cross
the boundary. Unfortunately, this doesn't help with the problem observed here:
gcc still complains.

To fix this, start the loop at index 0 instead of 1. Also, a conditional was
added to skip the case where the index is 0, so that the loop iterations follow
the desired logic, and it makes all versions of gcc happy.

Signed-off-by: pengfuyuan <pengfuyuan@xxxxxxxxxx>
---
 fs/btrfs/disk-io.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index fbf9006c6234..8b05d556d747 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -96,9 +96,13 @@ static void csum_tree_block(struct extent_buffer *buf, u8 *result)
 	crypto_shash_update(shash, kaddr + BTRFS_CSUM_SIZE,
 			    first_page_part - BTRFS_CSUM_SIZE);
 
-	for (i = 1; i < num_pages; i++) {
-		kaddr = page_address(buf->pages[i]);
-		crypto_shash_update(shash, kaddr, PAGE_SIZE);
+	for (i = 0; i < num_pages; i++) {
+		struct page *p = buf->pages[i];
+
+		if (i != 0) {
+			kaddr = page_address(p);
+			crypto_shash_update(shash, kaddr, PAGE_SIZE);
+		}
 	}
 	memset(result, 0, BTRFS_CSUM_SIZE);
 	crypto_shash_final(shash, result);
-- 
2.25.1

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