Fix the following out-of-bounds warnings by using a flexible-array
member *pages[] at the bottom of struct extent_buffer:

fs/btrfs/disk-io.c:225:34: warning: array subscript 1 is above array bounds of ‘struct page *[1]’ [-Warray-bounds]
fs/btrfs/struct-funcs.c:80:46: warning: array subscript 1 is above array bounds of ‘struct page *[1]’ [-Warray-bounds]
fs/btrfs/struct-funcs.c:101:32: warning: array subscript 1 is above array bounds of ‘struct page * const[1]’ [-Warray-bounds]
fs/btrfs/struct-funcs.c:133:46: warning: array subscript 1 is above array bounds of ‘struct page *[1]’ [-Warray-bounds]
fs/btrfs/struct-funcs.c:156:32: warning: array subscript 1 is above array bounds of ‘struct page * const[1]’ [-Warray-bounds]
fs/btrfs/struct-funcs.c:80:46: warning: array subscript 1 is above array bounds of ‘struct page *[1]’ [-Warray-bounds]
fs/btrfs/struct-funcs.c:101:32: warning: array subscript 1 is above array bounds of ‘struct page * const[1]’ [-Warray-bounds]
fs/btrfs/struct-funcs.c:133:46: warning: array subscript 1 is above array bounds of ‘struct page *[1]’ [-Warray-bounds]
fs/btrfs/struct-funcs.c:156:32: warning: array subscript 1 is above array bounds of ‘struct page * const[1]’ [-Warray-bounds]
fs/btrfs/struct-funcs.c:80:46: warning: array subscript 1 is above array bounds of ‘struct page *[1]’ [-Warray-bounds]
fs/btrfs/struct-funcs.c:101:32: warning: array subscript 1 is above array bounds of ‘struct page * const[1]’ [-Warray-bounds]
fs/btrfs/struct-funcs.c:133:46: warning: array subscript 1 is above array bounds of ‘struct page *[1]’ [-Warray-bounds]
fs/btrfs/struct-funcs.c:156:32: warning: array subscript 1 is above array bounds of ‘struct page * const[1]’ [-Warray-bounds]

Also, make use of the struct_size() helper to properly calculate the
total size of struct extent_buffer for the kmem cache allocation.

This is part of the ongoing efforts to globally enable -Warray-bounds.

The code was built with ppc64_defconfig and -Warray-bounds enabled by
default in the main Makefile.

Link: https://github.com/KSPP/linux/issues/109
Cc: Qu Wenruo <redacted>
Cc: David Sterba <dsterba@suse.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 fs/btrfs/extent_io.c | 5 +++--
 fs/btrfs/extent_io.h | 2 +-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 9e81d25dea70..4cf0b72fdd9f 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c

@@ -232,8 +232,9 @@ int __init extent_state_cache_init(void)
 int __init extent_io_init(void)
 {
 	extent_buffer_cache = kmem_cache_create("btrfs_extent_buffer",
-			sizeof(struct extent_buffer), 0,
-			SLAB_MEM_SPREAD, NULL);
+			struct_size((struct extent_buffer *)0, pages,
+				    INLINE_EXTENT_BUFFER_PAGES),
+			0, SLAB_MEM_SPREAD, NULL);
 	if (!extent_buffer_cache)
 		return -ENOMEM;
 

diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
index 62027f551b44..b82e8b694a3b 100644
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h

@@ -94,11 +94,11 @@ struct extent_buffer {
 
 	struct rw_semaphore lock;
 
-	struct page *pages[INLINE_EXTENT_BUFFER_PAGES];
 	struct list_head release_list;
 #ifdef CONFIG_BTRFS_DEBUG
 	struct list_head leak_list;
 #endif
+	struct page *pages[];
 };
 
 /*

-- 
2.27.0