[PATCH] bcachefs: Use struct_size()

[Christophe JAILLET]

Use struct_size() instead of hand writing it.
This is less verbose and more robust.

While at it, prepare for the coming implementation by GCC and Clang of the
__counted_by attribute. Flexible array members annotated with __counted_by
can have their accesses bounds-checked at run-time checking via
CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for
strcpy/memcpy-family functions).

Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
---
This patch is part of a work done in parallel of what is currently worked
on by Kees Cook.

My patches are only related to corner cases that do NOT match the
semantic of his Coccinelle script[1].

In this case, struct_size() was not used to compute the size needed for the
structure and its flex array.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci
---
 fs/bcachefs/disk_groups.c | 3 +--
 fs/bcachefs/super_types.h | 2 +-
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/bcachefs/disk_groups.c b/fs/bcachefs/disk_groups.c
index b292dbef7992..224efa917427 100644
--- a/fs/bcachefs/disk_groups.c
+++ b/fs/bcachefs/disk_groups.c
@@ -166,8 +166,7 @@ int bch2_sb_disk_groups_to_cpu(struct bch_fs *c)
 	if (!groups)
 		return 0;
 
-	cpu_g = kzalloc(sizeof(*cpu_g) +
-			sizeof(cpu_g->entries[0]) * nr_groups, GFP_KERNEL);
+	cpu_g = kzalloc(struct_size(cpu_g, entries, nr_groups), GFP_KERNEL);
 	if (!cpu_g)
 		return -BCH_ERR_ENOMEM_disk_groups_to_cpu;
 
diff --git a/fs/bcachefs/super_types.h b/fs/bcachefs/super_types.h
index 597a8db73585..78d6138db62d 100644
--- a/fs/bcachefs/super_types.h
+++ b/fs/bcachefs/super_types.h
@@ -46,7 +46,7 @@ struct bch_disk_group_cpu {
 struct bch_disk_groups_cpu {
 	struct rcu_head			rcu;
 	unsigned			nr;
-	struct bch_disk_group_cpu	entries[];
+	struct bch_disk_group_cpu	entries[] __counted_by(nr);
 };
 
 #endif /* _BCACHEFS_SUPER_TYPES_H */
-- 
2.34.1