[PATCH v2][next] jbd2: Avoid dozens of -Wflex-array-member-not-at-end warnings

From: Gustavo A. R. Silva
Date: Fri Nov 01 2024 - 16:45:39 EST


-Wflex-array-member-not-at-end was introduced in GCC-14, and we
are getting ready to enable it, globally.

Use the `DEFINE_RAW_FLEX()` helper for an on-stack definition of
a flexible structure (`struct shash_desc`) where the size of the
flexible-array member (`__ctx`) is known at compile-time, and
refactor the rest of the code, accordingly.

So, with this, fix 77 of the following warnings:

include/linux/jbd2.h:1800:35: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
---
Changes in v2:
- Use DIV_ROUND_UP() to compute the number of elements for the flex
array. (Jan Kara)

v1:
- Link: https://lore.kernel.org/linux-hardening/ZxvyavDjXDaV9cNg@kspp/

include/linux/jbd2.h | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index 8aef9bb6ad57..50f7ea8714bf 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1796,22 +1796,21 @@ static inline unsigned long jbd2_log_space_left(journal_t *journal)
static inline u32 jbd2_chksum(journal_t *journal, u32 crc,
const void *address, unsigned int length)
{
- struct {
- struct shash_desc shash;
- char ctx[JBD_MAX_CHECKSUM_SIZE];
- } desc;
+ DEFINE_RAW_FLEX(struct shash_desc, desc, __ctx,
+ DIV_ROUND_UP(JBD_MAX_CHECKSUM_SIZE,
+ sizeof(*((struct shash_desc *)0)->__ctx)));
int err;

BUG_ON(crypto_shash_descsize(journal->j_chksum_driver) >
JBD_MAX_CHECKSUM_SIZE);

- desc.shash.tfm = journal->j_chksum_driver;
- *(u32 *)desc.ctx = crc;
+ desc->tfm = journal->j_chksum_driver;
+ *(u32 *)desc->__ctx = crc;

- err = crypto_shash_update(&desc.shash, address, length);
+ err = crypto_shash_update(desc, address, length);
BUG_ON(err);

- return *(u32 *)desc.ctx;
+ return *(u32 *)desc->__ctx;
}

/* Return most recent uncommitted transaction */
--
2.43.0