On 31/10/24 15:32, Jan Kara wrote:
On Thu 31-10-24 09:54:36, Gustavo A. R. Silva wrote:
On 31/10/24 06:33, Jan Kara wrote:
On Fri 25-10-24 13:32:58, Gustavo A. R. Silva wrote:
-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>
---
include/linux/jbd2.h | 13 +++++--------
1 file changed, 5 insertions(+), 8 deletions(-)
diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index 8aef9bb6ad57..ce4560e62d3b 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1796,22 +1796,19 @@ 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, 1);
Am I missing some magic here or the 1 above should be
JBD_MAX_CHECKSUM_SIZE?
This seems to be 32-bit code, and the element type of the flex-array
member `__ctx` is `void *`. Therefore, we have:
Why do you think the code is 32-bit? It is used regardless of the
architecture...
Right, sorry, I got a bit confused...
`sizeof(ctx) == 4` when `char ctx[JBD_MAX_CHECKSUM_SIZE];`
To maintain the same size, we tell `DEFINE_RAW_FLEX()` to allocate `1`
element for the flex array, as in 32-bit `sizeof(void *) == 4`.
So I agree we end up allocating enough space on stack but it is pretty
subtle and if JBD_MAX_CHECKSUM_SIZE definition changes, we have a problem.
I think we need something like (JBD_MAX_CHECKSUM_SIZE + sizeof(*desc->__ctx)
- 1) / sizeof(*desc->__ctx))?
I see. Well, in that case it'd be something more like:
- struct {
- struct shash_desc shash;
- char ctx[JBD_MAX_CHECKSUM_SIZE];
- } desc;
+ DEFINE_RAW_FLEX(struct shash_desc, desc, __ctx,
+ (JBD_MAX_CHECKSUM_SIZE +
+ sizeof(*((struct shash_desc *)0)->__ctx)) /
+ sizeof(*((struct shash_desc *)0)->__ctx));
Notice that `desc` is created inside `DEFINE_RAW_FLEX()`
Thanks
--
Gustavo
