Re: [PATCH] arm: Silence gcc warnings about arch ABI drift

Arnd Bergmann
2024-02-19T06:21:35Z

On Mon, Feb 19, 2024, at 05:09, Calvin Owens wrote:
> 32-bit arm builds uniquely emit a lot of spam like this:
>
> fs/bcachefs/backpointers.c: In function ‘extent_matches_bp’:
> fs/bcachefs/backpointers.c:15:13: note: parameter passing for
> argument of type ‘struct bch_backpointer’ changed in GCC 9.1
>
> Apply the arm64 change from commit ebcc5928c5d9 ("arm64: Silence gcc
> warnings about arch ABI drift") to silence them. It seems like Dave's
> original rationale applies here too.
>
> Cc: Dave Martin <Dave.Martin@arm.com>
> Signed-off-by: Calvin Owens <jcalvinowens@gmail.com>
> ---

I think these should be addressed in bcachefs instead.
While it's not the fault of bcachefs that the calling
convention changed between gcc versions, have a look at
the actual structure layout:

struct bch_val {
__u64 __nothing[0];
};
struct bpos {
/*
* Word order matches machine byte order - btree code treats a bpos as a
* single large integer, for search/comparison purposes
*
* Note that wherever a bpos is embedded in another on disk data
* structure, it has to be byte swabbed when reading in metadata that
* wasn't written in native endian order:
*/
#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
__u32 snapshot;
__u64 offset;
__u64 inode;
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
__u64 inode;
__u64 offset; /* Points to end of extent - sectors */
__u32 snapshot;
#else
#error edit for your odd byteorder.
#endif
} __packed
struct bch_backpointer {
struct bch_val v;
__u8 btree_id;
__u8 level;
__u8 data_type;
__u64 bucket_offset:40;
__u32 bucket_len;
struct bpos pos;
} __packed __aligned(8);

This is not something that should ever be passed by value
into a function.

Arnd