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

Kent Overstreet
2024-02-19T06:25:29Z

On Mon, Feb 19, 2024 at 07:21:11AM +0100, Arnd Bergmann wrote:
> 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.

Why?