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

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Message-ID: <k6cclnrqu2h6mh4wb33hpakhfasq7jec7dnbaiaxlpw525adan@cor5l3bi35u2>
Date: Mon, 19 Feb 2024 01:25:22 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Arnd Bergmann <arnd@...db.de>
Cc: Calvin Owens <jcalvinowens@...il.com>, 
	Russell King <linux@...linux.org.uk>, Dave Martin <Dave.Martin@....com>, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	"linux-bcachefs@...r.kernel.org" <linux-bcachefs@...r.kernel.org>
Subject: Re: [PATCH] arm: Silence gcc warnings about arch ABI drift

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@....com>
> > Signed-off-by: Calvin Owens <jcalvinowens@...il.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?