On Sat, 6 Jan 2024 at 16:42, Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx> wrote:
>
> This is not really a kernel/driver bug, just sparse being over-eager
> with truncation detection. I wonder if we could make sparse skip this
> check on forced casts like this:
No, please don't.
Just face the fact that using integer casts to mask bits off is a bad idea.
Yes, we could say "explicit casting is ok", since it's really the
hidden implicit casts changing values that sparse complains about, but
your solution is really ugly:
> static inline void __put_unaligned_be24(const u32 val, u8 *p)
> {
> - *p++ = val >> 16;
> - *p++ = val >> 8;
> - *p++ = val;
> + *p++ = (__force u8)(val >> 16);
> + *p++ = (__force u8)(val >> 8);
> + *p++ = (__force u8)val;
> }
That's just disgusting.
The *natural* thing to do is to simply make the masking itself be
explicit - not the cast. IOW, just write it as
*p++ = (val >> 16) & 0xff;
*p++ = (val >> 8) & 0xff;
*p++ = val & 0xff;
and doesn't that look much more natural?
Sure, the compiler will then just notice "you're assigning to a char,
to I don't actually need to do any masking at all", but now sparse
won't complain because there's no "cast silently drops bits" issue any
more.
And while the code is a bit more to read, I think it is actually to
some degree more obvious to a human too what is going on.
No?
Linus
