Re: [PATCH v4 2/4] bitfield: Add non-constant field_{prep,get}() helpers

Geert Uytterhoeven
2025-10-22T10:07:17Z

Hi Yury,

On Wed, 22 Oct 2025 at 06:20, Yury Norov <yury.norov@gmail.com> wrote:
> On Mon, Oct 20, 2025 at 03:00:24PM +0200, Geert Uytterhoeven wrote:
> > On Fri, 17 Oct 2025 at 20:51, Yury Norov <yury.norov@gmail.com> wrote:
> > > On Fri, Oct 17, 2025 at 12:54:10PM +0200, Geert Uytterhoeven wrote:
> > > > The existing FIELD_{GET,PREP}() macros are limited to compile-time
> > > > constants. However, it is very common to prepare or extract bitfield
> > > > elements where the bitfield mask is not a compile-time constant.
> > > >
> > > > To avoid this limitation, the AT91 clock driver and several other
> > > > drivers already have their own non-const field_{prep,get}() macros.
> > > > Make them available for general use by consolidating them in
> > > > <linux/bitfield.h>, and improve them slightly:
> > > > 1. Avoid evaluating macro parameters more than once,
> > > > 2. Replace "ffs() - 1" by "__ffs()",
> > > > 3. Support 64-bit use on 32-bit architectures.
> > > >
> > > > This is deliberately not merged into the existing FIELD_{GET,PREP}()
> > > > macros, as people expressed the desire to keep stricter variants for
> > > > increased safety, or for performance critical paths.
> > > >
> > > > Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> > > > Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
> > > > Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > > > Acked-by: Crt Mori <cmo@melexis.com>
> > > > ---
> > > > v4:
> > > > - Add Acked-by,
> > > > - Rebase on top of commit 7c68005a46108ffa ("crypto: qat - relocate
> > > > power management debugfs helper APIs") in v6.17-rc1,
> > > > - Convert more recently introduced upstream copies:
> > > > - drivers/edac/ie31200_edac.c
> > > > - drivers/iio/dac/ad3530r.c
> > >
> > > Can you split out the part that actually introduces the new API?
> >
> > Unfortunately not, as that would cause build warnings/failures due
> > to conflicting redefinitions.
> > That is a reason why I want to apply this patch ASAP: new copies show
> > up all the time.
>
> In a preparation patch, for each driver:
>
> +#ifndef field_prep
> #define field_prep() ...
> +#endif
>
> Or simply
>
> +#undef field_prep
> #define field_prep() ...
>
> Then add the generic field_prep() in a separate patch. Then you can drop
> ifdefery in the drivers.
>
> Yeah, more patches, but the result is cleaner.

And we need 3 kernel releases, as the addition of the macros to
the header file now has a hard dependency on adding the #undefs?
Unless I still apply all of them to an immutable branch, but then what
is the point?

> > > > --- a/include/linux/bitfield.h
> > > > +++ b/include/linux/bitfield.h
> > > > @@ -220,4 +220,40 @@ __MAKE_OP(64)
> > > > #undef __MAKE_OP
> > > > #undef ____MAKE_OP
> > > >
> > > > +/**
> > > > + * field_prep() - prepare a bitfield element
> > > > + * @mask: shifted mask defining the field's length and position
> > > > + * @val: value to put in the field
> > > > + *
> > > > + * field_prep() masks and shifts up the value. The result should be
> > > > + * combined with other fields of the bitfield using logical OR.
> > > > + * Unlike FIELD_PREP(), @mask is not limited to a compile-time constant.
> > > > + */
> > > > +#define field_prep(mask, val) \
> > > > + ({ \
> > > > + __auto_type __mask = (mask); \
> > > > + typeof(mask) __val = (val); \
> > > > + unsigned int __shift = sizeof(mask) <= 4 ? \
> > > > + __ffs(__mask) : __ffs64(__mask); \
> > > > + (__val << __shift) & __mask; \
> > >
> > > __ffs(0) is undef. The corresponding comment in
> > > include/asm-generic/bitops/__ffs.h explicitly says: "code should check
> > > against 0 first".
> >
> > An all zeroes mask is a bug in the code that calls field_{get,prep}().
>
> It's a bug in FIELD_GET() - for sure. Because it's enforced in
> __BF_FIELD_CHECK(). field_get() doesn't enforce it, doesn't even
> mention that in the comment.
>
> I'm not fully convinced that empty runtime mask should be a bug.

Getting (and using) data from nowhere is a bug.
Storing data where there is no space to store is also a bug.

I will add a comment.

> Consider memcpy(dst, src, 0). This is a no-op, but not a bug as
> soon as the pointers are valid. If you _think_ it's a bug - please
> enforce it.

memcpy() with a fixed size of zero is probably a bug.
memcpy() with a variable size is usually used to copy "as much as is
needed", so zero is usually not a bug.

> > > I think mask = 0 is a sign of error here. Can you add a code catching
> > > it at compile time, and maybe at runtime too? Something like:
> > >
> > > #define __field_prep(mask, val)
> > > ({
> > > unsigned __shift = sizeof(mask) <= 4 ? __ffs(mask) : __ffs64(mask);
> > > (val << __shift) & mask;
> > > })
> > >
> > > #define field_prep(mask, val)
> > > ({
> > > unsigned int __shift;
> > > __auto_type __mask = (mask), __ret = 0;
> > > typeof(mask) __val = (val);
> > >
> > > BUILD_BUG_ON_ZERO(const_true(mask == 0));
> >
> > Futile, as code with a constant mask should use FIELD_PREP() instead.
>
> It's a weak argument. Sometimes compiler is smart enough to realize
> that something is a constant, while people won't. Sometimes code gets
> refactored. Sometimes people build complex expressions that should
> work both in run-time and compile time cases. Sometimes variables are
> compile- or run-time depending on config (nr_cpu_ids is an example).
>
> The field_prep() must handle const case just as good as capitalized
> version does.

OK, I will add the (build-time) check.

> > > if (WARN_ON_ONCE(mask == 0))
> > > goto out;
> > >
> > > __ret = __field_prep(__mask, __val);
> > > out:
> > > ret;
> > > })
> >
> > Should we penalize all users (this is a macro, thus inlined everywhere)
> > to protect against something that is clearly a bug in the caller?
>
> No. But we can wrap it with a config:
>
> #ifdef CONFIG_BITFIELD_HARDENING
> if (WARN_ON_ONCE(mask == 0))
> goto out;
> #endif

That can be done later, when hardening other bitfield functions
and macros.

> > These new macros are intended for the case where mask is not a constant.
> > So typically it is a variable of type u32 or u64.
>
> You never mentioned that. Anyways, it's again a weak argument.

I'll add more comments ;-)

> > > > + unsigned int __shift = sizeof(mask) <= 4 ? \
> > > > + __ffs(__mask) : __ffs64(__mask); \
> > >
> > > Can you use BITS_PER_TYPE() here?
> >
> > Yes, I could use BITS_PER_TYPE(unsigned long) here, to match the
> > parameter type of __ffs() (on 64-bit platforms, __ffs() can be used
> > unconditionally anyway), at the expense of making the line much longer
> > so it has to be split. Is that worthwhile?
>
> Not sure I understand... The
>
> "unsigned int __shift = BITS_PER_TYPE(mask) < 64 ?"
>
> is 49 chars long vs 42 in your version. Even if you add two tabs, it's
> still way below limits. And yes,

Oh, you meant instead of the size check.
I thought you objected to the hardcoded number 4.

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds