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

Geert Uytterhoeven
2025-10-20T13:00:40Z

Hi Yury,

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.

> > --- 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}().

> 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.

>
> 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?
E.g. do_div() does not check for a zero divisor either.

> > +
> > +/**
> > + * field_get() - extract a bitfield element
> > + * @mask: shifted mask defining the field's length and position
> > + * @reg: value of entire bitfield
> > + *
> > + * field_get() extracts the field specified by @mask from the
> > + * bitfield passed in as @reg by masking and shifting it down.
> > + * Unlike FIELD_GET(), @mask is not limited to a compile-time constant.
> > + */
> > +#define field_get(mask, reg) \
> > + ({ \
> > + __auto_type __mask = (mask); \
> > + typeof(mask) __reg = (reg); \
>
> This would trigger Wconversion warning. Consider
> unsigned reg = 0xfff;
> field_get(0xf, reg);
>
> :6:26: warning: conversion to 'int' from 'unsigned int' may change the sign of the result [-Wsign-conversion]
> 6 | typeof(mask) __reg = reg;
> | ^~~
>
> Notice, the __auto_type makes the __mask to be int, while the reg is

Apparently using typeof(mask) has the same "issue"...

> unsigned int. You need to do:
>
> typeof(mask) __reg = (typeof(mask))(reg);

... so the cast is just hiding the issue? Worse, the cast may prevent the
compiler from flagging other issues, e.g. when accidentally passing
a pointer for reg.

>
> Please enable higher warning levels for the next round.

Enabling -Wsign-conversion gives lots of other (false positive?)
warnings.

> Also, because for numerals __auto_type is int, when char is enough - are
> you sure that the macro generates the optimal code? User can workaround it
> with:
>
> field_get((u8)0xf, reg)
>
> but it may not be trivial. Can you add an example and explanation please?

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.

> > + 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?

>
> > + (__reg & __mask) >> __shift; \
> > + })
> > +
>
> When mask == 0, we shouldn't touch 'val' at all. Consider
>
> field_get(0, get_user(ptr))
>
> In this case, evaluating 'reg' is an error, similarly to memcpy().

Again, a zero mask is a bug.

Thanks!

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