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

Yury Norov
2025-10-22T04:20:25Z

On Mon, Oct 20, 2025 at 03:00:24PM +0200, Geert Uytterhoeven wrote:
> 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.

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.

> > > --- 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.
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.

> > 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.

> > 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

The real question here: do you want to help people to catch their bugs,
or you want them to fight it alone?

The _BF_FIELD_CHECK() authors are nice people and provide helpful guides.
(I don't insist, it's up to you.)

> 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.

Ok, makes sense.

> > 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.

You never mentioned that. Anyways, it's again a weak argument.

> > > + 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,

unsigned int __shift = sizeof(mask) <= 4 ? \
__ffs(__mask) : __ffs64(__mask); \

is worse than

unsigned int __shift = BITS_PER_TYPE(mask) < 64 ? \
__ffs(__mask) : __ffs64(__mask); \

> > > + (__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