Re: [PATCH v3 3/3] linux/bits.h: Add compile time sanity check of GENMASK inputs

[Masahiro Yamada]

On Tue, Oct 8, 2019 at 4:44 PM Masahiro Yamada
<yamada.masahiro@xxxxxxxxxxxxx> wrote:
>
> Hi Geert,
>
> On Tue, Oct 8, 2019 at 4:23 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> >
> > Hi Rikard,
> >
> > On Sun, Aug 11, 2019 at 8:52 PM Rikard Falkeborn
> > <rikard.falkeborn@xxxxxxxxx> wrote:
> > > GENMASK() and GENMASK_ULL() are supposed to be called with the high bit
> > > as the first argument and the low bit as the second argument. Mixing
> > > them will return a mask with zero bits set.
> > >
> > > Recent commits show getting this wrong is not uncommon, see e.g.
> > > commit aa4c0c9091b0 ("net: stmmac: Fix misuses of GENMASK macro") and
> > > commit 9bdd7bb3a844 ("clocksource/drivers/npcm: Fix misuse of GENMASK
> > > macro").
> > >
> > > To prevent such mistakes from appearing again, add compile time sanity
> > > checking to the arguments of GENMASK() and GENMASK_ULL(). If both
> > > arguments are known at compile time, and the low bit is higher than the
> > > high bit, break the build to detect the mistake immediately.
> > >
> > > Since GENMASK() is used in declarations, BUILD_BUG_ON_ZERO() must be
> > > used instead of BUILD_BUG_ON().
> > >
> > > __builtin_constant_p does not evaluate is argument, it only checks if it
> > > is a constant or not at compile time, and __builtin_choose_expr does not
> > > evaluate the expression that is not chosen. Therefore, GENMASK(x++, 0)
> > > does only evaluate x++ once.
> > >
> > > Commit 95b980d62d52 ("linux/bits.h: make BIT(), GENMASK(), and friends
> > > available in assembly") made the macros in linux/bits.h available in
> > > assembly. Since BUILD_BUG_OR_ZERO() is not asm compatible, disable the
> > > checks if the file is included in an asm file.
> > >
> > > Signed-off-by: Rikard Falkeborn <rikard.falkeborn@xxxxxxxxx>
> > > ---
> > > Changes in v3:
> > >   - Changed back to shorter macro argument names
> > >   - Remove casts and use 0 instead of UL(0) in GENMASK_INPUT_CHECK(),
> > >     since all results in GENMASK_INPUT_CHECK() are now ints. Update
> > >     commit message to reflect that.
> > >
> > > Changes in v2:
> > >   - Add comment about why inputs are not checked when used in asm file
> > >   - Use UL(0) instead of 0
> > >   - Extract mask creation in a separate macro to improve readability
> > >   - Use high and low instead of h and l (part of this was extracted to a
> > >     separate patch)
> > >   - Updated commit message
> > >
> > > Joe Perches sent a series to fix the existing misuses of GENMASK() that
> > > needs to be merged before this to avoid build failures. Currently, 5 of
> > > the patches are not in Linus tree, and 2 are not in linux-next. There is
> > > also a patch pending by Nathan Chancellor that also needs to be merged
> > > before this patch is merged to avoid build failures.
> > >
> > >  include/linux/bits.h | 21 +++++++++++++++++++--
> > >  1 file changed, 19 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/linux/bits.h b/include/linux/bits.h
> > > index 669d69441a62..4ba0fb609239 100644
> > > --- a/include/linux/bits.h
> > > +++ b/include/linux/bits.h
> > > @@ -18,12 +18,29 @@
> > >   * position @h. For example
> > >   * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000.
> > >   */
> > > -#define GENMASK(h, l) \
> > > +#ifndef __ASSEMBLY__
> > > +#include <linux/build_bug.h>
> > > +#define GENMASK_INPUT_CHECK(h, l) \
> > > +       (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > > +               __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> > > +#else
> > > +/*
> > > + * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
> > > + * disable the input check if that is the case.
> > > + */
> > > +#define GENMASK_INPUT_CHECK(h, l) 0
> > > +#endif
> > > +
> > > +#define __GENMASK(h, l) \
> > >         (((~UL(0)) - (UL(1) << (l)) + 1) & \
> > >          (~UL(0) >> (BITS_PER_LONG - 1 - (h))))
> > > +#define GENMASK(h, l) \
> > > +       (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> > >
> > > -#define GENMASK_ULL(h, l) \
> > > +#define __GENMASK_ULL(h, l) \
> > >         (((~ULL(0)) - (ULL(1) << (l)) + 1) & \
> > >          (~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h))))
> > > +#define GENMASK_ULL(h, l) \
> > > +       (GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l))
> > >
> > >  #endif /* __LINUX_BITS_H */
> >
> > This is now commit 0fd35cd30a2fece1 ("linux/bits.h: add compile time sanity
> > check of GENMASK inputs") in next-20191008.
> >
> > <noreply@xxxxxxxxxxxxxx> reported the following failure in sun3_defconfig,
> > which I managed to reproduce with gcc-4.6.3:
>
> Oh dear.
>
> I was able to reproduce this for gcc 4.7 or 4.8,
> but I did not see any problem for gcc 4.9+
>
> Perhaps, is this due to broken __builtin_choose_expr or __builtin_constant_p
> for old compilers?
>
>
>
>
>
>
>
> >
> >     lib/842/842_compress.c: In function '__split_add_bits':
> >     lib/842/842_compress.c:164:25: error: first argument to
> > '__builtin_choose_expr' not a constant
> >     lib/842/842_compress.c:164:25: error: bit-field '<anonymous>'
> > width not an integer constant
> >     scripts/Makefile.build:265: recipe for target
> > 'lib/842/842_compress.o' failed
> >
> > __split_add_bits() calls GENMASK_ULL() with a non-constant.
> > However __split_add_bits() itself is called with constants only.
> > Apparently gcc fails to inline __split_add_bits().
> > Adding inline or always_inline doesn't help.
> >


If this is broken for GCC < 4.9,
we might be able to workaround it as follows:




diff --git a/include/linux/bits.h b/include/linux/bits.h
index 4ba0fb609239..f00417baf545 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -18,7 +18,7 @@
  * position @h. For example
  * GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000.
  */
-#ifndef __ASSEMBLY__
+#if !defined(__ASSEMBLY__) && (!defined(CONFIG_CC_IS_GCC) ||
CONFIG_GCC_VERSION >= 49000)
 #include <linux/build_bug.h>
 #define GENMASK_INPUT_CHECK(h, l) \
        (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \


-- 
Best Regards
Masahiro Yamada