Re: [PATCH 11/12] tools: sync lib/find_bit implementation

Rikard Falkeborn
2021-05-10T22:52:10Z

On Mon, May 10, 2021 at 06:44:44PM +0300, Andy Shevchenko wrote:
> +Cc: Rikard
>
> On Mon, May 10, 2021 at 6:31 PM Tetsuo Handa
> <penguin-kernel@i-love.sakura.ne.jp> wrote:
> >
> > Commit eaae7841ba83bb42 ("tools: sync lib/find_bit implementation") broke
> > build of 5.13-rc1 using gcc (GCC) 8.3.1 20190311 (Red Hat 8.3.1-3).
> >
> > DESCEND objtool
> > CC /usr/src/linux/tools/objtool/exec-cmd.o
> > CC /usr/src/linux/tools/objtool/help.o
> > CC /usr/src/linux/tools/objtool/pager.o
> > CC /usr/src/linux/tools/objtool/parse-options.o
> > CC /usr/src/linux/tools/objtool/run-command.o
> > CC /usr/src/linux/tools/objtool/sigchain.o
> > CC /usr/src/linux/tools/objtool/subcmd-config.o
> > LD /usr/src/linux/tools/objtool/libsubcmd-in.o
> > AR /usr/src/linux/tools/objtool/libsubcmd.a
> > CC /usr/src/linux/tools/objtool/arch/x86/special.o
> > In file included from /usr/src/linux/tools/include/linux/kernel.h:8:0,
> > from /usr/src/linux/tools/include/linux/list.h:7,
> > from /usr/src/linux/tools/objtool/include/objtool/arch.h:10,
> > from /usr/src/linux/tools/objtool/include/objtool/check.h:11,
> > from /usr/src/linux/tools/objtool/include/objtool/special.h:10,
> > from arch/x86/special.c:4:
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h: In function 'find_next_bit':
> > /usr/src/linux/tools/include/linux/bits.h:24:21: error: first argument to '__builtin_choose_expr' not a constant
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > ^
> > /usr/src/linux/tools/include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO'
> > #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> > (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h:32:17: note: in expansion of macro 'GENMASK'
> > val = *addr & GENMASK(size - 1, offset);
> > ^
> > /usr/src/linux/tools/include/linux/build_bug.h:16:51: error: bit-field '<anonymous>' width not an integer constant
> > #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:24:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> > (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h:32:17: note: in expansion of macro 'GENMASK'
> > val = *addr & GENMASK(size - 1, offset);
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h: In function 'find_next_and_bit':
> > /usr/src/linux/tools/include/linux/bits.h:24:21: error: first argument to '__builtin_choose_expr' not a constant
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > ^
> > /usr/src/linux/tools/include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO'
> > #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> > (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h:62:27: note: in expansion of macro 'GENMASK'
> > val = *addr1 & *addr2 & GENMASK(size - 1, offset);
> > ^
> > /usr/src/linux/tools/include/linux/build_bug.h:16:51: error: bit-field '<anonymous>' width not an integer constant
> > #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:24:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> > (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h:62:27: note: in expansion of macro 'GENMASK'
> > val = *addr1 & *addr2 & GENMASK(size - 1, offset);
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h: In function 'find_next_zero_bit':
> > /usr/src/linux/tools/include/linux/bits.h:24:21: error: first argument to '__builtin_choose_expr' not a constant
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > ^
> > /usr/src/linux/tools/include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO'
> > #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> > (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h:90:18: note: in expansion of macro 'GENMASK'
> > val = *addr | ~GENMASK(size - 1, offset);
> > ^
> > /usr/src/linux/tools/include/linux/build_bug.h:16:51: error: bit-field '<anonymous>' width not an integer constant
> > #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:24:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > ^
> > /usr/src/linux/tools/include/linux/bits.h:38:3: note: in expansion of macro 'GENMASK_INPUT_CHECK'
> > (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
> > ^
> > /usr/src/linux/tools/include/asm-generic/bitops/find.h:90:18: note: in expansion of macro 'GENMASK'
> > val = *addr | ~GENMASK(size - 1, offset);
> > ^
> > make[5]: *** [/usr/src/linux/tools/objtool/arch/x86/special.o] Error 1
> > make[4]: *** [arch/x86] Error 2
> > make[3]: *** [/usr/src/linux/tools/objtool/objtool-in.o] Error 2
> > make[2]: *** [objtool] Error 2
> > make[1]: *** [tools/objtool] Error 2
> > make: *** [__sub-make] Error 2
> >
> >

Some background: when I added the input check to GENMASK there was
originally a check that disabled the input check if gcc versions earlier
than 4.9, since for old compilers, there was a bug [1] where for some
constructs, __builtin_constant_p() did not evaluate to a constant. This
was supposedly fixed in gcc 4.9, but apparently there are newer gcc
versions which suffer from this too.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449

> >
> > Applying below diff seems to solve the build failure.
>
> It will desynchronize this implementation with the mother's one (i.e.
> in bits.h).
>
> > Do we need to use BUILD_BUG_ON_ZERO() here?
>
> Rikard?

Yes, (as Yury pointed out), GENMASK is used in for example structure
initializers where BUIlD_BUG() can not be used.
>
> >
> > diff --git a/tools/include/linux/bits.h b/tools/include/linux/bits.h
> > index 7f475d59a097..0aba9294f29d 100644
> > --- a/tools/include/linux/bits.h
> > +++ b/tools/include/linux/bits.h
> > @@ -21,8 +21,7 @@
> > #if !defined(__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)))
> > + ({ BUILD_BUG_ON(__builtin_constant_p((l) > (h)) && ((l) > (h))); 0; })
> > #else
> > /*
> > * BUILD_BUG_ON_ZERO is not available in h files included from asm files,

Does the following work for you? For simplicity, I copied__is_constexpr from
include/linux/minmax.h (which isn't available in tools/). A proper patch
would reuse __is_constexpr (possibly refactoring it to a separate
header since bits.h including minmax.h for that only seems smelly) and fix
bits.h in the kernel header as well, to keep the files in sync.

diff --git a/tools/include/linux/bits.h b/tools/include/linux/bits.h
index 7f475d59a097..7bc4c31a7df0 100644
--- a/tools/include/linux/bits.h
+++ b/tools/include/linux/bits.h
@@ -19,10 +19,13 @@
* GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000.
*/
#if !defined(__ASSEMBLY__)
+
+#define __is_constexpr(x) \
+ (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
#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)))
+ __is_constexpr((l) > (h)), (l) > (h), 0)))
#else
/*
* BUILD_BUG_ON_ZERO is not available in h files included from asm files,

I think any solution which results in using __GENMASK to avoid the input
check will just result in similar reports in the future.

Rikard