On Thu, 2022-11-10 at 17:34 -0800, sdf@xxxxxxxxxx wrote:
> On 11/10, Stanislav Fomichev wrote:
> > On 11/11, Eduard Zingerman wrote:
> > > A fix for the LLVM compilation error while building bpftool.
> > > Replaces the expression:
> > >
> > > _Static_assert((p) == NULL || ...)
> > >
> > > by expression:
> > >
> > > _Static_assert((__builtin_constant_p((p)) ? (p) == NULL : 0) || ...)
>
> > IIUC, when __builtin_constant_p(p) returns false, we just ignore the NULL
> > check?
> > Do we have cases like that? If no, maybe it's safer to fail?
>
> > s/(p) == NULL : 0/(p) == NULL : 1/ ?
>
> I'm probably missing something, can you pls clarify? So the error is as
> follows:
>
> > > btf_dump.c:1546:2: error: static_assert expression is not an integral
> > > constant expression
> hashmap__find(name_map, orig_name, &dup_cnt);
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ./hashmap.h:169:35: note: expanded from macro 'hashmap__find'
> hashmap_find((map), (long)(key), hashmap_cast_ptr(value))
> ^~~~~~~~~~~~~~~~~~~~~~~
> ./hashmap.h:126:17: note: expanded from macro 'hashmap_cast_ptr'
> _Static_assert((p) == NULL || sizeof(*(p)) ==
> sizeof(long),
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> btf_dump.c:1546:2: note: cast from 'void *' is not allowed in a constant
> expression
> ./hashmap.h:169:35: note: expanded from macro 'hashmap__find'
> hashmap_find((map), (long)(key), hashmap_cast_ptr(value))
>
> This line in particular:
>
> btf_dump.c:1546:2: note: cast from 'void *' is not allowed in a constant
> expression
>
> And the code does:
>
> size_t dup_cnt = 0;
> hashmap__find(name_map, orig_name, &dup_cnt);
>
> So where is that cast from 'void *' is happening? Is it the NULL check
> itself?
>
> Are we simply guarding against the user calling hashmap_cast_ptr with
> explicit NULL argument?
In case if (p) is not a constant I want the second part of the || to kick-in.
The complete condition looks as follows:
_Static_assert((__builtin_constant_p((p)) ? (p) == NULL : 0) || \
sizeof(*(p)) == sizeof(long), "...error...")
The intent is to check that (p) is either NULL or a pointer to
something of size long. So, if (p) is not a constant the expression
would be equivalent to:
_Static_assert(0 || sizeof(*(p)) == sizeof(long), "...error...")
I just tried the following:
struct hashmap *name_map;
char x; // not a constant, wrong pointer size
...
hashmap__find(name_map, orig_name, &x);
And it fails with an error message as intended:
btf_dump.c:1548:2: error: static_assert failed due to requirement '(__builtin_constant_p((&x)) ? (&x) == ((void *)0) : 0) || sizeof (*(&x)) == sizeof(long)' "&x pointee should be a long-sized integer or a pointer"
hashmap__find(name_map, orig_name, &x);
./hashmap.h:170:35: note: expanded from macro 'hashmap__find'
hashmap_find((map), (long)(key), hashmap_cast_ptr(value))
./hashmap.h:126:2: note: expanded from macro 'hashmap_cast_ptr'
_Static_assert((__builtin_constant_p((p)) ? (p) == NULL : 0) ||
>
> > > When "p" is not a constant the former is not considered to be a
> > > constant expression by LLVM 14.
> > >
> > > The error was introduced in the following patch-set: [1].
> > > The error was reported here: [2].
> > >
> > > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > > Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
> > >
> > > [1]
> > https://lore.kernel.org/bpf/20221109142611.879983-1-eddyz87@xxxxxxxxx/
> > > [2] https://lore.kernel.org/all/202211110355.BcGcbZxP-lkp@xxxxxxxxx/
> > > ---
> > > tools/lib/bpf/hashmap.h | 3 ++-
> > > tools/perf/util/hashmap.h | 3 ++-
> > > 2 files changed, 4 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/tools/lib/bpf/hashmap.h b/tools/lib/bpf/hashmap.h
> > > index 3fe647477bad..0a5bf1937a7c 100644
> > > --- a/tools/lib/bpf/hashmap.h
> > > +++ b/tools/lib/bpf/hashmap.h
> > > @@ -123,7 +123,8 @@ enum hashmap_insert_strategy {
> > > };
> > >
> > > #define hashmap_cast_ptr(p) ({ \
> > > - _Static_assert((p) == NULL || sizeof(*(p)) == sizeof(long), \
> > > + _Static_assert((__builtin_constant_p((p)) ? (p) == NULL : 0) || \
> > > + sizeof(*(p)) == sizeof(long), \
> > > #p " pointee should be a long-sized integer or a pointer"); \
> > > (long *)(p); \
> > > })
> > > diff --git a/tools/perf/util/hashmap.h b/tools/perf/util/hashmap.h
> > > index 3fe647477bad..0a5bf1937a7c 100644
> > > --- a/tools/perf/util/hashmap.h
> > > +++ b/tools/perf/util/hashmap.h
> > > @@ -123,7 +123,8 @@ enum hashmap_insert_strategy {
> > > };
> > >
> > > #define hashmap_cast_ptr(p) ({ \
> > > - _Static_assert((p) == NULL || sizeof(*(p)) == sizeof(long), \
> > > + _Static_assert((__builtin_constant_p((p)) ? (p) == NULL : 0) || \
> > > + sizeof(*(p)) == sizeof(long), \
> > > #p " pointee should be a long-sized integer or a pointer"); \
> > > (long *)(p); \
> > > })
> > > --
> > > 2.34.1
> > >
