Re: [PATCH bpf-next] bpf: fix bpf_ksym_exists in GCC

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> On Sun, Apr 28, 2024 at 4:26 AM Jose E. Marchesi
> <jose.marchesi@oracle.com> wrote:
>>
>> The macro bpf_ksym_exists is defined in bpf_helpers.h as:
>>
>>   #define bpf_ksym_exists(sym) ({                                                               \
>>         _Static_assert(!__builtin_constant_p(!!sym), #sym " should be marked as __weak");       \
>>         !!sym;                                                                                  \
>>   })
>>
>> The purpose of the macro is to determine whether a given symbol has
>> been defined, given the address of the object associated with the
>> symbol.  It also has a compile-time check to make sure the object
>> whose address is passed to the macro has been declared as weak, which
>> makes the check on `sym' meaningful.
>>
>> As it happens, the check for weak doesn't work in GCC in all cases,
>> because __builtin_constant_p not always folds at parse time when
>> optimizing.  This is because optimizations that happen later in the
>> compilation process, like inlining, may make a previously non-constant
>> expression a constant.  This results in errors like the following when
>> building the selftests with GCC:
>>
>>   bpf_helpers.h:190:24: error: expression in static assertion is not constant
>>   190 |         _Static_assert(!__builtin_constant_p(!!sym), #sym " should be marked as __weak");       \
>>       |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> Fortunately recent versions of GCC support a __builtin_has_attribute
>> that can be used to directly check for the __weak__ attribute.  This
>> patch changes bpf_helpers.h to use that builtin when building with a
>> recent enough GCC, and to omit the check if GCC is too old to support
>> the builtin.
>>
>> The macro used for GCC becomes:
>>
>>   #define bpf_ksym_exists(sym) ({                                                                       \
>>         _Static_assert(__builtin_has_attribute (*sym, __weak__), #sym " should be marked as __weak");   \
>>         !!sym;                                                                                          \
>>   })
>>
>> Note that since bpf_ksym_exists is designed to get the address of the
>> object associated with symbol SYM, we pass *sym to
>> __builtin_has_attribute instead of sym.  When an expression is passed
>> to __builtin_has_attribute then it is the type of the passed
>> expression that is checked for the specified attribute.  The
>> expression itself is not evaluated.  This accommodates well with the
>> existing usages of the macro:
>>
>> - For function objects:
>>
>>   struct task_struct *bpf_task_acquire(struct task_struct *p) __ksym __weak;
>>   [...]
>>   bpf_ksym_exists(bpf_task_acquire)
>>
>> - For variable objects:
>>
>>   extern const struct rq runqueues __ksym __weak; /* typed */
>>   [...]
>>   bpf_ksym_exists(&runqueues)
>>
>> Note also that BPF support was added in GCC 10 and support for
>> __builtin_has_attribute in GCC 9.
>>
>> Locally tested in bpf-next master branch.
>> No regressions.
>>
>> Signed-of-by: Jose E. Marchesi <jose.marchesi@oracle.com>
>> Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
>> Cc: david.faust@oracle.com
>> Cc: cupertino.miranda@oracle.com
>> ---
>>  tools/lib/bpf/bpf_helpers.h | 9 +++++++++
>>  1 file changed, 9 insertions(+)
>>
>> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
>> index 62e1c0cc4a59..a720636a87d9 100644
>> --- a/tools/lib/bpf/bpf_helpers.h
>> +++ b/tools/lib/bpf/bpf_helpers.h
>> @@ -186,10 +186,19 @@ enum libbpf_tristate {
>>  #define __kptr __attribute__((btf_type_tag("kptr")))
>>  #define __percpu_kptr __attribute__((btf_type_tag("percpu_kptr")))
>>
>> +#if defined (__clang__)
>>  #define bpf_ksym_exists(sym) ({                                                                        \
>>         _Static_assert(!__builtin_constant_p(!!sym), #sym " should be marked as __weak");       \
>>         !!sym;                                                                                  \
>>  })
>> +#elif __GNUC__ > 8
>> +#define bpf_ksym_exists(sym) ({                                                                        \
>> +       _Static_assert(__builtin_has_attribute (*sym, __weak__), #sym " should be marked as __weak");   \
>> +       !!sym;                                                                                          \
>> +})
>
> I wrapped _Static_assert() to keep it under 100 characters (and fix
> one unaligned '\' while at it). Also, the patch prefix should be
> "libbpf: " as this is purely a libbpf header. Applied to bpf-next,
> thanks.

Thank you.
Sorry for the wrong prefix and for the style fixes.

>
>> +#else
>> +#define bpf_ksym_exists(sym) !!sym
>> +#endif
>>
>>  #define __arg_ctx __attribute__((btf_decl_tag("arg:ctx")))
>>  #define __arg_nonnull __attribute((btf_decl_tag("arg:nonnull")))
>> --
>> 2.30.2
>>
>>