> On 4/28/24 4:25 AM, Jose E. Marchesi 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.
>
> It would be great if you can share details with asm code and
> BTF so we can understand better. I am not 100% sure about
> whether __builtin_has_attribute builtin can help to do
> run-time ksym resolution with libbpf.
Hi Yonghong.
I am a bit confused. Is the _Static_assert supposed to contribute
anything to the generated code?
This is what GCC generates for pass_handler:
-----
pass_handler:
.LFB1:
r2 = 0
r1 = runqueues ll
call 153
if r0 == 0 goto .L2
r1 = runqueues ll
if r1 == 0 goto .L2
r2 = out__existing_typed ll
r0 = *(u32 *) (r0+2920)
*(u32 *) (r2+0) = r0
.L2:
r6 = out__non_existent_typed ll
r1 = bpf_link_fops2 ll
r3 = out__existing_typeless ll
r4 = bpf_prog_active ll
r5 = out__non_existent_typeless ll
r9 = bpf_link_fops1 ll
*(u64 *) (r3+0) = r4
*(u64 *) (r5+0) = r9
*(u64 *) (r6+0) = r1
if r1 == 0 goto .L3
r2 = 0
call 153
*(u64 *) (r6+0) = r0
.L3:
r1 = bpf_task_acquire ll
if r1 == 0 goto .L20
.L4:
r1 = bpf_testmod_test_mod_kfunc ll
if r1 == 0 goto .L21
.L5:
r1 = invalid_kfunc ll
if r1 == 0 goto .L6
call invalid_kfunc
.L6:
r0 = 0
exit
.L21:
call bpf_testmod_test_mod_kfunc
goto .L5
.L20:
call bpf_task_acquire
goto .L4
.LFE1:
.size pass_handler, .-pass_handler
-----
And the .ksyms datasec:
-----
[7693] DATASEC '.ksyms' size=0 vlen=7
type_id=7690 offset=0 size=0 (FUNC 'invalid_kfunc')
type_id=7691 offset=0 size=0 (FUNC 'bpf_testmod_test_mod_kfunc')
type_id=7692 offset=0 size=0 (FUNC 'bpf_task_acquire')
type_id=7530 offset=0 size=4 (VAR 'bpf_link_fops2')
type_id=7550 offset=0 size=1 (VAR 'bpf_link_fops1')
type_id=7475 offset=0 size=1 (VAR 'bpf_prog_active')
type_id=7535 offset=0 size=3456 (VAR 'runqueues')
-----
Is the entry for runqueues en the datasec enough for libbpf to patch the
ksym value in the corresponding `r1 = runqueues ll' instructions?
>
> The following is what clang does:
>
> For example, for progs/test_ksyms_weak.c, we have
> 43 if (rq && bpf_ksym_exists(&runqueues))
> 44 out__existing_typed = rq->cpu;
> ...
> 56 if (!bpf_ksym_exists(bpf_task_acquire))
> 57 /* dead code won't be seen by the verifier */
> 58 bpf_task_acquire(0);
>
> The asm code:
>
> .loc 0 42 20 prologue_end # progs/test_ksyms_weak.c:42:20
> .Ltmp0:
> r6 = runqueues ll
> r1 = runqueues ll
> w2 = 0
> call 153
> .Ltmp1:
> .Ltmp2:
> #DEBUG_VALUE: pass_handler:rq <- $r0
> .loc 0 43 9 # progs/test_ksyms_weak.c:43:9
> .Ltmp3:
> if r0 == 0 goto LBB0_3
> .Ltmp4:
> .Ltmp5:
> # %bb.1: # %entry
> #DEBUG_VALUE: pass_handler:rq <- $r0
> if r6 == 0 goto LBB0_3
> ...
> LBB0_5: # %if.end4
> .loc 0 56 6 is_stmt 1 # progs/test_ksyms_weak.c:56:6
> .Ltmp25:
> r1 = bpf_task_acquire ll
> if r1 != 0 goto LBB0_7
> # %bb.6: # %if.then9
>
> Here, 'runqueues' and 'bpf_task_acquire' will be changed by libbpf
> based on the *current* kernel state. The BTF datasec encodes such ksym
> information like below which will be used by libbpf:
>
> .long 13079 # BTF_KIND_DATASEC(id = 395)
> .long 251658247 # 0xf000007
> .long 0
> .long 377
> .long bpf_task_acquire
> .long 0
> .long 379
> .long bpf_testmod_test_mod_kfunc
> .long 0
> .long 381
> .long invalid_kfunc
> .long 0
> .long 387
> .long runqueues
> .long 3264
> .long 388
> .long bpf_prog_active
> .long 1
> .long 389
> .long bpf_link_fops1
> .long 1
> .long 391
> .long bpf_link_fops2
> .long 4
>
> What gcc generates for the above example? It would be great
> if this can be put in the commit message.
>
>>
>> Locally tested in bpf-next master branch.
>> No regressions.
>>
>> Signed-of-by: Jose E. Marchesi <jose.marchesi@xxxxxxxxxx>
>> Cc: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
>> Cc: david.faust@xxxxxxxxxx
>> Cc: cupertino.miranda@xxxxxxxxxx
>> ---
>> 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; \
>> +})
>> +#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")))
