Re: [PATCH bpf-next 1/3] bpf: support nocsr patterns for calls to kfuncs

[Yonghong Song <yonghong.song@linux.dev>]

On 8/15/24 2:24 PM, Andrii Nakryiko wrote:
> On Mon, Aug 12, 2024 at 4:44 PM Eduard Zingerman <eddyz87@gmail.com> wrote:
>> Recognize nocsr patterns around kfunc calls.
>> For example, suppose bpf_cast_to_kern_ctx() follows nocsr contract
>> (which it does, it is rewritten by verifier as "r0 = r1" insn),
>> in such a case, rewrite BPF program below:
>>
>>    r2 = 1;
>>    *(u64 *)(r10 - 32) = r2;
>>    call %[bpf_cast_to_kern_ctx];
>>    r2 = *(u64 *)(r10 - 32);
>>    r0 = r2;
>>
>> Removing the spill/fill pair:
>>
>>    r2 = 1;
>>    call %[bpf_cast_to_kern_ctx];
>>    r0 = r2;
>>
>> Add a KF_NOCSR flag to mark kfuncs that follow nocsr contract.
>>
>> Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
>> ---
>>   include/linux/btf.h   |  1 +
>>   kernel/bpf/verifier.c | 36 ++++++++++++++++++++++++++++++++++++
>>   2 files changed, 37 insertions(+)
>>
>> diff --git a/include/linux/btf.h b/include/linux/btf.h
>> index cffb43133c68..59ca37300423 100644
>> --- a/include/linux/btf.h
>> +++ b/include/linux/btf.h
>> @@ -75,6 +75,7 @@
>>   #define KF_ITER_NEXT    (1 << 9) /* kfunc implements BPF iter next method */
>>   #define KF_ITER_DESTROY (1 << 10) /* kfunc implements BPF iter destructor */
>>   #define KF_RCU_PROTECTED (1 << 11) /* kfunc should be protected by rcu cs when they are invoked */
>> +#define KF_NOCSR        (1 << 12) /* kfunc follows nocsr calling contract */
>>
>>   /*
>>    * Tag marking a kernel function as a kfunc. This is meant to minimize the
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index df3be12096cf..c579f74be3f9 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -16140,6 +16140,28 @@ static bool verifier_inlines_helper_call(struct bpf_verifier_env *env, s32 imm)
>>          }
>>   }
>>
>> +/* Same as helper_nocsr_clobber_mask() but for kfuncs, see comment above */
>> +static u32 kfunc_nocsr_clobber_mask(struct bpf_kfunc_call_arg_meta *meta)
>> +{
>> +       const struct btf_param *params;
>> +       u32 vlen, i, mask;
>> +
>> +       params = btf_params(meta->func_proto);
>> +       vlen = btf_type_vlen(meta->func_proto);
>> +       mask = 0;
>> +       if (!btf_type_is_void(btf_type_by_id(meta->btf, meta->func_proto->type)))
>> +               mask |= BIT(BPF_REG_0);
>> +       for (i = 0; i < vlen; ++i)
>> +               mask |= BIT(BPF_REG_1 + i);
> Somewhere deep in btf_dump implementation of libbpf, there is a
> special handling of `<whatever> func(void)` (no args) function as
> having vlen == 1 and type being VOID (i.e., zero). I don't know if
> that still can happen, but I believe at some point we could get this
> vlen==1 and type=VOID for no-args functions. So I wonder if we should
> handle that here as well, or is it some compiler atavism we can forget
> about?

The case to have vlen=1 and type=VOID only happens for
bpf programs with llvm19 and later.
For example,

$ cat t.c
int foo(); // a kfunc or a helper
int bar() {
   return foo(1, 2);
}

$ clang --target=bpf -O2 -g -c t.c && llvm-dwarfdump t.o
t.c:3:13: warning: passing arguments to 'foo' without a prototype is deprecated in all versions of C and is not supported in C23 [-Wdeprecated-non-prototype]
     3 |   return foo(1, 2);
       |             ^
1 warning generated.
t.o:    file format elf64-bpf
...
0x00000039:   DW_TAG_subprogram
                 DW_AT_name      ("foo")
                 DW_AT_decl_file ("/home/yhs/t.c")
                 DW_AT_decl_line (1)
                 DW_AT_type      (0x00000043 "int")
                 DW_AT_declaration       (true)
                 DW_AT_external  (true)

0x00000041:     DW_TAG_unspecified_parameters

0x00000042:     NULL
...

If we do see a BPF kfunc/helper with vlen=1 and type is VOID,
that means the number of arguments is actual UNKNOWN
based on dwarf DW_TAG_subprogram tag. Although it is unlikely
people to write code like above, it might be still useful
to add check with vlen=1 and type=VOID and reject such a case.


>
>> +       return mask;
>> +}
>> +
>> +/* Same as verifier_inlines_helper_call() but for kfuncs, see comment above */
>> +static bool verifier_inlines_kfunc_call(struct bpf_kfunc_call_arg_meta *meta)
>> +{
>> +       return false;
>> +}
>> +
>>   /* GCC and LLVM define a no_caller_saved_registers function attribute.
>>    * This attribute means that function scratches only some of
>>    * the caller saved registers defined by ABI.
>> @@ -16238,6 +16260,20 @@ static void mark_nocsr_pattern_for_call(struct bpf_verifier_env *env,
>>                                    bpf_jit_inlines_helper_call(call->imm));
>>          }
>>
>> +       if (bpf_pseudo_kfunc_call(call)) {
>> +               struct bpf_kfunc_call_arg_meta meta;
>> +               int err;
>> +
>> +               err = fetch_kfunc_meta(env, call, &meta, NULL);
>> +               if (err < 0)
>> +                       /* error would be reported later */
>> +                       return;
>> +
>> +               clobbered_regs_mask = kfunc_nocsr_clobber_mask(&meta);
>> +               can_be_inlined = (meta.kfunc_flags & KF_NOCSR) &&
>> +                                verifier_inlines_kfunc_call(&meta);
>> +       }
>> +
>>          if (clobbered_regs_mask == ALL_CALLER_SAVED_REGS)
>>                  return;
>>
>> --
>> 2.45.2
>>