Re: [PATCH bpf-next v6 04/17] bpf: Prepare verifier logs for upcoming kfunc stack arguments

Yonghong Song
2026-04-21T15:41:42Z

On 4/21/26 6:48 AM, Alexei Starovoitov wrote:
> On Mon, Apr 20, 2026 at 11:07 PM Yonghong Song <yonghong.song@linux.dev> wrote:
>>
>>
>> On 4/20/26 5:03 PM, Alexei Starovoitov wrote:
>>> On Sun Apr 19, 2026 at 9:33 AM PDT, Yonghong Song wrote:
>>>> This change prepares verifier log reporting for upcoming kfunc stack
>>>> argument support.
>>>>
>>>> Today verifier log code mostly assumes that an argument can be described
>>>> directly by a register number. That works for arguments passed in `R1`
>>>> to `R5`, but it does not work once kfunc arguments can also be
>>>> passed on the stack.
>>>>
>>>> Introduce an internal `argno` representation such that register-passed
>>>> arguments keep using their real register numbers, while stack-passed
>>>> arguments use an encoded value above a dedicated base.
>>>> `reg_arg_name()` converts this representation into either `R%d` or
>>>> `*(R11-off)` when emitting verifier logs. If a particular `argno`
>>>> is corresponding to a stack argument, print `*(R11-off)`. Otherwise,
>>>> print `R%d`. Here R11 presents the base of stack arguments.
>>>>
>>>> This keeps existing logs readable for register arguments and allows the
>>>> same log sites to handle future stack arguments without open-coding
>>>> special cases.
>>>>
>>>> Update selftests accordingly.
>>>>
>>>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>>>> ---
>>>> include/linux/bpf_verifier.h | 1 +
>>>> kernel/bpf/verifier.c | 649 ++++++++++--------
>>>> .../testing/selftests/bpf/prog_tests/bpf_nf.c | 22 +-
>>>> .../selftests/bpf/prog_tests/cb_refs.c | 2 +-
>>>> .../selftests/bpf/prog_tests/kfunc_call.c | 2 +-
>>>> .../selftests/bpf/prog_tests/linked_list.c | 4 +-
>>>> .../selftests/bpf/progs/cgrp_kfunc_failure.c | 14 +-
>>>> .../selftests/bpf/progs/cpumask_failure.c | 10 +-
>>>> .../testing/selftests/bpf/progs/dynptr_fail.c | 22 +-
>>>> .../selftests/bpf/progs/file_reader_fail.c | 4 +-
>>>> tools/testing/selftests/bpf/progs/irq.c | 4 +-
>>>> tools/testing/selftests/bpf/progs/iters.c | 6 +-
>>>> .../selftests/bpf/progs/iters_state_safety.c | 14 +-
>>>> .../selftests/bpf/progs/iters_testmod.c | 4 +-
>>>> .../selftests/bpf/progs/iters_testmod_seq.c | 4 +-
>>>> .../selftests/bpf/progs/map_kptr_fail.c | 2 +-
>>>> .../selftests/bpf/progs/percpu_alloc_fail.c | 4 +-
>>>> .../testing/selftests/bpf/progs/rbtree_fail.c | 6 +-
>>>> .../bpf/progs/refcounted_kptr_fail.c | 2 +-
>>>> .../testing/selftests/bpf/progs/stream_fail.c | 2 +-
>>>> .../selftests/bpf/progs/task_kfunc_failure.c | 18 +-
>>>> .../selftests/bpf/progs/task_work_fail.c | 6 +-
>>>> .../selftests/bpf/progs/test_bpf_nf_fail.c | 8 +-
>>>> .../bpf/progs/test_kfunc_dynptr_param.c | 2 +-
>>>> .../bpf/progs/test_kfunc_param_nullable.c | 2 +-
>>>> .../selftests/bpf/progs/verifier_bits_iter.c | 4 +-
>>>> .../bpf/progs/verifier_ref_tracking.c | 6 +-
>>>> .../selftests/bpf/progs/verifier_vfs_reject.c | 8 +-
>>>> .../testing/selftests/bpf/progs/wq_failures.c | 2 +-
>>>> tools/testing/selftests/bpf/verifier/calls.c | 14 +-
>>>> 30 files changed, 474 insertions(+), 374 deletions(-)
>>>>
>>>> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
>>>> index b148f816f25b..9fbbddc40d21 100644
>>>> --- a/include/linux/bpf_verifier.h
>>>> +++ b/include/linux/bpf_verifier.h
>>>> @@ -913,6 +913,7 @@ struct bpf_verifier_env {
>>>> * e.g., in reg_type_str() to generate reg_type string
>>>> */
>>>> char tmp_str_buf[TMP_STR_BUF_LEN];
>>>> + char tmp_reg_arg_name_buf[32];
>>> the name is too long.
>>> Just tmp_arg_name ?
>>>
>>>> struct bpf_insn insn_buf[INSN_BUF_SIZE];
>>>> struct bpf_insn epilogue_buf[INSN_BUF_SIZE];
>>>> struct bpf_scc_callchain callchain_buf;
>>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>>> index 3716d9688d00..6aa4dc161a56 100644
>>>> --- a/kernel/bpf/verifier.c
>>>> +++ b/kernel/bpf/verifier.c
>>>> @@ -1751,6 +1751,55 @@ static struct bpf_verifier_state *push_stack(struct bpf_verifier_env *env,
>>>> return &elem->st;
>>>> }
>>>>
>>>> +/*
>>>> + * Unified argument number encoding for verifier log messages.
>>>> + * Register args (arg_idx 0-4) use their register number (R1-R5).
>>>> + * Stack args (arg_idx 5+) are encoded as STACK_ARGNO_BASE + arg_idx
>>>> + * to avoid collision with register numbers. reg_arg_name() decodes
>>>> + * this back to a human-readable string like "*(R11-8)" for logs.
>>>> + */
>>>> +#define STACK_ARGNO_BASE 100
>>>> +
>>>> +static bool is_stack_argno(int argno)
>>>> +{
>>>> + return argno >= STACK_ARGNO_BASE;
>>>> +}
>>>> +
>>>> +static u32 make_argno(u32 arg_idx)
>>>> +{
>>>> + if (arg_idx < MAX_BPF_FUNC_REG_ARGS)
>>>> + return BPF_REG_1 + arg_idx;
>>>> + return STACK_ARGNO_BASE + arg_idx;
>>>> +}
>>>> +
>>>> +static u32 arg_idx_from_argno(int argno)
>>>> +{
>>>> + if (is_stack_argno(argno))
>>>> + return argno - STACK_ARGNO_BASE;
>>>> + return argno - BPF_REG_1;
>>>> +}
>>>> +
>>>> +static int next_argno(int argno)
>>>> +{
>>>> + return make_argno(arg_idx_from_argno(argno) + 1);
>>>> +}
>>> I don't like this +1, -1 dance all around. Makes the whole thing
>>> hard to follow.
>>> Keep argno starting at 1. So old regno == argno.
>> The above make_argno() returns an argno having regno == argno
>> for regno 1-5.
> No. It doesn't.
>
> it does:
>
> +static u32 make_argno(u32 arg_idx)
> +{
> + if (arg_idx < MAX_BPF_FUNC_REG_ARGS)
> + return BPF_REG_1 + arg_idx;
>
> so for argno == 0 it returns 1 and that's my objection.
> Make argno starting at 1.

Okay. Will do.