Re: [PATCH bpf-next 7/9] bpf: Prepare verifier logs for upcoming kfunc stack arguments

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Tue Apr 21, 2026 at 10:20 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.
>
> Acked-by: Puranjay Mohan <puranjay@kernel.org>
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> ---
>  include/linux/bpf_verifier.h                  |   1 +
>  kernel/bpf/verifier.c                         | 640 ++++++++++--------
>  .../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, 464 insertions(+), 375 deletions(-)
>
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index b148f816f25b..d5b4303315dd 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_arg_name[32];
>  	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 18ab92581452..82568a427211 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1742,6 +1742,44 @@ static struct bpf_verifier_state *push_stack(struct bpf_verifier_env *env,
>  	return &elem->st;
>  }
>  
> +#define STACK_ARGNO_BASE 100
> +
> +static bool is_stack_argno(int argno)
> +{
> +	return argno > STACK_ARGNO_BASE;
> +}
> +
> +/* arg starts at 1 */
> +static u32 make_argno(u32 arg)
> +{
> +	if (arg <= MAX_BPF_FUNC_REG_ARGS)
> +		return arg;
> +	return STACK_ARGNO_BASE + arg;
> +}

You can remove this and simplify everything further by

static bool is_stack_argno(int argno)
{
	return argno > MAX_BPF_FUNC_REG_ARGS;
}

> +
> +static u32 arg_from_argno(int argno)
> +{
> +	if (is_stack_argno(argno))
> +		return argno - STACK_ARGNO_BASE;
> +	return argno;
> +}

remove as well.

and a comment like:

/*
 * switch (argno) {
 * case 1: R1
 * case 5: R5
 * case 6: *(u64 *)(R11 +- 8)
 * case 7: *(u64 *)(R11 +- 16)
 */

> +static const char *reg_arg_name(struct bpf_verifier_env *env, int argno)
> +{
> +	char *buf = env->tmp_arg_name;
> +	int len = sizeof(env->tmp_arg_name);
> +	u32 arg;
> +
> +	if (!is_stack_argno(argno)) {
> +		snprintf(buf, len, "R%d", argno);
> +		return buf;
> +	}
> +
> +	arg = arg_from_argno(argno);

gone

> +	snprintf(buf, len, "*(R11-%u)", (arg - MAX_BPF_FUNC_REG_ARGS) * BPF_REG_SIZE);
> +	return buf;
> +}