netdev - RE: [PATCH bpf-next 2/3] bpf: Add get_func_[arg|ret|arg

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Message-ID: <61ae66d49f24f_c5bd208bf@john.notmuch>
Date:   Mon, 06 Dec 2021 11:39:00 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: RE: [PATCH bpf-next 2/3] bpf: Add get_func_[arg|ret|arg_cnt] helpers

Jiri Olsa wrote:
> Adding following helpers for tracing programs:
> 
> Get n-th argument of the traced function:
>   long bpf_get_func_arg(void *ctx, u32 n, u64 *value)
> 
> Get return value of the traced function:
>   long bpf_get_func_ret(void *ctx, u64 *value)
> 
> Get arguments count of the traced funtion:
>   long bpf_get_func_arg_cnt(void *ctx)
> 
> The trampoline now stores number of arguments on ctx-8
> address, so it's easy to verify argument index and find
> return value argument's position.
> 
> Moving function ip address on the trampoline stack behind
> the number of functions arguments, so it's now stored on
> ctx-16 address if it's needed.
> 
> All helpers above are inlined by verifier.
> 
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index c26871263f1f..d5a3791071d6 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -4983,6 +4983,31 @@ union bpf_attr {
>   *	Return
>   *		The number of loops performed, **-EINVAL** for invalid **flags**,
>   *		**-E2BIG** if **nr_loops** exceeds the maximum number of loops.
> + *
> + * long bpf_get_func_arg(void *ctx, u32 n, u64 *value)
> + *	Description
> + *		Get **n**-th argument (zero based) of the traced function (for tracing programs)
> + *		returned in **value**.
> + *
> + *	Return
> + *		0 on success.
> + *		**-EINVAL** if n >= arguments count of traced function.
> + *
> + * long bpf_get_func_ret(void *ctx, u64 *value)
> + *	Description
> + *		Get return value of the traced function (for tracing programs)
> + *		in **value**.
> + *
> + *	Return
> + *		0 on success.
> + *		**-EINVAL** for tracing programs other than BPF_TRACE_FEXIT or BPF_MODIFY_RETURN.


Can we just throw a verifier error if the program type doesn't support
this? Then weget a void and ther is no error case.

> + *
> + * long bpf_get_func_arg_cnt(void *ctx)
> + *	Description
> + *		Get number of arguments of the traced function (for tracing programs).
> + *
> + *	Return
> + *		The number of arguments of the traced function.
>   */
>  #define __BPF_FUNC_MAPPER(FN)		\
>  	FN(unspec),			\
> @@ -5167,6 +5192,9 @@ union bpf_attr {
>  	FN(kallsyms_lookup_name),	\
>  	FN(find_vma),			\
>  	FN(loop),			\
> +	FN(get_func_arg),		\
> +	FN(get_func_ret),		\
> +	FN(get_func_arg_cnt),		\
>  	/* */
>  
>  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 6522ffdea487..cf6853d3a8e9 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -12974,6 +12974,7 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env,
>  static int do_misc_fixups(struct bpf_verifier_env *env)
>  {
>  	struct bpf_prog *prog = env->prog;
> +	enum bpf_attach_type eatype = prog->expected_attach_type;
>  	bool expect_blinding = bpf_jit_blinding_enabled(prog);
>  	enum bpf_prog_type prog_type = resolve_prog_type(prog);
>  	struct bpf_insn *insn = prog->insnsi;
> @@ -13344,11 +13345,79 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>  			continue;
>  		}
>  

[...]

> +		/* Implement get_func_arg_cnt inline. */
> +		if (prog_type == BPF_PROG_TYPE_TRACING &&
> +		    insn->imm == BPF_FUNC_get_func_arg_cnt) {
> +			/* Load nr_args from ctx - 8 */
> +			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> +
> +			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, 1);
> +			if (!new_prog)
> +				return -ENOMEM;

How does this handle the !x86 case? The code above only touches the x86
jit? Perhaps its obvious with some code digging, but its not to me from
the patch description and code here.

> +
> +			env->prog = prog = new_prog;
> +			insn      = new_prog->insnsi + i + delta;
> +			continue;
> +		}
> +