linux-kernel - Re: [PATCH] bpf/btf: Fix is_int

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Message-ID: <1e01ab5a-c171-0b7a-751a-9ba7da4cd5dd@linux.dev>
Date:   Wed, 12 Oct 2022 17:50:33 -0700
From:   Martin KaFai Lau <martin.lau@...ux.dev>
To:     Chengming Zhou <zhouchengming@...edance.com>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, song@...nel.org,
        yhs@...com, john.fastabend@...il.com, kpsingh@...nel.org,
        sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org
Subject: Re: [PATCH] bpf/btf: Fix is_int_ptr()

On 10/12/22 5:58 AM, Chengming Zhou wrote:
> When tracing a kernel function with arg type is u32*, btf_ctx_access()
> would report error: arg2 type INT is not a struct.
> 
> The commit bb6728d75611 ("bpf: Allow access to int pointer arguments
> in tracing programs") added support for int pointer, but don't skip
> modifiers before checking it's type. This patch fixes it.

A selftest is needed.  You can refer to the selftest added in the patch set [0] 
of the commit bb6728d75611.

This belongs to bpf-next.  Please tag it as bpf-next and also v2 in the next 
revision:
Documentation/bpf/bpf_devel_QA.rst  (Q: How do I indicate which tree....)

[0]: https://lore.kernel.org/bpf/20211208193245.172141-2-jolsa@kernel.org/

> 
> Fixes: bb6728d75611 ("bpf: Allow access to int pointer arguments in tracing programs")
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> ---
>   kernel/bpf/btf.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index eba603cec2c5..2b343c42ed10 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -5316,8 +5316,8 @@ static bool is_int_ptr(struct btf *btf, const struct btf_type *t)
>   	/* t comes in already as a pointer */
>   	t = btf_type_by_id(btf, t->type);
>   
> -	/* allow const */
> -	if (BTF_INFO_KIND(t->info) == BTF_KIND_CONST)
> +	/* skip modifiers */
> +	while (btf_type_is_modifier(t))

There is btf_type_skip_modifiers() that should be useful here.

>   		t = btf_type_by_id(btf, t->type);
>   
>   	return btf_type_is_int(t);