linux-kernel - Re: [PATCH bpf-next v3] bpf: make the attach target more accurate

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Message-ID: <CAEf4BzZZRk0Ko64wy5E34wAa3psk07UhGg9DENU-CQYfLwT1ig@mail.gmail.com>
Date: Thu, 10 Jul 2025 16:24:58 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com, 
	andrii@...nel.org, martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org, 
	yonghong.song@...ux.dev, kpsingh@...nel.org, sdf@...ichev.me, 
	haoluo@...gle.com, jolsa@...nel.org, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Menglong Dong <dongml2@...natelecom.cn>
Subject: Re: [PATCH bpf-next v3] bpf: make the attach target more accurate

On Thu, Jul 10, 2025 at 12:10 AM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> For now, we lookup the address of the attach target in
> bpf_check_attach_target() with find_kallsyms_symbol_value or
> kallsyms_lookup_name, which is not accurate in some cases.
>
> For example, we want to attach to the target "t_next", but there are
> multiple symbols with the name "t_next" exist in the kallsyms, which makes
> the attach target ambiguous, and the attach should fail.
>
> Introduce the function bpf_lookup_attach_addr() to do the address lookup,
> which will return -EADDRNOTAVAIL when the symbol is not unique.
>
> We can do the testing with following shell:
>
> for s in $(cat /proc/kallsyms | awk '{print $3}' | sort | uniq -d)
> do
>   if grep -q "^$s\$" /sys/kernel/debug/tracing/available_filter_functions
>   then
>     bpftrace -e "fentry:$s {printf(\"1\");}" -v
>   fi
> done
>
> The script will find all the duplicated symbols in /proc/kallsyms, which
> is also in /sys/kernel/debug/tracing/available_filter_functions, and
> attach them with bpftrace.
>
> After this patch, all the attaching fail with the error:
>
> The address of function xxx cannot be found
> or
> No BTF found for xxx
>
> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> ---
> v3:
> - reject all the duplicated symbols
> v2:
> - Lookup both vmlinux and modules symbols when mod is NULL, just like
>   kallsyms_lookup_name().
>
>   If the btf is not a modules, shouldn't we lookup on the vmlinux only?
>   I'm not sure if we should keep the same logic with
>   kallsyms_lookup_name().
>
> - Return the kernel symbol that don't have ftrace location if the symbols
>   with ftrace location are not available
> ---
>  kernel/bpf/verifier.c | 71 ++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 66 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 53007182b46b..bf4951154605 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -23476,6 +23476,67 @@ static int check_non_sleepable_error_inject(u32 btf_id)
>         return btf_id_set_contains(&btf_non_sleepable_error_inject, btf_id);
>  }
>
> +struct symbol_lookup_ctx {
> +       const char *name;
> +       unsigned long addr;
> +};
> +
> +static int symbol_callback(void *data, unsigned long addr)
> +{
> +       struct symbol_lookup_ctx *ctx = data;
> +
> +       if (ctx->addr)
> +               return -EADDRNOTAVAIL;

#define ENOTUNIQ        76     /* Name not unique on network */

fits a bit better, no?

> +       ctx->addr = addr;
> +
> +       return 0;
> +}
> +

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