linux-kernel - Re: [PATCH] libbpf: resolve kernel function name optimization for kprobe

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Date:   Mon, 9 Jan 2023 12:29:12 -0800
From:   Yonghong Song <yhs@...a.com>
To:     menglong8.dong@...il.com, daniel@...earbox.net
Cc:     ast@...nel.org, andrii@...nel.org, martin.lau@...ux.dev,
        song@...nel.org, yhs@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com,
        jolsa@...nel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, Menglong Dong <imagedong@...cent.com>
Subject: Re: [PATCH] libbpf: resolve kernel function name optimization for
 kprobe



On 1/9/23 1:42 AM, menglong8.dong@...il.com wrote:
> From: Menglong Dong <imagedong@...cent.com>
> 
> The function name in kernel may be changed by the compiler. For example,
> the function 'ip_rcv_core' can be compiled to 'ip_rcv_core.isra.0'.
> 
> This kind optimization can happen in any kernel function. Therefor, we
> should conside this case.
> 
> If we failed to attach kprobe with a '-ENOENT', then we can lookup the
> kallsyms and check if there is a similar function end with '.xxx', and
> retry.

This might produce incorrect result, so this approach won't work
for all .isra.0 cases. When a function name is changed from
<func> to <func>.isra.<num>, it is possible that compiler may have
make some changes to the arguments, e.g., removing one argument,
chaning a semantics of argument, etc. if bpf program still
uses the original function signature, the bpf program may
produce unexpected result.

> 
> Signed-off-by: Menglong Dong <imagedong@...cent.com>
> ---
>   tools/lib/bpf/libbpf.c | 37 ++++++++++++++++++++++++++++++++++++-
>   1 file changed, 36 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index a5c67a3c93c5..fdfb1ca34ced 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -10375,12 +10375,30 @@ bpf_program__attach_kprobe_multi_opts(const struct bpf_program *prog,
>   	return libbpf_err_ptr(err);
>   }
>   
> +struct kprobe_resolve {
> +	char pattern[128];
> +	char name[128];
> +};
> +
> +static int kprobe_kallsyms_cb(unsigned long long sym_addr, char sym_type,
> +			      const char *sym_name, void *ctx)
> +{
> +	struct kprobe_resolve *res = ctx;
> +
> +	if (!glob_match(sym_name, res->pattern))
> +		return 0;
> +	strcpy(res->name, sym_name);
> +	return 1;
> +}
> +
>   static int attach_kprobe(const struct bpf_program *prog, long cookie, struct bpf_link **link)
>   {
>   	DECLARE_LIBBPF_OPTS(bpf_kprobe_opts, opts);
> +	struct kprobe_resolve res = {};
>   	unsigned long offset = 0;
>   	const char *func_name;
>   	char *func;
> +	int err;
>   	int n;
>   
>   	*link = NULL;
> @@ -10408,8 +10426,25 @@ static int attach_kprobe(const struct bpf_program *prog, long cookie, struct bpf
>   
>   	opts.offset = offset;
>   	*link = bpf_program__attach_kprobe_opts(prog, func, &opts);
> +	err = libbpf_get_error(*link);
> +
> +	if (!err || err != -ENOENT)
> +		goto out;
> +
> +	sprintf(res.pattern, "%s.*", func);
> +	if (!libbpf_kallsyms_parse(kprobe_kallsyms_cb, &res))
> +		goto out;
> +
> +	pr_warn("prog '%s': trying to create %s '%s+0x%zx' perf event instead\n",
> +		prog->name, opts.retprobe ? "kretprobe" : "kprobe",
> +		res.name, offset);
> +
> +	*link = bpf_program__attach_kprobe_opts(prog, res.name, &opts);
> +	err = libbpf_get_error(*link);
> +
> +out:
>   	free(func);
> -	return libbpf_get_error(*link);
> +	return err;
>   }
>   
>   static int attach_ksyscall(const struct bpf_program *prog, long cookie, struct bpf_link **link)