linux-kernel - Re: [PATCH bpf-next v1 03/10] bpf: Verifier support for KF_IMPLICIT

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Message-ID: <aff8eeed-414c-49b3-b7f0-c8c328ed5199@linux.dev>
Date: Tue, 13 Jan 2026 16:03:01 -0800
From: Ihor Solodrai <ihor.solodrai@...ux.dev>
To: Eduard Zingerman <eddyz87@...il.com>, Alexei Starovoitov
 <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Mykyta Yatsenko <yatsenko@...a.com>, Tejun Heo <tj@...nel.org>,
 Alan Maguire <alan.maguire@...cle.com>,
 Benjamin Tissoires <bentiss@...nel.org>, Jiri Kosina <jikos@...nel.org>,
 bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-input@...r.kernel.org, sched-ext@...ts.linux.dev
Subject: Re: [PATCH bpf-next v1 03/10] bpf: Verifier support for
 KF_IMPLICIT_ARGS

On 1/13/26 1:59 PM, Eduard Zingerman wrote:
> On Fri, 2026-01-09 at 10:48 -0800, Ihor Solodrai wrote:
> 
> [...]
> 
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -3271,6 +3271,38 @@ static struct btf *find_kfunc_desc_btf(struct bpf_verifier_env *env, s16 offset)
>>  	return btf_vmlinux ?: ERR_PTR(-ENOENT);
>>  }
>>  
>> +#define KF_IMPL_SUFFIX "_impl"
>> +
>> +static const struct btf_type *find_kfunc_impl_proto(struct bpf_verifier_env *env,
>> +						    struct btf *btf,
>> +						    const char *func_name)
>> +{
>> +	char impl_name[KSYM_SYMBOL_LEN];
> 
> Oh, as we discussed already, this should use env->tmp_str_buf.

The env->tmp_str_buf size is smaller:

	#define TMP_STR_BUF_LEN 320

*And* there is already a local char buffer of size KSYM_SYMBOL_LEN
already in use in verifier.c:

	int bpf_check_attach_target(...) {
		bool prog_extension = prog->type == BPF_PROG_TYPE_EXT;
		bool prog_tracing = prog->type == BPF_PROG_TYPE_TRACING;
		char trace_symbol[KSYM_SYMBOL_LEN];
	[...]

Since these are function names, the real limit is KSYM_SYMBOL_LEN,
right?

Sure >320 chars long kfunc name is unlikely, but technically possible.

> 
>> +	const struct btf_type *func;
>> +	s32 impl_id;
>> +	int len;
>> +
>> +	len = snprintf(impl_name, sizeof(impl_name), "%s%s", func_name, KF_IMPL_SUFFIX);
>> +	if (len < 0 || len >= sizeof(impl_name)) {
>> +		verbose(env, "function name %s%s is too long\n", func_name, KF_IMPL_SUFFIX);
>> +		return NULL;
>> +	}
> 
> [...]