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

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Message-ID: <5027595d4eff50d423af8ebc5fecd6a0f7229d60.camel@gmail.com>
Date: Tue, 13 Jan 2026 17:06:02 -0800
From: Eduard Zingerman <eddyz87@...il.com>
To: Ihor Solodrai <ihor.solodrai@...ux.dev>, 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 Tue, 2026-01-13 at 16:03 -0800, Ihor Solodrai wrote:
> 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.

320 is good enough, you'll be able to cover this:

kfunc_trace_long_descriptive_kernel_symbol_for_tracing_scheduler_memory_io_and_interrupt_paths_during_runtime_analysis_of_latency_throughput_and_resource_contention_on_large_scale_multiprocessor_linux_systems_using_bpf_and_kprobes_without_requiring_kernel_recompilation_or_system_restart_for_production_use_cases_v2x

But not this:

kfunc_trace_kernel_scheduler_and_memory_management_path_for_observing_task_lifecycle_events_context_switches_page_fault_handling_and_io_wait_states_while_debugging_performance_regressions_on_large_multiprocessor_systems_running_preemptible_linux_kernels_with_bpf_tracing_and_dynamic_instrumentation_enabled_for_deep_visibility_into_runtime_behavior_and_latency_sensitive_code_paths_without_recompilation.

Should suffice, I think.