Subject: Re: [PATCH bpf-next v2 4/6] bpf: Disallow !call_get_func_ip progs tail-calling call_get_func_ip progs

From: Leon Hwang <[email-masked]>
Date: Tue, 3 Mar 2026 09:47:39 +0800

On 2/3/26 23:53, [email-masked] wrote:
>> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
>> --- a/kernel/bpf/core.c
>> +++ b/kernel/bpf/core.c
> 
> [ ... ]
> 
>> @@ -2422,11 +2423,17 @@ static bool bpf_map_owner_matches(const struct bpf_map *map, const struct bpf_prog *fp,
>>  	case BPF_MAP_OWNER_MATCH_FOR_INIT:
>>  		if (owner->kprobe_write_ctx != aux->kprobe_write_ctx)
>>  			return false;
>> +		if (owner->call_get_func_ip != fp->call_get_func_ip)
>> +			return false;
>>  		break;
>>
>>  	case BPF_MAP_OWNER_MATCH_FOR_UPDATE:
>>  		if (!owner->kprobe_write_ctx && aux->kprobe_write_ctx)
>>  			return false;
>> +		if (bpf_prog_has_trampoline(fp)) {
>> +			if (!owner->call_get_func_ip && fp->call_get_func_ip)
>> +				return false;
>> +		}
>>  		break;
> 
> The FOR_UPDATE path correctly guards the call_get_func_ip check with
> bpf_prog_has_trampoline(fp), since only trampoline-based programs
> rely on the func IP stored on the trampoline stack. Kprobe programs
> get func_ip from kprobe_running()->addr via bpf_get_func_ip_kprobe(),
> which is independent of the trampoline.
> 
> Should the FOR_INIT path have the same bpf_prog_has_trampoline()
> guard? Without it, two kprobe programs that share a prog_array will
> be rejected if one calls bpf_get_func_ip() and the other does not,
> even though mixing is safe for kprobes.
> 
> bpf_check_tail_call() calls this with BPF_MAP_OWNER_MATCH_FOR_INIT:
> 
>     kprobe prog_a (call_get_func_ip=1) loads, references jmp_table
>       -> owner->call_get_func_ip = 1
>     kprobe prog_b (call_get_func_ip=0) loads, references same jmp_table
>       -> owner->call_get_func_ip != fp->call_get_func_ip
>       -> returns false, prog_b fails to load
> 
> Something like this would match the FOR_UPDATE scoping:
> 
>     case BPF_MAP_OWNER_MATCH_FOR_INIT:
>             if (owner->kprobe_write_ctx != aux->kprobe_write_ctx)
>                     return false;
>             if (bpf_prog_has_trampoline(fp)) {
>                     if (owner->call_get_func_ip != fp->call_get_func_ip)
>                             return false;
>             }
>             break;
> 

Correct.

The FOR_INIT path should have the same bpf_prog_has_trampoline() guard.

I'll do it in the next revision.

Thanks,
Leon

> 
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> 
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/22582690876
> 
> AI-authorship-score: medium
> AI-authorship-explanation: The v1 selftests carried Assisted-by Codex tags, and the kernel code follows a mechanical copy-paste pattern across commits; the v2 selftests were rewritten but the overall series structure suggests AI-assisted development.
> issues-found: 1
> issue-severity-score: low
> issue-severity-explanation: The FOR_INIT strict equality check on call_get_func_ip is missing a bpf_prog_has_trampoline() guard, causing unnecessary rejection of kprobe programs that share a prog_array with different call_get_func_ip values.