linux-kernel - Re: [PATCH bpf-next v7 2/3] bpf: clean-up bounds checking in __bpf_get

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Message-ID: <05b2c226-9a09-4541-a18c-8a21898846d0@kernel.org>
Date: Thu, 4 Sep 2025 15:40:43 -0700
From: Song Liu <song@...nel.org>
To: Arnaud Lecomte <contact@...aud-lcm.com>, alexei.starovoitov@...il.com,
 yonghong.song@...ux.dev
Cc: andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
 daniel@...earbox.net, eddyz87@...il.com, haoluo@...gle.com,
 john.fastabend@...il.com, jolsa@...nel.org, kpsingh@...nel.org,
 linux-kernel@...r.kernel.org, martin.lau@...ux.dev, sdf@...ichev.me,
 syzbot+c9b724fbb41cf2538b7b@...kaller.appspotmail.com,
 syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH bpf-next v7 2/3] bpf: clean-up bounds checking in
 __bpf_get_stack


On 9/3/25 4:40 PM, Arnaud Lecomte wrote:
> Clean-up bounds checking for trace->nr in
> __bpf_get_stack by limiting it only to
> max_depth.
>
> Signed-off-by: Arnaud Lecomte <contact@...aud-lcm.com>
> Cc: Song Lui <song@...nel.org>

Typo in my name, which is "Song Liu".

This looks right.

Acked-by: Song Liu <song@...nel.org>

> ---
>   kernel/bpf/stackmap.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index ed707bc07173..9f3ae426ddc3 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -462,13 +462,15 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>   	if (may_fault)
>   		rcu_read_lock(); /* need RCU for perf's callchain below */
>   
> -	if (trace_in)
> +	if (trace_in) {
>   		trace = trace_in;
> -	else if (kernel && task)
> +		trace->nr = min_t(u32, trace->nr, max_depth);
> +	} else if (kernel && task) {
>   		trace = get_callchain_entry_for_task(task, max_depth);
> -	else
> +	} else {
>   		trace = get_perf_callchain(regs, 0, kernel, user, max_depth,
>   					   crosstask, false);
> +	}
>   
>   	if (unlikely(!trace) || trace->nr < skip) {
>   		if (may_fault)
> @@ -477,7 +479,6 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>   	}
>   
>   	trace_nr = trace->nr - skip;
> -	trace_nr = (trace_nr <= num_elem) ? trace_nr : num_elem;
>   	copy_len = trace_nr * elem_size;
>   
>   	ips = trace->ip + skip;