From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from plesk.hostmyservers.fr (plesk.hostmyservers.fr [45.145.164.37]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6A9C02FE58F; Mon, 25 Aug 2025 16:39:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=45.145.164.37 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756139982; cv=none; b=B+YUKoCDrs+AEpuzClAz4TicAWeNycm/JWsiLqF70/WqXM13ipiG+XsoHrzdMSj/0P2AHmq2h/xXJK+Cj/n1GBewt9zwlHNUk5VbxC8qVd6w3VVboiwNSgioNS6OEwvpM6+EQWtaDS/1G5cJN7snPU5thN6ImggbcFr+y0g70sk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756139982; c=relaxed/simple; bh=F3qCyhsjaviup5ygBJcuz8aUOr/seEJq9d+yePUzDP8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=mrhdcs23adwLS2rBlVyq7A1bUVcljpGqKoi73v3AyWED0lGb+Mx47mWFYoVmdoU8ztpyQrIpmcaun38sRTjUhOm/gxtZZWRfoYdoIr5HVvmP+2pEVQ3gWH90U8xIrpRy+OQ4LCbUvhsVc0cJ5qAKpxVqTjDjpZNra9lKL9/WNm8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=arnaud-lcm.com; spf=pass smtp.mailfrom=arnaud-lcm.com; arc=none smtp.client-ip=45.145.164.37 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=arnaud-lcm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arnaud-lcm.com Received: from [10.17.113.181] (unknown [15.248.3.85]) by plesk.hostmyservers.fr (Postfix) with ESMTPSA id 81D85413A7; Mon, 25 Aug 2025 16:39:29 +0000 (UTC) Authentication-Results: Plesk; spf=pass (sender IP is 15.248.3.85) smtp.mailfrom=contact@arnaud-lcm.com smtp.helo=[10.17.113.181] Received-SPF: pass (Plesk: connection is authenticated) Message-ID: Date: Mon, 25 Aug 2025 17:39:28 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH bpf-next RESEND v4 1/2] bpf: refactor max_depth computation in bpf_get_stack() To: Martin KaFai Lau , yonghong.song@linux.dev Cc: andrii@kernel.org, ast@kernel.org, bpf@vger.kernel.org, daniel@iogearbox.net, eddyz87@gmail.com, haoluo@google.com, john.fastabend@gmail.com, jolsa@kernel.org, kpsingh@kernel.org, linux-kernel@vger.kernel.org, sdf@fomichev.me, syzbot+c9b724fbb41cf2538b7b@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com, song@kernel.org References: <20250819162652.8776-1-contact@arnaud-lcm.com> <3d8fe484-2889-4367-9405-91aeee7d2ef0@linux.dev> Content-Language: en-US From: "Lecomte, Arnaud" In-Reply-To: <3d8fe484-2889-4367-9405-91aeee7d2ef0@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-PPP-Message-ID: <175613997037.23104.10940002931951082170@Plesk> X-PPP-Vhost: arnaud-lcm.com On 19/08/2025 22:15, Martin KaFai Lau wrote: > On 8/19/25 9:26 AM, Arnaud Lecomte wrote: >> A new helper function stack_map_calculate_max_depth() that >> computes the max depth for a stackmap. >> >> Changes in v2: >>   - Removed the checking 'map_size % map_elem_size' from >>     stack_map_calculate_max_depth >>   - Changed stack_map_calculate_max_depth params name to be more generic >> >> Changes in v3: >>   - Changed map size param to size in max depth helper >> >> Changes in v4: >>   - Fixed indentation in max depth helper for args >> >> Link to v3: >> https://lore.kernel.org/all/09dc40eb-a84e-472a-8a68-36a2b1835308@linux.dev/ >> >> Signed-off-by: Arnaud Lecomte >> Acked-by: Yonghong Song >> --- >>   kernel/bpf/stackmap.c | 30 ++++++++++++++++++++++++------ >>   1 file changed, 24 insertions(+), 6 deletions(-) >> >> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c >> index 3615c06b7dfa..b9cc6c72a2a5 100644 >> --- a/kernel/bpf/stackmap.c >> +++ b/kernel/bpf/stackmap.c >> @@ -42,6 +42,27 @@ static inline int stack_map_data_size(struct >> bpf_map *map) >>           sizeof(struct bpf_stack_build_id) : sizeof(u64); >>   } >>   +/** >> + * stack_map_calculate_max_depth - Calculate maximum allowed stack >> trace depth >> + * @size:  Size of the buffer/map value in bytes >> + * @elem_size:  Size of each stack trace element >> + * @flags:  BPF stack trace flags (BPF_F_USER_STACK, >> BPF_F_USER_BUILD_ID, ...) >> + * >> + * Return: Maximum number of stack trace entries that can be safely >> stored >> + */ >> +static u32 stack_map_calculate_max_depth(u32 size, u32 elem_size, >> u64 flags) >> +{ >> +    u32 skip = flags & BPF_F_SKIP_FIELD_MASK; >> +    u32 max_depth; >> + >> +    max_depth = size / elem_size; >> +    max_depth += skip; >> +    if (max_depth > sysctl_perf_event_max_stack) >> +        return sysctl_perf_event_max_stack; > > hmm... this looks a bit suspicious. Is it possible that > sysctl_perf_event_max_stack is being changed to a larger value in > parallel? > Hi Martin, this is a valid concern as sysctl_perf_event_max_stack can be modified at runtime through /proc/sys/kernel/perf_event_max_stack. What we could maybe do instead is to create a copy: u32 current_max = READ_ONCE(sysctl_perf_event_max_stack); Any thoughts on this ? >> + >> +    return max_depth; >> +} >> + >>   static int prealloc_elems_and_freelist(struct bpf_stack_map *smap) >>   { >>       u64 elem_size = sizeof(struct stack_map_bucket) + >> @@ -406,7 +427,7 @@ static long __bpf_get_stack(struct pt_regs *regs, >> struct task_struct *task, >>                   struct perf_callchain_entry *trace_in, >>                   void *buf, u32 size, u64 flags, bool may_fault) >>   { >> -    u32 trace_nr, copy_len, elem_size, num_elem, max_depth; >> +    u32 trace_nr, copy_len, elem_size, max_depth; >>       bool user_build_id = flags & BPF_F_USER_BUILD_ID; >>       bool crosstask = task && task != current; >>       u32 skip = flags & BPF_F_SKIP_FIELD_MASK; >> @@ -438,10 +459,7 @@ static long __bpf_get_stack(struct pt_regs >> *regs, struct task_struct *task, >>           goto clear; >>       } >>   -    num_elem = size / elem_size; >> -    max_depth = num_elem + skip; >> -    if (sysctl_perf_event_max_stack < max_depth) >> -        max_depth = sysctl_perf_event_max_stack; >> +    max_depth = stack_map_calculate_max_depth(size, elem_size, flags); >>         if (may_fault) >>           rcu_read_lock(); /* need RCU for perf's callchain below */ >> @@ -461,7 +479,7 @@ 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; > > I suspect it was fine because trace_nr was still bounded by num_elem. > We should bring back the num_elem bound as an additional safe net. >> +    trace_nr = min(trace_nr, max_depth - skip); > > but now the min() is also based on max_depth which could be > sysctl_perf_event_max_stack. > > beside, if I read it correctly, in "max_depth - skip", the max_depth > could also be less than skip. I assume trace->nr is bound by > max_depth, so should be less of a problem but still a bit unintuitive > to read. > >>       copy_len = trace_nr * elem_size; >>         ips = trace->ip + skip; >