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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 [IPV6:2a02:8084:255b:aa00:5463:d190:b7cc:eed1] (unknown [IPv6:2a02:8084:255b:aa00:5463:d190:b7cc:eed1]) by plesk.hostmyservers.fr (Postfix) with ESMTPSA id 8D1D340099; Mon, 25 Aug 2025 20:07:11 +0000 (UTC) Authentication-Results: Plesk; spf=pass (sender IP is 2a02:8084:255b:aa00:5463:d190:b7cc:eed1) smtp.mailfrom=contact@arnaud-lcm.com smtp.helo=[IPV6:2a02:8084:255b:aa00:5463:d190:b7cc:eed1] Received-SPF: pass (Plesk: connection is authenticated) Message-ID: <0165bf55-4a46-4e75-91df-644b0281b247@arnaud-lcm.com> Date: Mon, 25 Aug 2025 21:07:10 +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: Yonghong Song , Martin KaFai Lau 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> <43b9d0ff-9922-490a-ac6b-7e8e7baa2247@linux.dev> Content-Language: en-US From: "Lecomte, Arnaud" In-Reply-To: <43b9d0ff-9922-490a-ac6b-7e8e7baa2247@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-PPP-Message-ID: <175615243259.32519.10645825439471745222@Plesk> X-PPP-Vhost: arnaud-lcm.com On 25/08/2025 19:27, Yonghong Song wrote: > > > On 8/25/25 9:39 AM, Lecomte, Arnaud wrote: >> >> 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 ? > > There is no need to have READ_ONCE. Jut do >     int curr_sysctl_max_stack = sysctl_perf_event_max_stack; >     if (max_depth > curr_sysctl_max_stack) >       return curr_sysctl_max_stack; > > Because of the above change, the patch is not a refactoring change any > more. > Why would you not consider it as a refactoring change anymore ? >> >>>> + >>>> +    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; >>> > >