linux-kernel - Re: [PATCH RESEND v2 1/2] bpf: refactor max_depth computation in bpf_get

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Message-ID: <084a70f2-96ef-43a1-a1d0-863496c4547e@arnaud-lcm.com>
Date: Tue, 12 Aug 2025 20:32:11 +0100
From: Arnaud Lecomte <contact@...aud-lcm.com>
To: Yonghong Song <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,
 song@...nel.org, syzbot+c9b724fbb41cf2538b7b@...kaller.appspotmail.com,
 syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH RESEND v2 1/2] bpf: refactor max_depth computation in
 bpf_get_stack()

Thanks Yonghong for your feedbacks and your patience !

On 12/08/2025 05:39, Yonghong Song wrote:
>
>
> On 8/9/25 5:09 AM, Arnaud Lecomte wrote:
>> A new helper function stack_map_calculate_max_depth() that
>> computes the max depth for a stackmap.
>
> Please add 'bpf-next' in the subject like [PATCH bpf-next v2 1/2]
> so CI can properly test the patch set.
>
>>
>> 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
>>
>> Signed-off-by: Arnaud Lecomte <contact@...aud-lcm.com>
>> ---
>>   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..532447606532 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
>> + * @map_size:        Size of the buffer/map value in bytes
>
> let us rename 'map_size' to 'size' since the size represents size of
> buffer or map, not just for map.
>
>> + * @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 map_size, u32 
>> elem_size, u64 flags)
>
> map_size -> size
> Also, you can replace 'flags' to 'skip', so below 'u32 skip = flags & 
> BPF_F_SKIP_FIELD_MASK'
> is not necessary.
>
>> +{
>> +    u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
>> +    u32 max_depth;
>> +
>> +    max_depth = map_size / elem_size;
>> +    max_depth += skip;
>> +    if (max_depth > sysctl_perf_event_max_stack)
>> +        return sysctl_perf_event_max_stack;
>> +
>> +    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;
>> +    trace_nr = min(trace_nr, max_depth - skip);
>>       copy_len = trace_nr * elem_size;
>>         ips = trace->ip + skip;
>
>