Re: [PATCH bpf-next RESEND v4 1/2] bpf: refactor max_depth computation in bpf_get_stack()

["Lecomte, Arnaud" <contact@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 <contact@arnaud-lcm.com>
>>>> Acked-by: Yonghong Song <yonghong.song@linux.dev>
>>>> ---
>>>>   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;
>>>
>
>