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

[Yonghong Song <yonghong.song@linux.dev>]

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.

>
>>> +
>>> +    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;
>>