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

["Lecomte, Arnaud" <contact@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 <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 ?

>> +
>> +    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;
>