Re: [PATCH bpf-next v7 1/3] bpf: refactor max_depth computation in bpf_get_stack()

[Song Liu <song@kernel.org>]

On 9/3/25 4:39 PM, 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
>
> Changes in v5:
>   - Bound back trace_nr to num_elem in __bpf_get_stack
>   - Make a copy of sysctl_perf_event_max_stack
>     in stack_map_calculate_max_depth
>
> Changes in v6:
>   - Restrained max_depth computation only when required
>   - Additional cleanup from Song in __bpf_get_stack
>
> Changes in v7:
>   - Removed additional cleanup from v6
>
> Link to v6: https://lore.kernel.org/all/20250903135323.97847-1-contact@arnaud-lcm.com/
>
> Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>
> Acked-by: Yonghong Song <yonghong.song@linux.dev>

Acked-by: Song Liu <song@kernel.org>


> ---
>   kernel/bpf/stackmap.c | 38 +++++++++++++++++++++++++++-----------
>   1 file changed, 27 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 3615c06b7dfa..ed707bc07173 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -42,6 +42,28 @@ 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;
> +	u32 curr_sysctl_max_stack = READ_ONCE(sysctl_perf_event_max_stack);
> +
> +	max_depth = size / elem_size;
> +	max_depth += skip;
> +	if (max_depth > curr_sysctl_max_stack)
> +		return curr_sysctl_max_stack;
> +
> +	return max_depth;
> +}
> +
>   static int prealloc_elems_and_freelist(struct bpf_stack_map *smap)
>   {
>   	u64 elem_size = sizeof(struct stack_map_bucket) +
> @@ -300,20 +322,17 @@ static long __bpf_get_stackid(struct bpf_map *map,
>   BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
>   	   u64, flags)
>   {
> -	u32 max_depth = map->value_size / stack_map_data_size(map);
> -	u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
> +	u32 elem_size = stack_map_data_size(map);
>   	bool user = flags & BPF_F_USER_STACK;
>   	struct perf_callchain_entry *trace;
>   	bool kernel = !user;
> +	u32 max_depth;
>   
>   	if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
>   			       BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID)))
>   		return -EINVAL;
>   
> -	max_depth += skip;
> -	if (max_depth > sysctl_perf_event_max_stack)
> -		max_depth = sysctl_perf_event_max_stack;
> -
> +	max_depth = stack_map_calculate_max_depth(map->value_size, elem_size, flags);
>   	trace = get_perf_callchain(regs, 0, kernel, user, max_depth,
>   				   false, false);
>   
> @@ -406,8 +425,8 @@ 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;
>   	bool user_build_id = flags & BPF_F_USER_BUILD_ID;
> +	u32 trace_nr, copy_len, elem_size, max_depth;
>   	bool crosstask = task && task != current;
>   	u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
>   	bool user = flags & BPF_F_USER_STACK;
> @@ -438,10 +457,7 @@ 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 */