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

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

On 8/7/25 10:50 AM, Arnaud Lecomte wrote:
> A new helper function stack_map_calculate_max_depth() that
> computes the max depth for a stackmap.
>
> Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>
> ---
>   kernel/bpf/stackmap.c | 38 ++++++++++++++++++++++++++++++--------
>   1 file changed, 30 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 3615c06b7dfa..14e034045310 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -42,6 +42,31 @@ 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
> + * @elem_size:       Size of each stack trace element
> + * @map_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,
> + * or -EINVAL if size is not a multiple of elem_size

-EINVAL is not needed here. See below.

> + */
> +static u32 stack_map_calculate_max_depth(u32 map_size, u32 map_elem_size, u64 map_flags)

map_elem_size -> elem_size

> +{
> +	u32 max_depth;
> +	u32 skip = map_flags & BPF_F_SKIP_FIELD_MASK;

reverse Christmas tree?

> +
> +	if (unlikely(map_size%map_elem_size))
> +		return -EINVAL;

The above should not be here. The checking 'map_size % map_elem_size' is only needed
for bpf_get_stack(), not applicable for bpf_get_stackid().

> +
> +	max_depth = map_size / map_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 +431,7 @@ 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;
> @@ -423,8 +448,6 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>   		goto clear;
>   
>   	elem_size = user_build_id ? sizeof(struct bpf_stack_build_id) : sizeof(u64);
> -	if (unlikely(size % elem_size))
> -		goto clear;

Please keep this one.

>   
>   	/* cannot get valid user stack for task without user_mode regs */
>   	if (task && user && !user_mode(regs))
> @@ -438,10 +461,9 @@ 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 (max_depth < 0)
> +		goto err_fault;

max_depth is never less than 0.

>   
>   	if (may_fault)
>   		rcu_read_lock(); /* need RCU for perf's callchain below */
> @@ -461,7 +483,7 @@ 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;