Commit e17d62fe authored by Arnaud Lecomte's avatar Arnaud Lecomte Committed by Andrii Nakryiko
Browse files

bpf: Refactor stack map trace depth calculation into helper function 



Extract the duplicated maximum allowed depth computation for stack
traces stored in BPF stacks from bpf_get_stackid() and __bpf_get_stack()
into a dedicated stack_map_calculate_max_depth() helper function.

This unifies the logic for:
- The max depth computation
- Enforcing the sysctl_perf_event_max_stack limit

No functional changes for existing code paths.

Signed-off-by: default avatarArnaud Lecomte <contact@arnaud-lcm.com>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Acked-by: default avatarYonghong Song <yonghong.song@linux.dev>
Acked-by: default avatarSong Liu <song@kernel.org>
Link: https://lore.kernel.org/bpf/20251025192858.31424-1-contact@arnaud-lcm.com
parent 88427328

kernel/bpf/stackmap.c

+32 −15
Original line number Diff line number Diff line
@@ -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, kernel, user, max_depth,

				   false, false);
@@ -406,7 +425,7 @@ 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;
@@ -438,21 +457,20 @@ 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 */



	if (trace_in)

	if (trace_in) {

		trace = trace_in;

	else if (kernel && task)

		trace->nr = min_t(u32, trace->nr, max_depth);

	} else if (kernel && task) {

		trace = get_callchain_entry_for_task(task, max_depth);

	else

	} else {

		trace = get_perf_callchain(regs, kernel, user, max_depth,

					   crosstask, false);

	}



	if (unlikely(!trace) || trace->nr < skip) {

		if (may_fault)
@@ -461,7 +479,6 @@ 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;

	copy_len = trace_nr * elem_size;



	ips = trace->ip + skip;