bpf: refactor max_depth computation in bpf_get_stack()

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

Link to v4: https://lore.kernel.org/all/[email protected]/

Signed-off-by: Arnaud Lecomte <[email protected]>
Acked-by: Yonghong Song <[email protected]>

Author: ArnaudLcm

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) +
@@ -438,10 +460,7 @@ 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 */
@@ -460,6 +479,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
 		goto err_fault;
 	}
 
+	num_elem = size / elem_size;
 	trace_nr = trace->nr - skip;
 	trace_nr = (trace_nr <= num_elem) ? trace_nr : num_elem;
 	copy_len = trace_nr * elem_size;