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: Arnaud Lecomte <contact@arnaud-lcm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/bpf/20251025192858.31424-1-contact@arnaud-lcm.com

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 4d53cdd1374cf713afd6f2b57c9193819f9180a8..5e9ad050333c25e577d10d210f26da19283603a3 100644 (file)
--- 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, 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;