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bpf: Refactor stack map trace depth calculation into helper function

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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
This commit is contained in:
Arnaud Lecomte 2025-10-25 19:28:58 +00:00 committed by Andrii Nakryiko
parent 88427328e3
commit e17d62fedd
1 changed files with 32 additions and 15 deletions
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47
kernel/bpf/stackmap.c
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@ -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;