linux-kernel - [PATCH RESEND v2 1/2] bpf: refactor max_depth computation in bpf_get

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Message-ID: <20250809120911.88670-1-contact@arnaud-lcm.com>
Date: Sat,  9 Aug 2025 13:09:11 +0100
From: Arnaud Lecomte <contact@...aud-lcm.com>
To: yonghong.song@...ux.dev
Cc: andrii@...nel.org,
	ast@...nel.org,
	bpf@...r.kernel.org,
	daniel@...earbox.net,
	eddyz87@...il.com,
	haoluo@...gle.com,
	john.fastabend@...il.com,
	jolsa@...nel.org,
	kpsingh@...nel.org,
	linux-kernel@...r.kernel.org,
	martin.lau@...ux.dev,
	sdf@...ichev.me,
	song@...nel.org,
	syzbot+c9b724fbb41cf2538b7b@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com,
	Arnaud Lecomte <contact@...aud-lcm.com>
Subject: [PATCH RESEND v2 1/2] 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

Signed-off-by: Arnaud Lecomte <contact@...aud-lcm.com>
---
 kernel/bpf/stackmap.c | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 3615c06b7dfa..532447606532 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -42,6 +42,27 @@ 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
+ * @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 map_size, u32 elem_size, u64 flags)
+{
+	u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
+	u32 max_depth;
+
+	max_depth = map_size / 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 +427,7 @@ 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;
@@ -438,10 +459,7 @@ 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 */
@@ -461,7 +479,7 @@ 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;
-- 
2.43.0