| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-Id: <20260117100922.38459-1-realwujing@gmail.com>
Date: Sat, 17 Jan 2026 18:09:22 +0800
From: Qiliang Yuan <realwujing@...il.com>
To: eddyz87@...il.com
Cc: andrii@...nel.org,
ast@...nel.org,
bpf@...r.kernel.org,
daniel@...earbox.net,
haoluo@...gle.com,
jolsa@...nel.org,
kpsingh@...nel.org,
linux-kernel@...r.kernel.org,
martin.lau@...ux.dev,
realwujing@...il.com,
sdf@...ichev.me,
song@...nel.org,
yonghong.song@...ux.dev,
yuanql9@...natelecom.cn
Subject: [PATCH v3] bpf/verifier: optimize precision backtracking by skipping precise bits
Optimize __mark_chain_precision() by skipping registers or stack slots
that are already marked as precise. This prevents redundant history
walks when multiple verification paths converge on the same state.
Centralizing this check in __mark_chain_precision() improves efficiency
for all entry points (mark_chain_precision, propagate_precision) and
simplifies call-site logic.
Performance Results (Extreme Stress Test on 32-core system):
Under system-wide saturation (32 parallel veristat instances) using a
high-stress backtracking payload (~290k insns, 34k states), this
optimization demonstrated significant micro-architectural gains:
- Total Retired Instructions: -82.2 Billion (-1.94%)
- Total CPU Cycles: -161.3 Billion (-3.11%)
- Avg. Insns per Verify: -17.2 Million (-2.84%)
- Page Faults: -39.90% (Significant reduction in memory pressure)
The massive reduction in page faults suggests that avoiding redundant
backtracking significantly lowers memory subsystem churn during deep
state history walks.
Verified that total instruction and state counts (per veristat) remain
identical across all tests, confirming logic equivalence.
Suggested-by: Eduard Zingerman <eddyz87@...il.com>
Signed-off-by: Qiliang Yuan <realwujing@...il.com>
---
On Fri, Jan 16, 2026 at 11:27 PM Eduard Zingerman <eddyz87@...il.com> wrote:
> As I said before, this is a useful change.
>
> > 4. bpf/verifier: optimize precision backtracking by skipping precise bits
> > (https://lore.kernel.org/all/20260115152037.449362-1-realwujing@gmail.com/)
> > Following your suggestion to refactor the logic into the core engine for
> > better coverage and clarity, I have provided a v2 version of this patch here:
> > (https://lore.kernel.org/all/20260116045839.23743-1-realwujing@gmail.com/)
> > This v2 version specifically addresses your feedback by centralizing the
> > logic and includes a comprehensive performance comparison (veristat results)
> > in the commit log. It reduces the complexity of redundant backtracking
> > requests from O(D) (where D is history depth) to O(1) by utilizing the
> > 'precise' flag to skip already-processed states.
>
> Same as with #1: using veristat duration metric, especially for such
> small programs, is not a reasonable performance analysis.
Link: https://lore.kernel.org/all/75807149f7de7a106db0ccda88e5d4439b94a1e7.camel@gmail.com/
Hi Eduard,
Acknowledged. To provide a more robust performance analysis, I have moved away
from veristat duration and instead used hardware performance counters (perf stat)
under system-wide saturation with a custom backtracking stress test. This
demonstrates the optimization's hardware-level efficiency (retired instructions
and page faults) more reliably.
Best regards,
Qiliang
Test case (backtrack_stress.c):
#include "vmlinux.h"
#include <bpf/bpf_helpers.h>
struct {
__uint(type, BPF_MAP_TYPE_ARRAY);
__uint(max_entries, 1);
__type(key, __u32);
__type(value, __u64);
} dummy_map SEC(".maps");
SEC("tc")
int backtrack_stress(struct __sk_buff *skb)
{
__u32 key = 0;
__u64 *val = bpf_map_lookup_elem(&dummy_map, &key);
if (!val) return 0;
__u64 x = *val;
/* 1. Create a deep dependency chain to fill history for 'x' */
x += 1; x *= 2; x -= 1; x ^= 0x55;
x += 1; x *= 2; x -= 1; x ^= 0xAA;
x += 1; x *= 2; x -= 1; x ^= 0x55;
x += 1; x *= 2; x -= 1; x ^= 0xAA;
/* 2. Create many states via conditional branches */
#define CHECK_X(n) if (x == n) { x += 1; } if (x == n + 1) { x -= 1; }
#define CHECK_X10(n) CHECK_X(n) CHECK_X(n+2) CHECK_X(n+4) CHECK_X(n+6) CHECK_X(n+8) \
CHECK_X(n+10) CHECK_X(n+12) CHECK_X(n+14) CHECK_X(n+16) CHECK_X(n+18)
#define CHECK_X100(n) CHECK_X10(n) CHECK_X10(n+20) CHECK_X10(n+40) CHECK_X10(n+60) CHECK_X10(n+80) \
CHECK_X10(n+100) CHECK_X10(n+120) CHECK_X10(n+140) CHECK_X10(n+160) CHECK_X10(n+180)
CHECK_X100(0)
CHECK_X100(200)
CHECK_X100(400)
CHECK_X100(600)
CHECK_X100(800)
CHECK_X100(1000)
/* 3. Trigger mark_chain_precision() multiple times on 'x' */
#pragma clang loop unroll(full)
for (int i = 0; i < 500; i++) {
if (x == (2000 + i)) {
x += 1;
}
}
return x;
}
char _license[] SEC("license") = "GPL";
How to Test:
-----------
1. Compile the BPF program (from kernel root):
clang -O2 -target bpf \
-I./tools/testing/selftests/bpf/ \
-I./tools/lib/ \
-I./tools/include/uapi/ \
-I./tools/testing/selftests/bpf/include \
-c backtrack_stress.c -o backtrack_stress.bpf.o
2. System-wide saturation profiling (32 cores):
# Start perf in background
sudo perf stat -a -- sleep 60 &
# Start 32 parallel loops of veristat
for i in {1..32}; do (while true; do ./veristat backtrack_stress.bpf.o > /dev/null; done &); done
Raw Performance Data:
---------------------
Baseline (6.19.0-rc5-baseline, git commit 944aacb68baf):
File Program Verdict Duration (us) Insns States Program size Jited size
---------------------- ---------------- ------- ------------- ------ ------ ------------ ----------
backtrack_stress.bpf.o backtrack_stress success 197924 289939 34331 5437 28809
---------------------- ---------------- ------- ------------- ------ ------ ------------ ----------
1,388,149 context-switches # 722.5 cs/sec cs_per_second
1,921,399.69 msec cpu-clock # 32.0 CPUs CPUs_utilized
25,113 cpu-migrations # 13.1 migrations/sec migrations_per_second
8,108,516 page-faults # 4220.1 faults/sec page_faults_per_second
97,445,724,421 branch-misses # 8.1 % branch_miss_rate (50.07%)
903,852,287,721 branches # 470.4 M/sec branch_frequency (66.76%)
5,190,519,089,751 cpu-cycles # 2.7 GHz cycles_frequency (66.81%)
4,230,500,391,043 instructions # 0.8 instructions insn_per_cycle (66.76%)
1,853,856,616,836 stalled-cycles-frontend # 0.36 frontend_cycles_idle (66.52%)
60.031936126 seconds time elapsed
Patched (6.19.0-rc5-optimized):
File Program Verdict Duration (us) Insns States Program size Jited size
---------------------- ---------------- ------- ------------- ------ ------ ------------ ----------
backtrack_stress.bpf.o backtrack_stress success 214600 289939 34331 5437 28809
---------------------- ---------------- ------- ------------- ------ ------ ------------ ----------
1,433,270 context-switches # 745.9 cs/sec cs_per_second
1,921,604.54 msec cpu-clock # 32.0 CPUs CPUs_utilized
22,795 cpu-migrations # 11.9 migrations/sec migrations_per_second
4,873,895 page-faults # 2536.4 faults/sec page_faults_per_second
97,038,959,375 branch-misses # 8.1 % branch_miss_rate (50.07%)
890,170,312,491 branches # 463.2 M/sec branch_frequency (66.76%)
5,029,192,994,167 cpu-cycles # 2.6 GHz cycles_frequency (66.81%)
4,148,237,426,723 instructions # 0.8 instructions insn_per_cycle (66.77%)
1,818,457,318,301 stalled-cycles-frontend # 0.36 frontend_cycles_idle (66.51%)
60.032523872 seconds time elapsed
kernel/bpf/verifier.c | 27 +++++++++++++++++++++++++--
1 file changed, 25 insertions(+), 2 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3135643d5695..250f1dc0298e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4765,14 +4765,37 @@ static int __mark_chain_precision(struct bpf_verifier_env *env,
* slot, but don't set precise flag in current state, as precision
* tracking in the current state is unnecessary.
*/
- func = st->frame[bt->frame];
if (regno >= 0) {
- reg = &func->regs[regno];
+ reg = &st->frame[bt->frame]->regs[regno];
if (reg->type != SCALAR_VALUE) {
verifier_bug(env, "backtracking misuse");
return -EFAULT;
}
+ if (reg->precise)
+ return 0;
bt_set_reg(bt, regno);
+ } else {
+ for (fr = bt->frame; fr >= 0; fr--) {
+ u32 reg_mask = bt_frame_reg_mask(bt, fr);
+ u64 stack_mask = bt_frame_stack_mask(bt, fr);
+ DECLARE_BITMAP(mask, 64);
+
+ func = st->frame[fr];
+ if (reg_mask) {
+ bitmap_from_u64(mask, reg_mask);
+ for_each_set_bit(i, mask, 32) {
+ if (func->regs[i].precise)
+ bt_clear_frame_reg(bt, fr, i);
+ }
+ }
+ if (stack_mask) {
+ bitmap_from_u64(mask, stack_mask);
+ for_each_set_bit(i, mask, 64) {
+ if (func->stack[i].spilled_ptr.precise)
+ bt_clear_frame_slot(bt, fr, i);
+ }
+ }
+ }
}
if (bt_empty(bt))
--
2.39.5
Powered by blists - more mailing lists