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Subject[BUG soft lockup] Re: [PATCH bpf-next v3] bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH
On Tue, Feb 02, 2021 at 01:50:02PM +0000, Brendan Jackman wrote:

SNIP

> diff --git a/tools/testing/selftests/bpf/verifier/atomic_bounds.c b/tools/testing/selftests/bpf/verifier/atomic_bounds.c
> new file mode 100644
> index 000000000000..e82183e4914f
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/verifier/atomic_bounds.c
> @@ -0,0 +1,27 @@
> +{
> + "BPF_ATOMIC bounds propagation, mem->reg",
> + .insns = {
> + /* a = 0; */
> + /*
> + * Note this is implemented with two separate instructions,
> + * where you might think one would suffice:
> + *
> + * BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
> + *
> + * This is because BPF_ST_MEM doesn't seem to set the stack slot
> + * type to 0 when storing an immediate.
> + */
> + BPF_MOV64_IMM(BPF_REG_0, 0),
> + BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_0, -8),
> + /* b = atomic_fetch_add(&a, 1); */
> + BPF_MOV64_IMM(BPF_REG_1, 1),
> + BPF_ATOMIC_OP(BPF_DW, BPF_ADD | BPF_FETCH, BPF_REG_10, BPF_REG_1, -8),
> + /* Verifier should be able to tell that this infinite loop isn't reachable. */
> + /* if (b) while (true) continue; */
> + BPF_JMP_IMM(BPF_JNE, BPF_REG_1, 0, -1),
> + BPF_EXIT_INSN(),
> + },
> + .result = ACCEPT,
> + .result_unpriv = REJECT,
> + .errstr_unpriv = "back-edge",
> +},
> base-commit: 61ca36c8c4eb3bae35a285b1ae18c514cde65439
> --
> 2.30.0.365.g02bc693789-goog
>

hi,
I tracked soft lock up on powerpc to this test:

[root@ibm-p9z-07-lp1 bpf]# ./test_verifier 25
#25/u BPF_ATOMIC bounds propagation, mem->reg SKIP
#25/p BPF_ATOMIC bounds propagation, mem->reg

Message from syslogd@ibm-p9z-07-lp1 at Jun 27 11:24:34 ...
kernel:watchdog: BUG: soft lockup - CPU#2 stuck for 26s! [test_verifier:1055]

Message from syslogd@ibm-p9z-07-lp1 at Jun 27 11:25:04 ...
kernel:watchdog: BUG: soft lockup - CPU#2 stuck for 48s! [test_verifier:1055]

please check the console output below.. it looks like the verifier
allowed the loop to happen for some reason on powerpc.. any idea?

I'm on latest bpf-next/master, I can send the config if needed

thanks,
jirka


---
ibm-p9z-07-lp1 login: [ 184.108655] watchdog: BUG: soft lockup - CPU#2 stuck for 26s! [test_verifier:1055]
[ 184.108679] Modules linked in: snd_seq_dummy(E) snd_hrtimer(E) snd_seq(E) snd_seq_device(E) snd_timer(E) snd(E) soundcore(E) bonding(E) tls(E) rfkill(E) pseries_rng(E) drm(E) fuse(E) drm_panel_orientation_quirks(E) xfs(E) libcrc32c(E) sd_mod(E) t10_pi(E) ibmvscsi(E) ibmveth(E) scsi_transport_srp(E) vmx_crypto(E) dm_mirror(E) dm_region_hash(E) dm_log(E) dm_mod(E)
[ 184.108722] CPU: 2 PID: 1055 Comm: test_verifier Tainted: G E 5.13.0-rc3+ #3
[ 184.108728] NIP: c00800000131314c LR: c000000000c56918 CTR: c008000001313118
[ 184.108733] REGS: c0000000119ef820 TRAP: 0900 Tainted: G E (5.13.0-rc3+)
[ 184.108739] MSR: 8000000000009033 <SF,EE,ME,IR,DR,RI,LE> CR: 44222840 XER: 20040003
[ 184.108752] CFAR: c008000001313150 IRQMASK: 0
[ 184.108752] GPR00: c000000000c5671c c0000000119efac0 c000000002a08400 0000000000000001
[ 184.108752] GPR04: c0080000010c0048 ffffffffffffffff 0000000001f3f8ec 0000000000000008
[ 184.108752] GPR08: 0000000000000000 c0000000119efae8 0000000000000001 49adb8fcb8417937
[ 184.108752] GPR12: c008000001313118 c00000001ecae400 0000000000000000 0000000000000000
[ 184.108752] GPR16: 0000000000000000 0000000000000000 0000000000000000 c0000000021cf6f8
[ 184.108752] GPR20: 0000000000000000 c0000000119efc34 c0000000119efc30 c0080000010c0048
[ 184.108752] GPR24: c00000000a1dc100 0000000000000001 c000000011fadc80 c0000000021cf638
[ 184.108752] GPR28: c0080000010c0000 0000000000000001 c0000000021cf638 c0000000119efaf0
[ 184.108812] NIP [c00800000131314c] bpf_prog_a2eb9104e5e8a5bf+0x34/0xcee8
[ 184.108819] LR [c000000000c56918] bpf_test_run+0x2f8/0x470
[ 184.108826] Call Trace:
[ 184.108828] [c0000000119efac0] [c0000000119efb30] 0xc0000000119efb30 (unreliable)
[ 184.108835] [c0000000119efb30] [c000000000c5671c] bpf_test_run+0xfc/0x470
[ 184.108841] [c0000000119efc10] [c000000000c57b6c] bpf_prog_test_run_skb+0x38c/0x660
[ 184.108848] [c0000000119efcb0] [c00000000035de6c] __sys_bpf+0x46c/0xd60
[ 184.108854] [c0000000119efd90] [c00000000035e810] sys_bpf+0x30/0x40
[ 184.108859] [c0000000119efdb0] [c00000000002ea34] system_call_exception+0x144/0x280
[ 184.108866] [c0000000119efe10] [c00000000000c570] system_call_vectored_common+0xf0/0x268
[ 184.108874] --- interrupt: 3000 at 0x7fff8bb3ef24
[ 184.108878] NIP: 00007fff8bb3ef24 LR: 0000000000000000 CTR: 0000000000000000
[ 184.108883] REGS: c0000000119efe80 TRAP: 3000 Tainted: G E (5.13.0-rc3+)
[ 184.108887] MSR: 800000000280f033 <SF,VEC,VSX,EE,PR,FP,ME,IR,DR,RI,LE> CR: 28000848 XER: 00000000
[ 184.108903] IRQMASK: 0
[ 184.108903] GPR00: 0000000000000169 00007fffe4577710 00007fff8bc27200 000000000000000a
[ 184.108903] GPR04: 00007fffe45777b8 0000000000000080 0000000000000001 0000000000000008
[ 184.108903] GPR08: 000000000000000a 0000000000000000 0000000000000000 0000000000000000
[ 184.108903] GPR12: 0000000000000000 00007fff8be1c400 0000000000000000 0000000000000000
[ 184.108903] GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[ 184.108903] GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[ 184.108903] GPR24: 0000000000000000 0000000000000000 0000000000000000 000000001000d1d0
[ 184.108903] GPR28: 0000000000000002 00007fffe4578128 00007fffe45782c0 00007fffe4577710
[ 184.108960] NIP [00007fff8bb3ef24] 0x7fff8bb3ef24
[ 184.108964] LR [0000000000000000] 0x0
[ 184.108967] --- interrupt: 3000
[ 184.108970] Instruction dump:
[ 184.108974] 60000000 f821ff91 fbe10068 3be10030 39000000 f91ffff8 38600001 393ffff8
[ 184.108985] 7d4048a8 7d4a1a14 7d4049ad 4082fff4 <28230000> 4082fffc 60000000 ebe10068
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