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Date:   Sun, 27 Jun 2021 17:34:14 +0200
From:   Jiri Olsa <jolsa@redhat.com>
To:     Brendan Jackman <jackmanb@google.com>
Cc:     bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        Florent Revest <revest@chromium.org>,
        John Fastabend <john.fastabend@gmail.com>,
        linux-kernel@vger.kernel.org,
        "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
        Sandipan Das <sandipan@linux.ibm.com>
Subject: [BUG soft lockup] Re: [PATCH bpf-next v3] bpf: Propagate stack
 bounds to registers in atomics w/ BPF_FETCH
Message-ID: <YNiadhIbJBBPeOr6@krava>
References: <20210202135002.4024825-1-jackmanb@google.com>
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In-Reply-To: <20210202135002.4024825-1-jackmanb@google.com>
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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