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Date:   Tue, 15 Sep 2020 22:23:11 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Will Deacon <will@...nel.org>
Cc:     bpf@...r.kernel.org, ardb@...nel.org, naresh.kamboju@...aro.org,
        Jiri Olsa <jolsa@...nel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Yauheni Kaliuta <yauheni.kaliuta@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Zi Shen Lim <zlim.lnx@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64: bpf: Fix branch offset in JIT

Hi Will, 

On Tue, Sep 15, 2020 at 03:17:08PM +0100, Will Deacon wrote:
> On Tue, Sep 15, 2020 at 04:53:44PM +0300, Ilias Apalodimas wrote:
> > On Tue, Sep 15, 2020 at 02:11:03PM +0100, Will Deacon wrote:
> > > Hi Ilias,
> > > 
> > > On Mon, Sep 14, 2020 at 07:03:55PM +0300, Ilias Apalodimas wrote:
> > > > Running the eBPF test_verifier leads to random errors looking like this:
> > > > 
> > > > [ 6525.735488] Unexpected kernel BRK exception at EL1
> > > > [ 6525.735502] Internal error: ptrace BRK handler: f2000100 [#1] SMP
> > > 
> > > Does this happen because we poison the BPF memory with BRK instructions?
> > > Maybe we should look at using a special immediate so we can detect this,
> > > rather than end up in the ptrace handler.
> > 
> > As discussed offline this is what aarch64_insn_gen_branch_imm() will return for
> > offsets > 128M and yes replacing the handler with a more suitable message would 
> > be good.
> 
> Can you give the diff below a shot, please? Hopefully printing a more useful
> message will mean these things get triaged/debugged better in future.

[...]

The error print is going to be helpful imho. At least it will help
people notice something is wrong a lot faster than the previous one.


[  575.273203] BPF JIT generated an invalid instruction at bpf_prog_64e6f4ba80861823_F+0x2e4/0x9a4!
[  575.281996] Unexpected kernel BRK exception at EL1
[  575.286786] Internal error: BRK handler: f2000100 [#5] PREEMPT SMP
[  575.292965] Modules linked in: crct10dif_ce drm ip_tables x_tables ipv6 btrfs blake2b_generic libcrc32c xor xor_neon zstd_compress raid6_pq nvme nvme_core realtek
[  575.307516] CPU: 21 PID: 11760 Comm: test_verifier Tainted: G      D W         5.9.0-rc3-01410-ged6d9b022813-dirty #1
[  575.318125] Hardware name: Socionext SynQuacer E-series DeveloperBox, BIOS build #1 Jun  6 2020
[  575.326825] pstate: 20000005 (nzCv daif -PAN -UAO BTYPE=--)
[  575.332396] pc : bpf_prog_64e6f4ba80861823_F+0x2e4/0x9a4
[  575.337705] lr : bpf_prog_d3e125b76c96daac+0x40/0xdec
[  575.342752] sp : ffff8000144e3ba0
[  575.346061] x29: ffff8000144e3bd0 x28: 0000000000000000
[  575.351371] x27: 00000085f19dc08d x26: 0000000000000000
[  575.356681] x25: ffff8000144e3ba0 x24: ffff800011fdf038
[  575.361991] x23: ffff8000144e3d20 x22: 0000000000000001
[  575.367301] x21: ffff800011fdf000 x20: ffff0009609d4740
[  575.372611] x19: 0000000000000000 x18: 0000000000000000
[  575.377921] x17: 0000000000000000 x16: 0000000000000000
[  575.383231] x15: 0000000000000000 x14: 0000000000000000
[  575.388540] x13: 0000000000000000 x12: 0000000000000000
[  575.393850] x11: 0000000000000000 x10: ffff8000000bc65c
[  575.399160] x9 : 0000000000000000 x8 : ffff8000144e3c58
[  575.404469] x7 : 0000000000000000 x6 : 0000000dd7ae967a
[  575.409779] x5 : 00ffffffffffffff x4 : 0007fabd6992cf96
[  575.415088] x3 : 0000000000000018 x2 : ffff8000000ba214
[  575.420398] x1 : 000000000000000a x0 : 0000000000000009
[  575.425708] Call trace:
[  575.428152]  bpf_prog_64e6f4ba80861823_F+0x2e4/0x9a4
[  575.433114]  bpf_prog_d3e125b76c96daac+0x40/0xdec
[  575.437822]  bpf_dispatcher_xdp_func+0x10/0x1c
[  575.442265]  bpf_test_run+0x80/0x240
[  575.445838]  bpf_prog_test_run_xdp+0xe8/0x190
[  575.450196]  __do_sys_bpf+0x8e8/0x1b00
[  575.453943]  __arm64_sys_bpf+0x24/0x510
[  575.457780]  el0_svc_common.constprop.0+0x6c/0x170
[  575.462570]  do_el0_svc+0x24/0x90
[  575.465883]  el0_sync_handler+0x90/0x19c
[  575.469802]  el0_sync+0x158/0x180
[  575.473118] Code: d4202000 d4202000 d4202000 d4202000 (d4202000)
[  575.479211] ---[ end trace 8cd54c7d5c0ffda4 ]---

Cheers
/Ilias

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