Re: [PATCH v2] arm64: bpf: Fix branch offset in JIT

[Yauheni Kaliuta]

Hi, Ilias!

>>>>> On Tue, 15 Sep 2020 22:23:11 +0300, Ilias Apalodimas  wrote:

 > 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.


If you start to amend extables, could you consider a change like

05a68e892e89 ("s390/kernel: expand exception table logic to allow new handling options")

and implementation of BPF_PROBE_MEM then?

 > [ 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


-- 
WBR,
Yauheni Kaliuta