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

[Ilias Apalodimas]

Hi Will,

On Mon, Sep 14, 2020 at 03:35:04PM +0300, Ilias Apalodimas wrote:
> On Mon, Sep 14, 2020 at 01:20:43PM +0100, Will Deacon wrote:
> > On Mon, Sep 14, 2020 at 11:36:21AM +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
> > > [ 6525.741609] Modules linked in: nls_utf8 cifs libdes libarc4 dns_resolver fscache binfmt_misc nls_ascii nls_cp437 vfat fat aes_ce_blk crypto_simd cryptd aes_ce_cipher ghash_ce gf128mul efi_pstore sha2_ce sha256_arm64 sha1_ce evdev efivars efivarfs ip_tables x_tables autofs4 btrfs blake2b_generic xor xor_neon zstd_compress raid6_pq libcrc32c crc32c_generic ahci xhci_pci libahci xhci_hcd igb libata i2c_algo_bit nvme realtek usbcore nvme_core scsi_mod t10_pi netsec mdio_devres of_mdio gpio_keys fixed_phy libphy gpio_mb86s7x
> > > [ 6525.787760] CPU: 3 PID: 7881 Comm: test_verifier Tainted: G        W         5.9.0-rc1+ #47
> > > [ 6525.796111] Hardware name: Socionext SynQuacer E-series DeveloperBox, BIOS build #1 Jun  6 2020
> > > [ 6525.804812] pstate: 20000005 (nzCv daif -PAN -UAO BTYPE=--)
> > > [ 6525.810390] pc : bpf_prog_c3d01833289b6311_F+0xc8/0x9f4
> > > [ 6525.815613] lr : bpf_prog_d53bb52e3f4483f9_F+0x38/0xc8c
> > > [ 6525.820832] sp : ffff8000130cbb80
> > > [ 6525.824141] x29: ffff8000130cbbb0 x28: 0000000000000000
> > > [ 6525.829451] x27: 000005ef6fcbf39b x26: 0000000000000000
> > > [ 6525.834759] x25: ffff8000130cbb80 x24: ffff800011dc7038
> > > [ 6525.840067] x23: ffff8000130cbd00 x22: ffff0008f624d080
> > > [ 6525.845375] x21: 0000000000000001 x20: ffff800011dc7000
> > > [ 6525.850682] x19: 0000000000000000 x18: 0000000000000000
> > > [ 6525.855990] x17: 0000000000000000 x16: 0000000000000000
> > > [ 6525.861298] x15: 0000000000000000 x14: 0000000000000000
> > > [ 6525.866606] x13: 0000000000000000 x12: 0000000000000000
> > > [ 6525.871913] x11: 0000000000000001 x10: ffff8000000a660c
> > > [ 6525.877220] x9 : ffff800010951810 x8 : ffff8000130cbc38
> > > [ 6525.882528] x7 : 0000000000000000 x6 : 0000009864cfa881
> > > [ 6525.887836] x5 : 00ffffffffffffff x4 : 002880ba1a0b3e9f
> > > [ 6525.893144] x3 : 0000000000000018 x2 : ffff8000000a4374
> > > [ 6525.898452] x1 : 000000000000000a x0 : 0000000000000009
> > > [ 6525.903760] Call trace:
> > > [ 6525.906202]  bpf_prog_c3d01833289b6311_F+0xc8/0x9f4
> > > [ 6525.911076]  bpf_prog_d53bb52e3f4483f9_F+0x38/0xc8c
> > > [ 6525.915957]  bpf_dispatcher_xdp_func+0x14/0x20
> > > [ 6525.920398]  bpf_test_run+0x70/0x1b0
> > > [ 6525.923969]  bpf_prog_test_run_xdp+0xec/0x190
> > > [ 6525.928326]  __do_sys_bpf+0xc88/0x1b28
> > > [ 6525.932072]  __arm64_sys_bpf+0x24/0x30
> > > [ 6525.935820]  el0_svc_common.constprop.0+0x70/0x168
> > > [ 6525.940607]  do_el0_svc+0x28/0x88
> > > [ 6525.943920]  el0_sync_handler+0x88/0x190
> > > [ 6525.947838]  el0_sync+0x140/0x180
> > > [ 6525.951154] Code: d4202000 d4202000 d4202000 d4202000 (d4202000)
> > > [ 6525.957249] ---[ end trace cecc3f93b14927e2 ]---
> > > 
> > > The reason seems to be the offset[] creation and usage ctx->offset[]
> > 
> > "seems to be"? Are you unsure?
> 
> Reading the history and other ports of the JIT implementation, I couldn't 
> tell if the decision on skipping the 1st entry was deliberate or not on 
> Aarch64. Reading through the mailist list didn't help either [1].
> Skipping the 1st entry seems indeed to cause the problem.
> I did run the patch though the BPF tests and showed no regressions + fixing 
> the error.

I'll correct myself here.
Looking into 7c2e988f400e ("bpf: fix x64 JIT code generation for jmp to 1st insn")
explains things a bit better.
Jumping back to the 1st insn wasn't allowed until eBPF bounded loops were 
introduced. That's what the 1st instruction was not saved in the original code.

> > 
> > No Fixes: tag?
> 
> I'll re-spin and apply one 
> 
Any suggestion on any Fixes I should apply? The original code was 'correct' and
broke only when bounded loops and their self-tests were introduced.

Thanks
/Ilias