Messages in this thread Patch in this message | | | Date | Thu, 01 Jul 2021 16:32:03 +0530 | From | "Naveen N. Rao" <> | Subject | Re: [BUG soft lockup] Re: [PATCH bpf-next v3] bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH |
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Hi Brendan, Hi Jiri,
Brendan Jackman wrote: > On Wed, 30 Jun 2021 at 14:42, Jiri Olsa <jolsa@redhat.com> wrote: >> >> On Wed, Jun 30, 2021 at 12:34:58PM +0200, Brendan Jackman wrote: >> > On Tue, 29 Jun 2021 at 23:09, Jiri Olsa <jolsa@redhat.com> wrote: >> > > >> > > On Tue, Jun 29, 2021 at 06:41:24PM +0200, Jiri Olsa wrote: >> > > > On Tue, Jun 29, 2021 at 06:25:33PM +0200, Brendan Jackman wrote: >> > > > > On Tue, 29 Jun 2021 at 18:04, Jiri Olsa <jolsa@redhat.com> wrote: >> > > > > > On Tue, Jun 29, 2021 at 04:10:12PM +0200, Jiri Olsa wrote: >> > > > > > > On Mon, Jun 28, 2021 at 11:21:42AM +0200, Brendan Jackman wrote: >> > >> > > > > > > > atomics in .imm). Any idea if this test was ever passing on PowerPC? >> > > > > > > > >> > > > > > > >> > > > > > > hum, I guess not.. will check >> > > > > > >> > > > > > nope, it locks up the same: >> > > > > >> > > > > Do you mean it locks up at commit 91c960b0056 too? >> > >> > Sorry I was being stupid here - the test didn't exist at this commit >> > >> > > > I tried this one: >> > > > 37086bfdc737 bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH >> > > > >> > > > I will check also 91c960b0056, but I think it's the new test issue >> > >> > So yeah hard to say whether this was broken on PowerPC all along. How >> > hard is it for me to get set up to reproduce the failure? Is there a >> > rootfs I can download, and some instructions for running a PowerPC >> > QEMU VM? If so if you can also share your config and I'll take a look. >> > >> > If it's not as simple as that, I'll stare at the code for a while and >> > see if anything jumps out. >> > >> >> I have latest fedora ppc server and compile/install latest bpf-next tree >> I think it will be reproduced also on vm, I attached my config > > OK, getting set up to boot a PowerPC QEMU isn't practical here unless > someone's got commands I can copy-paste (suspect it will need .config > hacking too). Looks like you need to build a proper bootloader, and > boot an installer disk.
There are some notes put up here, though we can do better: https://github.com/linuxppc/wiki/wiki/Booting-with-Qemu
If you are familiar with ubuntu/fedora cloud images (and cloud-init), you should be able to grab one of the ppc64le images and boot it in qemu: https://cloud-images.ubuntu.com/releases/hirsute/release/ https://alt.fedoraproject.org/alt/
> > Looked at the code for a bit but nothing jumped out. It seems like the > verifier is seeing a BPF_ADD | BPF_FETCH, which means it doesn't > detect an infinite loop, but then we lose the BPF_FETCH flag somewhere > between do_check in verifier.c and bpf_jit_build_body in > bpf_jit_comp64.c. That would explain why we don't get the "eBPF filter > atomic op code %02x (@%d) unsupported", and would also explain the > lockup because a normal atomic add without fetch would leave BPF R1 > unchanged. > > We should be able to confirm that theory by disassembling the JITted > code that gets hexdumped by bpf_jit_dump when bpf_jit_enable is set to > 2... at least for PowerPC 32-bit... maybe you could paste those lines > into the 64-bit version too? Here's some notes I made for > disassembling the hexdump on x86, I guess you'd just need to change > the objdump flags: > > -- > > - Enable console JIT output: > ```shell > echo 2 > /proc/sys/net/core/bpf_jit_enable > ``` > - Load & run the program of interest. > - Copy the hex code from the kernel console to `/tmp/jit.txt`. Here's what a > short program looks like. This includes a line of context - don't paste the > `flen=` line. > ``` > [ 79.381020] flen=8 proglen=54 pass=4 image=000000001af6f390 > from=test_verifier pid=258 > [ 79.389568] JIT code: 00000000: 0f 1f 44 00 00 66 90 55 48 89 e5 48 81 ec 08 00 > [ 79.397411] JIT code: 00000010: 00 00 48 c7 45 f8 64 00 00 00 bf 04 00 00 00 48 > [ 79.405965] JIT code: 00000020: f7 df f0 48 29 7d f8 8b 45 f8 48 83 f8 60 74 02 > [ 79.414719] JIT code: 00000030: c9 c3 31 c0 eb fa > ``` > - This incantation will split out and decode the hex, then disassemble the > result: > ```shell > cat /tmp/jit.txt | cut -d: -f2- | xxd -r >/tmp/obj && objdump -D -b > binary -m i386:x86-64 /tmp/obj > ``` > > -- > > Sandipan, Naveen, do you know of anything in the PowerPC code that > might be leading us to drop the BPF_FETCH flag from the atomic > instruction in tools/testing/selftests/bpf/verifier/atomic_bounds.c?
Yes, I think I just found the issue. We aren't looking at the correct BPF instruction when checking the IMM value.
--- a/arch/powerpc/net/bpf_jit_comp64.c +++ b/arch/powerpc/net/bpf_jit_comp64.c @@ -673,7 +673,7 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context * * BPF_STX ATOMIC (atomic ops) */ case BPF_STX | BPF_ATOMIC | BPF_W: - if (insn->imm != BPF_ADD) { + if (insn[i].imm != BPF_ADD) { pr_err_ratelimited( "eBPF filter atomic op code %02x (@%d) unsupported\n", code, i); @@ -695,7 +695,7 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context * PPC_BCC_SHORT(COND_NE, tmp_idx); break; case BPF_STX | BPF_ATOMIC | BPF_DW: - if (insn->imm != BPF_ADD) { + if (insn[i].imm != BPF_ADD) { pr_err_ratelimited( "eBPF filter atomic op code %02x (@%d) unsupported\n", code, i);
Thanks, Naveen
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