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SubjectRe: [BUG soft lockup] Re: [PATCH bpf-next v3] bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH
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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