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

[Brendan Jackman]

On Wed, 30 Jun 2021 at 14:42, Jiri Olsa <jolsa@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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.

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?