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Date: Mon, 16 Aug 2021 21:45:22 +0900
From: Kuniyuki Iwashima <kuniyu@...zon.co.jp>
To: <yhs@...com>
CC: <andrii.nakryiko@...il.com>, <andrii@...nel.org>, <ast@...nel.org>,
<benh@...zon.com>, <bpf@...r.kernel.org>, <daniel@...earbox.net>,
<davem@...emloft.net>, <john.fastabend@...il.com>, <kafai@...com>,
<kpsingh@...nel.org>, <kuba@...nel.org>, <kuni1840@...il.com>,
<kuniyu@...zon.co.jp>, <netdev@...r.kernel.org>,
<songliubraving@...com>
Subject: Re: [PATCH v5 bpf-next 3/4] selftest/bpf: Implement sample UNIX domain socket iterator program.
From: Yonghong Song <yhs@...com>
Date: Sun, 15 Aug 2021 11:10:49 -0700
> On 8/13/21 5:21 PM, Kuniyuki Iwashima wrote:
> > From: Andrii Nakryiko <andrii.nakryiko@...il.com>
> > Date: Fri, 13 Aug 2021 16:25:53 -0700
> >> On Thu, Aug 12, 2021 at 9:46 AM Kuniyuki Iwashima <kuniyu@...zon.co.jp> wrote:
> >>>
> >>> The iterator can output almost the same result compared to /proc/net/unix.
> >>> The header line is aligned, and the Inode column uses "%8lu" because "%5lu"
> >>> can be easily overflown.
> >>>
> >>> # cat /sys/fs/bpf/unix
> >>> Num RefCount Protocol Flags Type St Inode Path
> >>
> >> It's totally my OCD, but why the column name is not aligned with
> >> values? I mean the "Inode" column. It's left aligned, but values
> >> (numbers) are right-aligned? I'd fix that while applying, but I can't
> >> apply due to selftests failures, so please take a look.
> >
> > Ah, honestly, I've felt something strange about the column... will fix it!
> >
> >
> >>
> >>
> >>> ffff963c06689800: 00000002 00000000 00010000 0001 01 18697 private/defer
> >>> ffff963c7c979c00: 00000002 00000000 00000000 0001 01 598245 @Hello@...ld@
> >>>
> >>> # cat /proc/net/unix
> >>> Num RefCount Protocol Flags Type St Inode Path
> >>> ffff963c06689800: 00000002 00000000 00010000 0001 01 18697 private/defer
> >>> ffff963c7c979c00: 00000002 00000000 00000000 0001 01 598245 @Hello@...ld@
> >>>
> >>> Note that this prog requires the patch ([0]) for LLVM code gen. Thanks to
> >>> Yonghong Song for analysing and fixing.
> >>>
> >>> [0] https://reviews.llvm.org/D107483
> >>>
> >>> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.co.jp>
> >>> Acked-by: Yonghong Song <yhs@...com>
> >>> ---
> >>
> >> This selftests breaks test_progs-no_alu32 ([0], the error log is super
> >> long and can freeze browser; it looks like an infinite loop and BPF
> >> verifier just keeps reporting it until it runs out of 1mln
> >> instructions or something). Please check what's going on there, I
> >> can't land it as it is right now.
> >>
> >> [0] https://github.com/kernel-patches/bpf/runs/3326071112?check_suite_focus=true#step:6:124288
> >>
> >>
> >>> tools/testing/selftests/bpf/README.rst | 38 +++++++++
> >>> .../selftests/bpf/prog_tests/bpf_iter.c | 16 ++++
> >>> tools/testing/selftests/bpf/progs/bpf_iter.h | 8 ++
> >>> .../selftests/bpf/progs/bpf_iter_unix.c | 77 +++++++++++++++++++
> >>> .../selftests/bpf/progs/bpf_tracing_net.h | 4 +
> >>> 5 files changed, 143 insertions(+)
> >>> create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_unix.c
> >>>
> >>
> >> [...]
> >>
> >>> + /* The name of the abstract UNIX domain socket starts
> >>> + * with '\0' and can contain '\0'. The null bytes
> >>> + * should be escaped as done in unix_seq_show().
> >>> + */
> >>> + int i, len;
> >>> +
> >>
> >> no_alu32 variant probably isn't happy about using int for this, it
> >> probably does << 32, >> 32 dance and loses track of actual value in
> >> the loop. You can try using u64 instead.
> >
> > Sorry, I missed the no_alu32 test.
> > Changing int to __u64 fixed the error, thanks!
>
> Indeed for no_alu32, the index has << 32 and >> 32, which makes
> verifier *equivalent* register tracking not effective, see below:
>
> 96: r1 = r8
>
> 97: r1 <<= 32
>
> 98: r2 = r1
>
> 99: r2 >>= 32
>
> 100: if r2 > 109 goto +19 <LBB0_21>
>
> 101: r1 s>>= 32
>
> 102: if r1 s< 2 goto +17 <LBB0_21>
>
> 103: r9 = 1
>
> 104: r8 <<= 32
>
> 105: r8 >>= 32
>
> Because these shifting, r1/r2/r8 equivalence cannot be
> easily established, so verifier ends with conservative
> r8 and cannot verify program successfully.
>
> Using __u64 for 'i' and 'len', the upper bound is directly
> tested:
> 98: if r8 > 109 goto +16 <LBB0_21>
>
> 99: if r8 < 2 goto +15 <LBB0_21>
> and verifier is very happy with this.
Thanks for explanation!
I understand that the shift dance is to mimic the overflow of int because
actually 64-bit register is allocated to 'i' and 32-bit operations cannot
be used in no_alu32 test, so using __64 to remove the dance resolves it.
>
> >
> >
> >>
> >>> + len = unix_sk->addr->len - sizeof(short);
> >>> +
> >>> + BPF_SEQ_PRINTF(seq, " @");
> >>> +
> >>> + /* unix_mkname() tests this upper bound. */
> >>> + if (len < sizeof(struct sockaddr_un))
> >>> + for (i = 1; i < len; i++)
> >>
> >> if you move above if inside the loop to break out of the loop, does it
> >> change how Clang generates code?
> >>
> >> for (i = 1; i < len i++) {
> >> if (i >= sizeof(struct sockaddr_un))
> >> break;
> >> BPF_SEQ_PRINTF(...);
> >> }
> >
> > Yes, but there seems little defference.
> > Which is preferable?
> >
> > ---8<---
> > before (for inside if) <- -> after (if inside loop)
> > 96: 07 08 00 00 fe ff ff ff r8 += -2 | ; for (i = 1; i < len; i++) {
> > ; if (len < sizeof(struct sockaddr_un)) | 97: bf 81 00 00 00 00 00 00 r1 = r8
> > 97: 25 08 10 00 6d 00 00 00 if r8 > 109 goto +16 <LBB0_21> | 98: 07 01 00 00 fc ff ff ff r1 += -4
> > ; for (i = 1; i < len; i++) | 99: 25 01 12 00 6b 00 00 00 if r1 > 107 goto +18 <LBB0_21>
> > 98: a5 08 0f 00 02 00 00 00 if r8 < 2 goto +15 <LBB0_21> | 100: 07 08 00 00 fe ff ff ff r8 += -2
> > 99: b7 09 00 00 01 00 00 00 r9 = 1 | 101: b7 09 00 00 01 00 00 00 r9 = 1
> > 100: 05 00 16 00 00 00 00 00 goto +22 <LBB0_18> | 102: b7 06 00 00 02 00 00 00 r6 = 2
> > | 103: 05 00 17 00 00 00 00 00 goto +23 <LBB0_17>
> > ...
> > 111: 85 00 00 00 7e 00 00 00 call 126 | 113: b4 05 00 00 08 00 00 00 w5 = 8
> > ; for (i = 1; i < len; i++) | 114: 85 00 00 00 7e 00 00 00 call 126
> > 112: 07 09 00 00 01 00 00 00 r9 += 1 | ; for (i = 1; i < len; i++) {
> > 113: ad 89 09 00 00 00 00 00 if r9 < r8 goto +9 <LBB0_18> | 115: 25 08 02 00 6d 00 00 00 if r8 > 109 goto +2 <LBB0_21>
> > > 116: 07 09 00 00 01 00 00 00 r9 += 1
> > > ; for (i = 1; i < len; i++) {
> > > 117: ad 89 09 00 00 00 00 00 if r9 < r8 goto +9 <LBB0_17>
> > ---8<---
> >
> >
> >>
> >>
> >>> + BPF_SEQ_PRINTF(seq, "%c",
> >>> + unix_sk->addr->name->sun_path[i] ?:
> >>> + '@');
> >>> + }
> >>> + }
> >>> +
> >>> + BPF_SEQ_PRINTF(seq, "\n");
> >>> +
> >>> + return 0;
> >>> +}
> >>> diff --git a/tools/testing/selftests/bpf/progs/bpf_tracing_net.h b/tools/testing/selftests/bpf/progs/bpf_tracing_net.h
> >>> index 3af0998a0623..eef5646ddb19 100644
> >>> --- a/tools/testing/selftests/bpf/progs/bpf_tracing_net.h
> >>> +++ b/tools/testing/selftests/bpf/progs/bpf_tracing_net.h
> >>> @@ -5,6 +5,10 @@
> >>> #define AF_INET 2
> >>> #define AF_INET6 10
> >>>
> >>> +#define __SO_ACCEPTCON (1 << 16)
> >>> +#define UNIX_HASH_SIZE 256
> >>> +#define UNIX_ABSTRACT(unix_sk) (unix_sk->addr->hash < UNIX_HASH_SIZE)
> >>> +
> >>> #define SOL_TCP 6
> >>> #define TCP_CONGESTION 13
> >>> #define TCP_CA_NAME_MAX 16
> >>> --
> >>> 2.30.2
> >>>
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