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From: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
To: <yhs@fb.com>
Cc: <andrii.nakryiko@gmail.com>, <andrii@kernel.org>,
	<ast@kernel.org>, <benh@amazon.com>, <bpf@vger.kernel.org>,
	<daniel@iogearbox.net>, <davem@davemloft.net>,
	<john.fastabend@gmail.com>, <kafai@fb.com>, <kpsingh@kernel.org>,
	<kuba@kernel.org>, <kuni1840@gmail.com>, <kuniyu@amazon.co.jp>,
	<netdev@vger.kernel.org>, <songliubraving@fb.com>
Subject: Re: [PATCH v5 bpf-next 3/4] selftest/bpf: Implement sample UNIX domain socket iterator program.
Date: Mon, 16 Aug 2021 21:45:22 +0900	[thread overview]
Message-ID: <20210816124522.38487-1-kuniyu@amazon.co.jp> (raw)
In-Reply-To: <c5b4e17b-97b3-061a-6956-6f21c5ad9581@fb.com>

From:   Yonghong Song <yhs@fb.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@gmail.com>
> > Date:   Fri, 13 Aug 2021 16:25:53 -0700
> >> On Thu, Aug 12, 2021 at 9:46 AM Kuniyuki Iwashima <kuniyu@amazon.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@World@
> >>>
> >>>    # 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@World@
> >>>
> >>> 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@amazon.co.jp>
> >>> Acked-by: Yonghong Song <yhs@fb.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
> >>>

  reply	other threads:[~2021-08-16 12:45 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-12 16:45 [PATCH v5 bpf-next 0/4] BPF iterator for UNIX domain socket Kuniyuki Iwashima
2021-08-12 16:45 ` [PATCH v5 bpf-next 1/4] bpf: af_unix: Implement " Kuniyuki Iwashima
2021-08-12 16:45 ` [PATCH v5 bpf-next 2/4] bpf: Support "%c" in bpf_bprintf_prepare() Kuniyuki Iwashima
2021-08-12 16:45 ` [PATCH v5 bpf-next 3/4] selftest/bpf: Implement sample UNIX domain socket iterator program Kuniyuki Iwashima
2021-08-13 23:25   ` Andrii Nakryiko
2021-08-14  0:21     ` Kuniyuki Iwashima
2021-08-14  0:26       ` Andrii Nakryiko
2021-08-14  1:00         ` Kuniyuki Iwashima
2021-08-15 18:10       ` Yonghong Song
2021-08-16 12:45         ` Kuniyuki Iwashima [this message]
2021-08-12 16:45 ` [PATCH v5 bpf-next 4/4] selftest/bpf: Extend the bpf_snprintf() test for "%c" Kuniyuki Iwashima
2021-08-13 23:28   ` Andrii Nakryiko
2021-08-13 23:30     ` Andrii Nakryiko
2021-08-14  0:37       ` Kuniyuki Iwashima

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