Re: [PATCH bpf-next v5 6/6] selftests/bpf: Add a series of tests for bpf_snprintf

From: Florent Revest <revest@chromium.org>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>, Yonghong Song <yhs@fb.com>,
	KP Singh <kpsingh@kernel.org>,
	Brendan Jackman <jackmanb@chromium.org>,
	open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH bpf-next v5 6/6] selftests/bpf: Add a series of tests for bpf_snprintf
Date: Mon, 26 Apr 2021 12:10:04 +0200	[thread overview]
Message-ID: <CABRcYm+=XSt_U-19eYXU8+XwDUXoBGQMROMbm6xk9P9OHnUW_A@mail.gmail.com> (raw)
In-Reply-To: <CAEf4BzZUM4hb9owhompwARabRvRbCYxBrpgXSdXM8RRm42tU1A@mail.gmail.com>

On Sat, Apr 24, 2021 at 12:38 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Mon, Apr 19, 2021 at 8:52 AM Florent Revest <revest@chromium.org> wrote:
> >
> > The "positive" part tests all format specifiers when things go well.
> >
> > The "negative" part makes sure that incorrect format strings fail at
> > load time.
> >
> > Signed-off-by: Florent Revest <revest@chromium.org>
> > ---
> >  .../selftests/bpf/prog_tests/snprintf.c       | 125 ++++++++++++++++++
> >  .../selftests/bpf/progs/test_snprintf.c       |  73 ++++++++++
> >  .../bpf/progs/test_snprintf_single.c          |  20 +++
> >  3 files changed, 218 insertions(+)
> >  create mode 100644 tools/testing/selftests/bpf/prog_tests/snprintf.c
> >  create mode 100644 tools/testing/selftests/bpf/progs/test_snprintf.c
> >  create mode 100644 tools/testing/selftests/bpf/progs/test_snprintf_single.c
> >
> > diff --git a/tools/testing/selftests/bpf/prog_tests/snprintf.c b/tools/testing/selftests/bpf/prog_tests/snprintf.c
> > new file mode 100644
> > index 000000000000..a958c22aec75
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/prog_tests/snprintf.c
> > @@ -0,0 +1,125 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/* Copyright (c) 2021 Google LLC. */
> > +
> > +#include <test_progs.h>
> > +#include "test_snprintf.skel.h"
> > +#include "test_snprintf_single.skel.h"
> > +
> > +#define EXP_NUM_OUT  "-8 9 96 -424242 1337 DABBAD00"
> > +#define EXP_NUM_RET  sizeof(EXP_NUM_OUT)
> > +
> > +#define EXP_IP_OUT   "127.000.000.001 0000:0000:0000:0000:0000:0000:0000:0001"
> > +#define EXP_IP_RET   sizeof(EXP_IP_OUT)
> > +
> > +/* The third specifier, %pB, depends on compiler inlining so don't check it */
> > +#define EXP_SYM_OUT  "schedule schedule+0x0/"
> > +#define MIN_SYM_RET  sizeof(EXP_SYM_OUT)
> > +
> > +/* The third specifier, %p, is a hashed pointer which changes on every reboot */
> > +#define EXP_ADDR_OUT "0000000000000000 ffff00000add4e55 "
> > +#define EXP_ADDR_RET sizeof(EXP_ADDR_OUT "unknownhashedptr")
> > +
> > +#define EXP_STR_OUT  "str1 longstr"
> > +#define EXP_STR_RET  sizeof(EXP_STR_OUT)
> > +
> > +#define EXP_OVER_OUT "%over"
> > +#define EXP_OVER_RET 10
> > +
> > +#define EXP_PAD_OUT "    4 000"
>
> Roughly 50% of the time I get failure for this test case:
>
> test_snprintf_positive:FAIL:pad_out unexpected pad_out: actual '    4
> 0000' != expected '    4 000'
>
> Re-running this test case immediately passes. Running again most
> probably fails. Please take a look.

Do you have more information on how to reproduce this ?
I spinned up a VM at 87bd9e602 with ./vmtest -s and then run this script:

#!/bin/sh
for i in `seq 1000`
do
  ./test_progs -t snprintf
  if [ $? -ne 0 ];
  then
    echo FAILURE
    exit 1
  fi
done

The thousand executions passed.

This is a bit concerning because your unexpected_pad_out seems to have
an extra '0' so it ends up with strlen(pad_out)=11 but
sizeof(pad_out)=10. The actual string writing is not really done by
our helper code but by the snprintf implementation (str and str_size
are only given to snprintf()) so I'd expect the truncation to work
well there. I'm a bit puzzled