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

From: Andrii Nakryiko
Date: Tue Apr 27 2021 - 14:03:48 EST

On Tue, Apr 27, 2021 at 2:51 AM Florent Revest <revest@xxxxxxxxxxxx> wrote:
>
> On Tue, Apr 27, 2021 at 8:35 AM Rasmus Villemoes
> <linux@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > On 26/04/2021 23.08, Florent Revest wrote:
> > > On Mon, Apr 26, 2021 at 6:19 PM Andrii Nakryiko
> > > <andrii.nakryiko@xxxxxxxxx> wrote:
> > >>
> > >> On Mon, Apr 26, 2021 at 3:10 AM Florent Revest <revest@xxxxxxxxxxxx> wrote:
> > >>>
> > >>> On Sat, Apr 24, 2021 at 12:38 AM Andrii Nakryiko
> > >>> <andrii.nakryiko@xxxxxxxxx> wrote:
> > >>>>
> > >>>> On Mon, Apr 19, 2021 at 8:52 AM Florent Revest <revest@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > >>>>> ---
> > >>>>> .../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
> > >>
> > >> I'm puzzled too, have no idea. I also can't repro this with vmtest.sh.
> > >> But I can quite reliably reproduce with my local ArchLinux-based qemu
> > >> image with different config (see [0] for config itself). So please try
> > >> with my config and see if that helps to repro. If not, I'll have to
> > >> debug it on my own later.
> > >>
> > >> [0] https://gist.github.com/anakryiko/4b6ae21680842bdeacca8fa99d378048
> > >
> > > I tried that config on the same commit 87bd9e602 (bpf-next/master)
> > > with my debian-based qemu image and I still can't reproduce the issue
> > > :| If I can be of any help let me know, I'd be happy to help
> > >
> >
> > It's not really clear to me if this is before or after the rewrite to
> > use bprintf, but regardless, in those two patches this caught my attention:
>
> I tried to reproduce Andrii's bug both before and after the bprintf
> rewrite but I think he meant before.

I'm running on the latest bpf-next master, but I don't think it's
related to bprintf change.

>
> > u64 args[MAX_TRACE_PRINTK_VARARGS] = { arg1, arg2, arg3 };
> > - enum bpf_printf_mod_type mod[MAX_TRACE_PRINTK_VARARGS];
> > + u32 *bin_args;
> > static char buf[BPF_TRACE_PRINTK_SIZE];
> > unsigned long flags;
> > int ret;
> >
> > - ret = bpf_printf_prepare(fmt, fmt_size, args, args, mod,
> > - MAX_TRACE_PRINTK_VARARGS);
> > + ret = bpf_bprintf_prepare(fmt, fmt_size, args, &bin_args,
> > + MAX_TRACE_PRINTK_VARARGS);
> > if (ret < 0)
> > return ret;
> >
> > - ret = snprintf(buf, sizeof(buf), fmt, BPF_CAST_FMT_ARG(0, args, mod),
> > - BPF_CAST_FMT_ARG(1, args, mod), BPF_CAST_FMT_ARG(2, args, mod));
> > - /* snprintf() will not append null for zero-length strings */
> > - if (ret == 0)
> > - buf[0] = '\0';
> > + ret = bstr_printf(buf, sizeof(buf), fmt, bin_args);
> >
> > raw_spin_lock_irqsave(&trace_printk_lock, flags);
> > trace_bpf_trace_printk(buf);
> > raw_spin_unlock_irqrestore(&trace_printk_lock, flags);
> >
> > Why isn't the write to buf[] protected by that spinlock? Or put another
> > way, what protects buf[] from concurrent writes?
>
> You're right, that is a bug, I missed that buf was static and thought
> it was just on the stack. That snprintf call should be after the
> raw_spin_lock_irqsave. I'll send a patch. Thank you Rasmus. (before my
> snprintf series, there was a vsprintf after the raw_spin_lock_irqsave)

Can you please also clean up unnecessary ()s you added in at least a
few places. Thanks.

>
> > Probably the test cases are not run in parallel, but this is the kind of
> > thing that would give those symptoms.
>
> I think it's a separate issue from what Andrii reported though because
> the flaky test exercises the bpf_snprintf helper and this buf spinlock
> bug you just found only affects the bpf_trace_printk helper.
>
> That being said, it does smell a little bit like a concurrency issue
> too, indeed. The bpf_snprintf test program is a raw_tp/sys_enter so it
> attaches to all syscall entries and most likely gets executed many
> more times than necessary and probably on parallel CPUs. The "pad_out"
> buffer they write to is unique and not locked so maybe the test's
> userspace reads pad_out while another CPU is writing on it and if the
> string output goes through a stage where it is " 4 0000" before
> being " 4 000", we might read at the wrong time. That being said, I
> would find it weird that this happens as much as 50% of the time and
> always specifically on that test case.
>
> Andrii could you maybe try changing the prog type to
> "tp/syscalls/sys_enter_nanosleep" on the machine where you can
> reproduce this bug ?

Yes, it helps. I can't repro it easily anymore. I think the right fix,
though, should be to filter by tid, not change the tracepoint.

I think what's happening is we see the string right before bstr_printf
does zero-termination with end[-1] = '\0'; So in some cases we see
truncated string, in others we see untruncated one.

>
> > Rasmus