Re: [PATCH bpf-next 3/6] bpf: switch BPF verifier log to be a rotating log by default

Andrii Nakryiko
2023-03-17T16:45:30Z

On Thu, Mar 16, 2023 at 8:54 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Thu, Mar 16, 2023 at 11:30:10AM -0700, Andrii Nakryiko wrote:
> > Currently, if user-supplied log buffer to collect BPF verifier log turns
> > out to be too small to contain full log, bpf() syscall return -ENOSPC,
> > fails BPF program verification/load, but preserves first N-1 bytes of
> > verifier log (where N is the size of user-supplied buffer).
> >
> > This is problematic in a bunch of common scenarios, especially when
> > working with real-world BPF programs that tend to be pretty complex as
> > far as verification goes and require big log buffers. Typically, it's
> > when debugging tricky cases at log level 2 (verbose). Also, when BPF program
> > is successfully validated, log level 2 is the only way to actually see
> > verifier state progression and all the important details.
> >
> > Even with log level 1, it's possible to get -ENOSPC even if the final
> > verifier log fits in log buffer, if there is a code path that's deep
> > enough to fill up entire log, even if normally it would be reset later
> > on (there is a logic to chop off successfully validated portions of BPF
> > verifier log).
> >
> > In short, it's not always possible to pre-size log buffer. Also, in
> > practice, the end of the log most often is way more important than the
> > beginning.
> >
> > This patch switches BPF verifier log behavior to effectively behave as
> > rotating log. That is, if user-supplied log buffer turns out to be too
> > short, instead of failing with -ENOSPC, verifier will keep overwriting
> > previously written log, effectively treating user's log buffer as a ring
> > buffer.
> >
> > Importantly, though, to preserve good user experience and not require
> > every user-space application to adopt to this new behavior, before
> > exiting to user-space verifier will rotate log (in place) to make it
> > start at the very beginning of user buffer as a continuous
> > zero-terminated string. The contents will be a chopped off N-1 last
> > bytes of full verifier log, of course.
> >
> > Given beginning of log is sometimes important as well, we add
> > BPF_LOG_FIXED (which equals 8) flag to force old behavior, which allows
> > tools like veristat to request first part of verifier log, if necessary.
> >
> > On the implementation side, conceptually, it's all simple. We maintain
> > 64-bit logical start and end positions. If we need to truncate the log,
> > start position will be adjusted accordingly to lag end position by
> > N bytes. We then use those logical positions to calculate their matching
> > actual positions in user buffer and handle wrap around the end of the
> > buffer properly. Finally, right before returning from bpf_check(), we
> > rotate user log buffer contents in-place as necessary, to make log
> > contents contiguous. See comments in relevant functions for details.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > ---
> > include/linux/bpf_verifier.h | 32 ++-
> > kernel/bpf/log.c | 182 +++++++++++++++++-
> > kernel/bpf/verifier.c | 17 +-
> > .../selftests/bpf/prog_tests/log_fixup.c | 1 +
> > 4 files changed, 209 insertions(+), 23 deletions(-)
> >
> > diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> > index 83dff25545ee..cff11c99ed9a 100644
> > --- a/include/linux/bpf_verifier.h
> > +++ b/include/linux/bpf_verifier.h
> > @@ -491,25 +491,42 @@ struct bpf_insn_aux_data {
> > #define BPF_VERIFIER_TMP_LOG_SIZE 1024
> >
> > struct bpf_verifier_log {
> > - u32 level;
> > - char kbuf[BPF_VERIFIER_TMP_LOG_SIZE];
> > + /* Logical start and end positions of a "log window" of the verifier log.
> > + * start_pos == 0 means we haven't truncated anything.
> > + * Once truncation starts to happen, start_pos + len_total == end_pos,
> > + * except during log reset situations, in which (end_pos - start_pos)
> > + * might get smaller than len_total (see bpf_vlog_reset()).
> > + * Generally, (end_pos - start_pos) gives number of useful data in
> > + * user log buffer.
> > + */
> > + u64 start_pos;
> > + u64 end_pos;
> ...
> >
> > -void bpf_vlog_reset(struct bpf_verifier_log *log, u32 new_pos)
> > +void bpf_vlog_reset(struct bpf_verifier_log *log, u64 new_pos)
> > {
> > char zero = 0;
> >
> > if (!bpf_verifier_log_needed(log))
> > return;
> >
> > - log->len_used = new_pos;
> > + /* if position to which we reset is beyond current log window, then we
> > + * didn't preserve any useful content and should adjust adjust
> > + * start_pos to end up with an empty log (start_pos == end_pos)
> > + */
> > + log->end_pos = new_pos;
> > + if (log->end_pos < log->start_pos)
> > + log->start_pos = log->end_pos;
> > +
> > if (put_user(zero, log->ubuf + new_pos))
>
> Haven't analyzed the code in details and asking based on comments...
> new_pos can be bigger than ubuf size and above write will be out of bounds, no?

Oh, great catch! Yes, new_pos is "logical position", so definitely can
go beyond buffer bounds. It should have been `log->ubuf + (new_pos %
log->len_total)`, of course. I'll improve tests to validate that the
buffer beyond allowed boundaries isn't modified at all, to catch this.