It's trivial for user to trigger "verifier log line truncated" warning,
as verifier has a fixed-sized buffer of 1024 bytes (as of now), and there are at
least two pieces of user-provided information that can be output through
this buffer, and both can be arbitrarily sized by user:
- BTF names;
- BTF.ext source code lines strings.
Verifier log buffer should be properly sized for typical verifier state
output. But it's sort-of expected that this buffer won't be long enough
in some circumstances. So let's drop the check. In any case code will
work correctly, at worst truncating a part of a single line output.
Reported-by: syzbot+8b2a08dfbd25fd933d75@syzkaller.appspotmail.com
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230516180409.3549088-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Drop the log_size>0 and log_buf!=NULL condition when log_level>0. This
allows users to request log_true_size of a full log without providing
actual (even if small) log buffer. Verifier log handling code was mostly
ready to handle NULL log->ubuf, so only few small changes were necessary
to prevent NULL log->ubuf from causing problems.
Note, that if user provided NULL log_buf with log_level>0 we don't
consider this a log truncation, and thus won't return -ENOSPC.
We also enforce that either (log_buf==NULL && log_size==0) or
(log_buf!=NULL && log_size>0).
Suggested-by: Lorenz Bauer <lmb@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/bpf/20230406234205.323208-15-andrii@kernel.org
Simplify internal verifier log API down to bpf_vlog_init() and
bpf_vlog_finalize(). The former handles input arguments validation in
one place and makes it easier to change it. The latter subsumes -ENOSPC
(truncation) and -EFAULT handling and simplifies both caller's code
(bpf_check() and btf_parse()).
For btf_parse(), this patch also makes sure that verifier log
finalization happens even if there is some error condition during BTF
verification process prior to normal finalization step.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/bpf/20230406234205.323208-14-andrii@kernel.org
Change how we do accounting in BPF_LOG_FIXED mode and adopt log->end_pos
as *logical* log position. This means that we can go beyond physical log
buffer size now and be able to tell what log buffer size should be to
fit entire log contents without -ENOSPC.
To do this for BPF_LOG_FIXED mode, we need to remove a short-circuiting
logic of not vsnprintf()'ing further log content once we filled up
user-provided buffer, which is done by bpf_verifier_log_needed() checks.
We modify these checks to always keep going if log->level is non-zero
(i.e., log is requested), even if log->ubuf was NULL'ed out due to
copying data to user-space, or if entire log buffer is physically full.
We adopt bpf_verifier_vlog() routine to work correctly with
log->ubuf == NULL condition, performing log formatting into temporary
kernel buffer, doing all the necessary accounting, but just avoiding
copying data out if buffer is full or NULL'ed out.
With these changes, it's now possible to do this sort of determination of
log contents size in both BPF_LOG_FIXED and default rolling log mode.
We need to keep in mind bpf_vlog_reset(), though, which shrinks log
contents after successful verification of a particular code path. This
log reset means that log->end_pos isn't always increasing, so to return
back to users what should be the log buffer size to fit all log content
without causing -ENOSPC even in the presence of log resetting, we need
to keep maximum over "lifetime" of logging. We do this accounting in
bpf_vlog_update_len_max() helper.
A related and subtle aspect is that with this logical log->end_pos even in
BPF_LOG_FIXED mode we could temporary "overflow" buffer, but then reset
it back with bpf_vlog_reset() to a position inside user-supplied
log_buf. In such situation we still want to properly maintain
terminating zero. We will eventually return -ENOSPC even if final log
buffer is small (we detect this through log->len_max check). This
behavior is simpler to reason about and is consistent with current
behavior of verifier log. Handling of this required a small addition to
bpf_vlog_reset() logic to avoid doing put_user() beyond physical log
buffer dimensions.
Another issue to keep in mind is that we limit log buffer size to 32-bit
value and keep such log length as u32, but theoretically verifier could
produce huge log stretching beyond 4GB. Instead of keeping (and later
returning) 64-bit log length, we cap it at UINT_MAX. Current UAPI makes
it impossible to specify log buffer size bigger than 4GB anyways, so we
don't really loose anything here and keep everything consistently 32-bit
in UAPI. This property will be utilized in next patch.
Doing the same determination of maximum log buffer for rolling mode is
trivial, as log->end_pos and log->start_pos are already logical
positions, so there is nothing new there.
These changes do incidentally fix one small issue with previous logging
logic. Previously, if use provided log buffer of size N, and actual log
output was exactly N-1 bytes + terminating \0, kernel logic coun't
distinguish this condition from log truncation scenario which would end
up with truncated log contents of N-1 bytes + terminating \0 as well.
But now with log->end_pos being logical position that could go beyond
actual log buffer size, we can distinguish these two conditions, which
we do in this patch. This plays nicely with returning log_size_actual
(implemented in UAPI in the next patch), as we can now guarantee that if
user takes such log_size_actual and provides log buffer of that exact
size, they will not get -ENOSPC in return.
All in all, all these changes do conceptually unify fixed and rolling
log modes much better, and allow a nice feature requested by users:
knowing what should be the size of the buffer to avoid -ENOSPC.
We'll plumb this through the UAPI and the code in the next patch.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/bpf/20230406234205.323208-12-andrii@kernel.org
Verifier log position reset is meaningless in BPF_LOG_KERNEL mode, so
just exit early in bpf_vlog_reset() if log->level is BPF_LOG_KERNEL.
This avoid meaningless put_user() into NULL log->ubuf.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/bpf/20230406234205.323208-8-andrii@kernel.org
Currently, if user-supplied log buffer to collect BPF verifier log turns
out to be too small to contain full log, bpf() syscall returns -ENOSPC,
fails BPF program verification/load, and preserves first N-1 bytes of
the 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, what's
worse, in practice, the end of the log most often is way more important
than the beginning, but verifier stops emitting log as soon as initial
log buffer is filled up.
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, verifier will keep overwriting previously written log,
effectively treating user's log buffer as a ring buffer. -ENOSPC is
still going to be returned at the end, to notify user that log contents
was truncated, but the important last N bytes of the log would be
returned, which might be all that user really needs. This consistent
-ENOSPC behavior, regardless of rotating or fixed log behavior, allows
to prevent backwards compatibility breakage. The only user-visible
change is which portion of verifier log user ends up seeing *if buffer
is too small*. Given contents of verifier log itself is not an ABI,
there is no breakage due to this behavior change. Specialized tools that
rely on specific contents of verifier log in -ENOSPC scenario are
expected to be easily adapted to accommodate old and new behaviors.
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.
BPF_LOG_FIXED flag is also a simple and straightforward way to check if
BPF verifier supports rotating behavior.
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>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/bpf/20230406234205.323208-4-andrii@kernel.org
It's not clear why we have 128 as minimum size, but it makes testing
harder and seems unnecessary, as we carefully handle truncation
scenarios and use proper snprintf variants. So remove this limitation
and just enforce positive length for log buffer.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/bpf/20230406234205.323208-3-andrii@kernel.org
kernel/bpf/verifier.c file is large and growing larger all the time. So
it's good to start splitting off more or less self-contained parts into
separate files to keep source code size (somewhat) somewhat under
control.
This patch is a one step in this direction, moving some of BPF verifier log
routines into a separate kernel/bpf/log.c. Right now it's most low-level
and isolated routines to append data to log, reset log to previous
position, etc. Eventually we could probably move verifier state
printing logic here as well, but this patch doesn't attempt to do that
yet.
Subsequent patches will add more logic to verifier log management, so
having basics in a separate file will make sure verifier.c doesn't grow
more with new changes.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/bpf/20230406234205.323208-2-andrii@kernel.org
