Re: [PATCH bpf-next] bpf: btf: don't log ignored BTF mismatches

[Connor O'Brien]

On Thu, Dec 8, 2022 at 4:03 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, Dec 8, 2022 at 10:01 AM Yonghong Song <yhs@xxxxxxxx> wrote:
> >
> >
> >
> > On 12/7/22 6:19 PM, Connor O'Brien wrote:
> > > On Wed, Nov 9, 2022 at 8:45 AM Yonghong Song <yhs@xxxxxxxx> wrote:
> > >>
> > >>
> > >>
> > >> On 11/8/22 6:41 PM, Connor O'Brien wrote:
> > >>> Enabling CONFIG_MODULE_ALLOW_BTF_MISMATCH is an indication that BTF
> > >>> mismatches are expected and module loading should proceed
> > >>> anyway. Logging with pr_warn() on every one of these "benign"
> > >>> mismatches creates unnecessary noise when many such modules are
> > >>> loaded. Instead, limit logging to the case where a BTF mismatch
> > >>> actually prevents a module form loading.
> > >>>
> > >>> Signed-off-by: Connor O'Brien <connoro@xxxxxxxxxx>
> > >>> ---
> > >>>    kernel/bpf/btf.c | 7 ++++---
> > >>>    1 file changed, 4 insertions(+), 3 deletions(-)
> > >>>
> > >>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > >>> index 5579ff3a5b54..406370487413 100644
> > >>> --- a/kernel/bpf/btf.c
> > >>> +++ b/kernel/bpf/btf.c
> > >>> @@ -7190,11 +7190,12 @@ static int btf_module_notify(struct notifier_block *nb, unsigned long op,
> > >>>                }
> > >>>                btf = btf_parse_module(mod->name, mod->btf_data, mod->btf_data_size);
> > >>>                if (IS_ERR(btf)) {
> > >>> -                     pr_warn("failed to validate module [%s] BTF: %ld\n",
> > >>> -                             mod->name, PTR_ERR(btf));
> > >>
> > >> I think such warning still useful even with
> > >> CONFIG_MODULE_ALLOW_BTF_MISMATCH.
> > >> Can we use pr_warn_ratelimited instead of pr_warn in the above to
> > >> avoid excessive warnings?
> > >
> > > I gave this a try on a Pixel 6 but I'm not sure it quite addresses the
> > > issue. The amount of logging doesn't seem to decrease much, I think
> > > because the interval between loading one mismatched module and the
> > > next can be greater than the default rate limit. To my mind, the issue
> > > is the total volume of these messages more so than their rate.
> > >
> > > For context, Android devices using the GKI may load hundreds of
> > > separately-built modules, and BTF mismatches are possible for any/all
> > > of these. It was pointed out to me that btf_verifier_log_type can also
> > > print several more lines per mismatched module. ~5 lines of logging
> > > for each mismatched module can start to add up, in terms of both
> > > overhead and the noise added to the kernel logs.
> > >
> > > This is more subjective but I think the warnings also read as though
> > > this is a more serious failure that might prevent affected modules
> > > from working correctly; anecdotally, I've gotten multiple questions
> > > about them asking if something is broken. This can be a red herring
> > > for anyone unfamiliar with BTF who is reading the logs to debug
> > > unrelated issues. In the CONFIG_MODULE_ALLOW_BTF_MISMATCH=y case the
> > > flood of warnings seems out of proportion to the actual result
> > > (modules still load successfully, just without debug info) especially
> > > since the user has explicitly enabled a config saying they expect
> > > mismatches.
> > >
> > > If there needs to be some logging in the "mismatch allowed" case,
> > > could an acceptable middle ground be to use pr_warn_once to send a
> >
> > So it looks like pr_warn_ratelimited still produces a lot of warning.
> > In this case, I guess pr_warn_once should be okay.
>
> Maybe pr_warn_once generic "some kernel module BTF mismatched". And
> also warn per-module message with details if
> CONFIG_MODULE_ALLOW_BTF_MISMATCH is not set?
>

Yeah, I figured the logging behavior should remain unchanged when
CONFIG_MODULE_ALLOW_BTF_MISMATCH is unset, the existing verbose
logging makes sense when every mismatch stops a module from loading.

Then for the pr_warn_once case something like "Kernel module BTF
mismatch detected, BTF debug info may be unavailable for some modules"



> >
> > > single message reporting that mismatches were detected & module BTF
> > > debug info might be unavailable? Alternatively, if we could opt out of
> > > module BTF loading then that would also avoid this issue, but that's
> > > already been proposed before ([1], [2]) so I thought working with the
> > > existing config option might be preferred.
> > >
> > > [1] https://lore.kernel.org/bpf/20220209052141.140063-1-connoro@xxxxxxxxxx/
> > > [2] https://lore.kernel.org/bpf/20221004222725.2813510-1-sdf@xxxxxxxxxx/