Re: [PATCH 1/2] bpf: Adjust BPF stack helper functions to accommodate skip > 0

[Namhyung Kim]

Hello,

On Thu, Mar 10, 2022 at 2:55 PM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 3/10/22 12:22 AM, Namhyung Kim wrote:
> > Let's say that the caller has storage for num_elem stack frames.  Then,
> > the BPF stack helper functions walk the stack for only num_elem frames.
> > This means that if skip > 0, one keeps only 'num_elem - skip' frames.
> >
> > This is because it sets init_nr in the perf_callchain_entry to the end
> > of the buffer to save num_elem entries only.  I believe it was because
> > the perf callchain code unwound the stack frames until it reached the
> > global max size (sysctl_perf_event_max_stack).
> >
> > However it now has perf_callchain_entry_ctx.max_stack to limit the
> > iteration locally.  This simplifies the code to handle init_nr in the
> > BPF callstack entries and removes the confusion with the perf_event's
> > __PERF_SAMPLE_CALLCHAIN_EARLY which sets init_nr to 0.
> >
> > Also change the comment on bpf_get_stack() in the header file to be
> > more explicit what the return value means.
> >
> > Based-on-patch-by: Eugene Loh <eugene.loh@xxxxxxxxxx>
> > Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
>
> The change looks good to me. This patch actually fixed a bug
> discussed below:
>
>
> https://lore.kernel.org/bpf/30a7b5d5-6726-1cc2-eaee-8da2828a9a9c@xxxxxxxxxx/
>
> A reference to the above link in the commit message
> will be useful for people to understand better with an
> example.

Ok, will do.

>
> Also, the following fixes tag should be added:
>
> Fixes: c195651e565a ("bpf: add bpf_get_stack helper")

Sure.

>
> Since the bug needs skip > 0 which is seldomly used,
> and the current returned stack is still correct although
> with less entries, I guess that is why less people
> complains.
>
> Anyway, ack the patch:
> Acked-by: Yonghong Song <yhs@xxxxxx>

Thanks for your review!

Namhyung


>
>
> > ---
> >   include/uapi/linux/bpf.h |  4 +--
> >   kernel/bpf/stackmap.c    | 56 +++++++++++++++++-----------------------
> >   2 files changed, 26 insertions(+), 34 deletions(-)
> >
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index b0383d371b9a..77f4a022c60c 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -2975,8 +2975,8 @@ union bpf_attr {
> >    *
> >    *                  # sysctl kernel.perf_event_max_stack=<new value>
> >    *  Return
> > - *           A non-negative value equal to or less than *size* on success,
> > - *           or a negative error in case of failure.
> > + *           The non-negative copied *buf* length equal to or less than
> > + *           *size* on success, or a negative error in case of failure.
> >    *
> >    * long bpf_skb_load_bytes_relative(const void *skb, u32 offset, void *to, u32 len, u32 start_header)
> [...]