On Thu, Mar 10, 2022 at 2:54 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. > > Also, the following fixes tag should be added: > > Fixes: c195651e565a ("bpf: add bpf_get_stack helper") > > 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> > > > > --- > > 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) Namhyung, I think you also need to mirror the change in tools/include/uapi/linux/bpf.h
