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

[Yonghong Song]

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)
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