Re: [PATCH v2 bpf 03/11] bpftool: use a local bpf_perf_event_value to fix accessing its fields

2023-06-06 14:02 UTC-0700 ~ Nick Desaulniers [off-list ref]

On Thu, Apr 21, 2022 at 12:39:04AM +0000, Alexander Lobakin wrote:

quoted

Fix the following error when building bpftool:

  CLANG   profiler.bpf.o
  CLANG   pid_iter.bpf.o
skeleton/profiler.bpf.c:18:21: error: invalid application of 'sizeof' to an incomplete type 'struct bpf_perf_event_value'
        __uint(value_size, sizeof(struct bpf_perf_event_value));
                           ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:13:39: note: expanded from macro '__uint'
tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helper_defs.h:7:8: note: forward declaration of 'struct bpf_perf_event_value'
struct bpf_perf_event_value;
       ^

struct bpf_perf_event_value is being used in the kernel only when
CONFIG_BPF_EVENTS is enabled, so it misses a BTF entry then.
Define struct bpf_perf_event_value___local with the
`preserve_access_index` attribute inside the pid_iter BPF prog to
allow compiling on any configs. It is a full mirror of a UAPI
structure, so is compatible both with and w/o CO-RE.
bpf_perf_event_read_value() requires a pointer of the original type,
so a cast is needed.

Hi Alexander,
What's the status of this series? I wasn't able to find a v3 on lore.

We received a report that OpenMandriva is carrying around this patch.
https://github.com/ClangBuiltLinux/linux/issues/1805.

+ Tomasz

Tomasz, do you have more info which particular configs can reproduce
this issue? Is this patch still necessary?

quoted

Fixes: 47c09d6a9f67 ("bpftool: Introduce "prog profile" command")

Hi Nick,

This patch is still necessary if you attempt to compile bpftool with
skeletons support, on a host with a kernel version lower than 5.15.

I took over on the bpftool patches from this series, and sent a new
version last month. Given that it only contains the bpftool patches, the
series has a different title and is not tagged as v3, but you can find
it here:

https://lore.kernel.org/all/20230512103354.48374-1-quentin@isovalent.com/t/#u (local)

Jiri (+Cc) found an issue with this set when CONFIG_BPF_EVENTS is
disabled. I need to replicate and investigate, but I've been short on
time to do that over the last few weeks.

Best,
Quentin