On Sat, Aug 22, 2020 at 12:49 AM Hao Luo [off-list ref] wrote:

On Fri, Aug 21, 2020 at 8:31 PM Andrii Nakryiko
[off-list ref] wrote:

quoted

On Fri, Aug 21, 2020 at 8:26 PM Andrii Nakryiko
[off-list ref] wrote:

quoted

On Wed, Aug 19, 2020 at 3:42 PM Hao Luo [off-list ref] wrote:

quoted

Add bpf_per_cpu_ptr() to help bpf programs access percpu vars.
bpf_per_cpu_ptr() has the same semantic as per_cpu_ptr() in the kernel
except that it may return NULL. This happens when the cpu parameter is
out of range. So the caller must check the returned value.

Signed-off-by: Hao Luo <redacted>
---

The logic looks correct, few naming nits, but otherwise:

Acked-by: Andrii Nakryiko <redacted>

quoted

 include/linux/bpf.h      |  3 ++
 include/linux/btf.h      | 11 +++++++
 include/uapi/linux/bpf.h | 14 +++++++++
 kernel/bpf/btf.c         | 10 -------
 kernel/bpf/verifier.c    | 64 ++++++++++++++++++++++++++++++++++++++--
 kernel/trace/bpf_trace.c | 18 +++++++++++
 6 files changed, 107 insertions(+), 13 deletions(-)

[...]

quoted

btw, having bpf_this_cpu_ptr(const void *ptr) seems worthwhile as well, WDYT?

It's probably not a good idea, IMHO. How does it interact with
preemption? Should we treat it as __this_cpu_ptr()? If so, I feel it's
easy to be misused, if the bpf program is called in a preemptible
context.

Btw, is bpf programs always called with preemption disabled? How about
interrupts? I haven't thought about these questions before but I think
they matter as we start to have more ways for bpf programs to interact
with the kernel.

non-sleepable BPF is always disabling CPU migration, so there is no
problem with this_cpu_ptr. For sleepable not sure, but we can disable
this helper for sleepable BPF programs, if that's a problem.

Best,
Hao