On Sat, Aug 22, 2020 at 12:49 AM Hao Luo haoluo@google.com wrote:
On Fri, Aug 21, 2020 at 8:31 PM Andrii Nakryiko andrii.nakryiko@gmail.com wrote:
On Fri, Aug 21, 2020 at 8:26 PM Andrii Nakryiko andrii.nakryiko@gmail.com wrote:
On Wed, Aug 19, 2020 at 3:42 PM Hao Luo haoluo@google.com wrote:
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 haoluo@google.com
The logic looks correct, few naming nits, but otherwise:
Acked-by: Andrii Nakryiko andriin@fb.com
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(-)
[...]
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