Re: accessing global and per-cpu vars

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Thu, May 21, 2020 at 11:58:47AM -0700, Andrii Nakryiko escreveu:
> On Thu, May 21, 2020 at 10:07 AM Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> > 2. teach pahole to store ' A ' annotated kallsyms into vmlinux BTF as
> > BTF_KIND_VAR.
> > There are ~300 of them, so should be minimal increase in size.
> 
> I thought we'd do that based on section name? Or we will actually
> teach pahole to extract kallsyms from vmlinux image?

No need to touch kallsyms:

  net/core/filter.c
  
  DEFINE_PER_CPU(struct bpf_redirect_info, bpf_redirect_info);
  
  # grep -w bpf_redirect_info /proc/kallsyms
  000000000002a160 A bpf_redirect_info
  #
  # readelf -s ~acme/git/build/v5.7-rc2+/vmlinux | grep bpf_redirect_info
  113637: 000000000002a2e0    32 OBJECT  GLOBAL DEFAULT   34 bpf_redirect_info
  #

Its in the ELF symtab.

[root@quaco ~]# grep ' A ' /proc/kallsyms | wc -l
351
[root@quaco ~]# readelf -s ~acme/git/build/v5.7-rc2+/vmlinux | grep "OBJECT  GLOBAL" | wc -l
3221
[root@quaco ~]#

So ' A ' in kallsyms needs some extra info from the symtab in addition
to being OBJECT GLOBAL, checking...
 
> There was step 1.5 (or even 0.5) to see if it's feasible to add not
> just per-CPU variables as well.

- Arnaldo