libbpf/bpftool inconsistent handling og .data and .bss ?

[Luigi Rizzo <rizzo@iet.unipi.it>]

I am experiencing some weirdness in global variables handling
in bpftool and libbpf, as described below.

This happens happen with code in foo_bpf.c compiled with
   clang-10 -O2 -Wall -Werror -target bpf ...
and subsequently exported with
   bpftool gen skeleton ...
(i have tried bpftool 5.8.7 and 5.9.0-rc6)

1. uninitialized globals are not recognised
   The following code in the bpf program

     int x;
     SEC("fentry/bar")
     int BPF_PROG(bar) { return 0;}

   compiles ok but bpftool then complains

      libbpf: prog 'bar': invalid relo against 'x' in special section
0xfff2; forgot to initialize global var?..

   The error disappears if I initialize x=0 or x=1
   (in the skeleton, x=0 ends up in .bss, x=1 ends up in .data)

2. .bss overrides from userspace are not seen in bpf at runtime

    In foo_bpf.c I have "int x = 0;"
    In the userspace program, before foo_bpf__load(), I do
       obj->bss->x = 1
    but after attach, the bpf code does not see the change, ie
        "if (x == 0) { .. } else { .. }"
    always takes the first branch.

    If I initialize "int x = 2" and then do
       obj->data->x = 1
    the update is seen correctly ie
          "if (x == 2) { .. } else { .. }"
     takes one or the other depending on whether userspace overrides
     the value before foo_bpf__load()

3. .data overrides do not seem to work for non-scalar types
    In foo_bpf.c I have
          struct one { int a; }; // type also visible to userspace
          struct one x { .a = 2 }; // avoid bugs #1 and #2
    If in userspace I do
          obj->data->x.a = 1
    the update is not seen in the kernel, ie
            "if (x.a == 2) { .. } else { .. }"
     always takes the first branch

Are these known issues ?

thanks
luigi