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

[Yonghong Song <yhs@fb.com>]

On 10/7/20 7:01 AM, Luigi Rizzo wrote:
> 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)

Yes, this is a particular issue with llvm10 which without "=0", put
into "common" section which we do not support. The issue is correct
in llvm11 if I remember correctly. But please just use "x = 0" it works
for all versions of compilers.

> 
> 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
>