Re: [RFC] libbbpf/bpftool: Support 32-bit Architectures.

[Stanislav Fomichev <sdf@google.com>]

On 02/15, Puranjay Mohan wrote:
> The BPF selftests fail to compile on 32-bit architectures as the skeleton
> generated by bpftool doesn’t take into consideration the size difference  
> of
> variables on 32-bit/64-bit architectures.

> As an example,
> If a bpf program has a global variable of type: long, its skeleton will  
> include
> a bss map that will have a field for this variable. The long variable in  
> BPF is
> 64-bit. if we are working on a 32-bit machine, the generated skeleton has  
> to
> compile for that machine where long is 32-bit.

> A reproducer for this issue:
>          root@56ec59aa632f:~# cat test.bpf.c
>          long var;

>          root@56ec59aa632f:~# clang -target bpf -g -c test.bpf.c

>          root@56ec59aa632f:~# bpftool btf dump file test.bpf.o format raw
>          [1] INT 'long int' size=8 bits_offset=0 nr_bits=64 encoding=SIGNED
>          [2] VAR 'var' type_id=1, linkage=global
>          [3] DATASEC '.bss' size=0 vlen=1
>                 type_id=2 offset=0 size=8 (VAR 'var')

>         root@56ec59aa632f:~# bpftool gen skeleton test.bpf.o > skeleton.h

>         root@56ec59aa632f:~# echo "#include \"skeleton.h\"" > test.c

>         root@56ec59aa632f:~# gcc test.c
>         In file included from test.c:1:
>         skeleton.h: In function 'test_bpf__assert':
>         skeleton.h:231:2: error: static assertion failed: "unexpected
> size of \'var\'"
>           231 |  _Static_assert(sizeof(s->bss->var) == 8, "unexpected
> size of 'var'");
>                  |  ^~~~~~~~~~~~~~

> One naive solution for this would be to map ‘long’ to ‘long long’ and
> ‘unsigned long’ to ‘unsigned long long’. But this doesn’t solve everything
> because this problem is also seen with pointers that are 64-bit in BPF and
> 32-bit in 32-bit machines.

> I want to work on solving this and am looking for ideas to solve it  
> efficiently.
> The main goal is to make libbbpf/bpftool host architecture agnostic.

Looks like bpftool needs to be aware of the target architecture. The
same way gcc is doing with build-host-target triplet. I don't
think this can be solved with a bunch of typedefs? But I've long
forgotten how a pure 32-bit machine looks, so I can't give any
useful input :-(


> Thanks,
> Puranjay Mohan.