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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Feb 15, 2023 at 5:48 PM Stanislav Fomichev <sdf@google.com> wrote:
>
> 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 :-(

Yeah, I'd rather avoid making bpftool aware of target architecture.
Three is 32 vs 64 bitness, there is also little/big endianness, etc.

So I'd recommend never using "long" (and similar types that depend on
bitness of the platform, like size_t, etc) for global variables. Also
don't use pointer types as types of the variable. Stick to __u64,
__u32, etc.

Note that all this is irrelevant for static global variables, as they
are not exposed in the BPF skeleton.

In general, mixing 32-bit host architecture with (always) 64-bit BPF
architecture always requires more care. And BPF skeleton is just one
aspect of this.

>
>
> > Thanks,
> > Puranjay Mohan.