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

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Hi,
Thanks for the response.

On Thu, Feb 16, 2023 at 11:13 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Wed, Feb 15, 2023 at 5:48 PM Stanislav Fomichev <sdf@xxxxxxxxxx> 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.

I feel if we follow. this convention then it will work out but
currently a lot of selftests use these
architecture dependent variable types and therefore don't even compile
for 32-bit architectures
because of the _Static_asserts in the skeleton.

Do you suggest replacing all these with __u64, __u32, etc. in the
selftests so that they compile on every architecture?

>
> 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.



-- 
Thanks and Regards

Yours Truly,

Puranjay Mohan




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