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. 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.
