2023-02-17 11:25 UTC+0100 ~ Puranjay Mohan <puranjay12@xxxxxxxxx> > 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. I'd rather avoid that too, but for addressing the endianness issue with cross-compiling, reported by Christophe and where the bytecode is not stored with the right endianness in the skeleton file, do you see an alternative? >> >> 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. > > >
