Re: BPF selftests and strict aliasing

[Yonghong Song <yonghong.song@linux.dev>]

On 1/27/24 11:59 AM, Jose E. Marchesi wrote:
> Hello.
> The following BPF selftests perform type-punning:
>
>    progs/bind4_prog.c
>    136 |         user_ip4 |= ((volatile __u16 *)&ctx->user_ip4)[0] << 0;
>
>    progs/bind6_prog.c
>    149 |                 user_ip6 |= ((volatile __u16 *)&ctx->user_ip6[i])[0] << 0;
>
>    progs/dynptr_fail.c
>    549 |         val = *(int *)&ptr;
>
>    progs/linked_list_fail.c
>    318 |         return *(int *)&f->head;
>    329 |         *(int *)&f->head = 0;
>    338 |         f = bpf_obj_new(typeof(*f));
>    341 |         return *(int *)&f->node2;
>    349 |         f = bpf_obj_new(typeof(*f));
>    352 |         *(int *)&f->node2 = 0;
>
>    progs/map_kptr_fail.c
>     34 |         *(u32 *)&v->unref_ptr = 0;
>
>    progs/syscall.c
>    172 |         attr->map_id = ((struct bpf_map *)&outer_array_map)->id;
>
>    progs/test_pkt_md_access.c
>     13 |                 TYPE tmp = *(volatile TYPE *)&skb->FIELD;               \
>
>    progs/test_sk_lookup.c
>     31 |         (((__u16 *)&(value))[LSE_INDEX((index), sizeof(value) / 2)])
>    427 |         val_u32 = *(__u32 *)&ctx->remote_port;
>
>    progs/timer_crash.c
>     38 |         *(void **)&value = (void *)0xdeadcaf3;
>
> This results in GCC warnings with -Wall but violating strict aliasing
> may also result in the compiler incorrectly optimizing something.
>
> There are some alternatives to deal with this:
>
> a) To rewrite the tests to conform to strict aliasing rules.
>
> b) To build these tests using -fno-strict-aliasing to make sure the
>     compiler will not rely on strict aliasing while optimizing.
>
> c) To add pragmas to these test files to avoid the warning:
>     _Pragma("GCC diagnostic ignored \"-Wstrict-aliasing\"")
>
> I think b) is probably the best way to go, because it will avoid the
> warnings, will void potential problems with optimizations triggered by
> strict aliasing, and will not require to rewrite the tests.

I tried with latest clang with -fstrict-aliasing:

[~/work/bpf-next/tools/testing/selftests/bpf (master)]$ cat run.sh
clang  -g -Wall -Werror -D__TARGET_ARCH_x86 -mlittle-endian -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf/tools/include \
   -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf -I/home/yhs/work/bpf-next/tools/include/uapi
   -I/home/yhs/work/bpf-next/tools/testing/selftests/usr/include
   -idirafter /home/yhs/work/llvm-project/llvm/build.19/install/lib/clang/19/include
   -idirafter /usr/local/include -idirafter /usr/include   -Wno-compare-distinct-pointer-types
   -DENABLE_ATOMICS_TESTS -O2 -fstrict-aliasing --target=bpf -c progs/bind4_prog.c -mcpu=v3
   -o /home/yhs/work/bpf-next/tools/testing/selftests/bpf/bind4_prog.bpf.o
[~/work/bpf-next/tools/testing/selftests/bpf (master)]$ ./run.sh
[~/work/bpf-next/tools/testing/selftests/bpf (master)]$

I does not have compilation failure. I am wondering why -fstrict-aliasing won't have warning in clang side
but have warning in gcc side.
Your suggestion 'b' seems okay or we could even add -fno-strict-aliasing into common compilation flags,
but I would like to understand more about -fstrict-aliasing difference between gcc and clang.

>
> Provided [1] gets applied, I can prepare a patch that adds the following
> to selftests/bpf/Makefile:
>
>    progs/bin4_prog.c-CFLAGS := -fno-strict-aliasing
>    progs/bind6_prog.c-CFLAGS := -fno-strict-aliasing
>    progs/dynptr_fail.cw-CFLAGS := -fno-strict-aliasing
>    progs/linked_list_fail.c-CFLAGS := -fno-strict-aliasing
>    progs/map_kptr_fail.c-CFLAGS := -fno-strict-aliasing
>    progs/syscall.c-CFLAGS := -fno-strict-aliasing
>    progs/test_pkt_md_access.c-CFLAGS := -fno-strict-aliasing
>    progs/test_sk_lookup.c-CFLAGS := -fno-strict-aliasing
>    progs/timer_crash.c-CFLAGS := -fno-strict-aliasing
>
> [1] https://lore.kernel.org/bpf/20240127100702.21549-1-jose.marchesi@oracle.com/T/#u
>