From: Arnd Bergmann
> Sent: 26 October 2020 21:03
>
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> Building with -Wextra shows lots of warnings in the bpf
> code such as
>
> kernel/bpf/verifier.c: In function ‘jit_subprogs’:
> include/linux/filter.h:345:4: warning: cast between incompatible function types from ‘unsigned int
> (*)(const void *, const struct bpf_insn *)’ to ‘u64 (*)(u64, u64, u64, u64, u64)’ {aka ‘long long
> unsigned int (*)(long long unsigned int, long long unsigned int, long long unsigned int, long long
> unsigned int, long long unsigned int)’} [-Wcast-function-type]
> 345 | ((u64 (*)(u64, u64, u64, u64, u64))(x))
> | ^
> kernel/bpf/verifier.c:10706:16: note: in expansion of macro ‘BPF_CAST_CALL’
> 10706 | insn->imm = BPF_CAST_CALL(func[subprog]->bpf_func) -
> | ^~~~~~~~~~~~~
>
> This appears to be intentional, so change the cast in a way that
> suppresses the warning.
It is also utterly horrid.
If the value is ever cast back then there is a lot of scope
for it going badly wrong somewhere.
It certainly isn't valid to use the target type to call the original function.
David
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