RE: [PATCH] bpf: suppress -Wcast-function-type warning

[David Laight <David.Laight@ACULAB.COM>]

From: Arnd Bergmann
> Sent: 26 October 2020 21:03
> 
> From: Arnd Bergmann <arnd@arndb.de>
> 
> 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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