Re: [PATCH 05/15] x86: Implement function_nocfi

[Thomas Gleixner]

On Sat, Apr 17 2021 at 17:11, Andy Lutomirski wrote:
> On Sat, Apr 17, 2021 at 4:53 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> which works for
>>
>>       foo = function_nocfi(bar);
>
> I agree in general.  But right now, we have, in asm/proto.h:
>
> void entry_SYSCALL_64(void);
>
> and that's pure nonsense.  Depending on your point of view,
> entry_SYSCALL_64 is a symbol that resolves to an integer or it's an
> array of bytes containing instructions, but it is most definitely not
> a function void (void).  So, regardless of any CFI stuff, I propose
> that we standardize our handling of prototypes of symbols that are
> opaque to the C compiler.  Here are a couple of choices:
>
> Easy one:
>
> extern u8 entry_SYSCALL_64[];
>
> Slightly more complicated:
>
> struct opaque_symbol;
> extern struct opaque_symbol entry_SYSCALL_64;
>
> The opaque_symbol variant avoids any possible confusion over the weird
> status of arrays in C, and it's hard to misuse, since struct
> opaque_symbol is an incomplete type.

Makes sense.