linux-kernel - Re: [PATCH v2 07/14] x86: Use an opaque type for functions not callable from C

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Date:   Thu, 26 Aug 2021 09:54:11 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>, x86@...nel.org
Cc:     Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH v2 07/14] x86: Use an opaque type for functions not
 callable from C

On 8/23/21 10:13 AM, Sami Tolvanen wrote:
> The kernel has several assembly functions that are not directly callable
> from C. Use an opaque type for these function prototypes to make misuse
> harder, and to avoid the need to annotate references to these functions
> for Clang's Control-Flow Integrity (CFI).

You have:

typedef const u8 *asm_func_t;

This is IMO a bit confusing.  asm_func_t like this is an *address* of a
function, not a function.

To be fair, C is obnoxious, but I think this will lead to more confusion
than is idea.  For example:

> -extern void __fentry__(void);
> +DECLARE_ASM_FUNC_SYMBOL(__fentry__);

Okay, __fentry__ is the name of a symbol, and the expression __fentry__
is a pointer (or an array that decays to a pointer, thanks C), which is
at least somewhat sensible.  But:

> -extern void (*paravirt_iret)(void);
> +extern asm_func_t paravirt_iret;

Now paravirt_iret is a global variable that points to an asm func.  I
bet people will read this wrong and, worse, type it wrong.

I think that there a couple ways to change this that would be a bit nicer.

1. typedef const u8 asm_func_t[];

This is almost nice, but asm_func_t will still be accepted as a function
argument, and the automatic decay rules will probably be confusing.

2. Rename asm_func_t to asm_func_ptr.  Then it's at least a bit more clear.

3. Use an incomplete struct:

struct asm_func;

typedef struct asm_func asm_func;

extern asm_func some_func;

void *get_ptr(void)
{
    return &some_func;
}

No macros required, and I think it's quite hard to misuse this by
accident.  asm_func can't be passed as an argument or used as a variable
because it has incomplete type, and there are no arrays so the decay
rules aren't in effect.

--Andy