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From: Andy Lutomirski <luto@kernel.org>
To: Sami Tolvanen <samitolvanen@google.com>, x86@kernel.org
Cc: Kees Cook <keescook@chromium.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org,
	clang-built-linux@googlegroups.com
Subject: Re: [PATCH v2 07/14] x86: Use an opaque type for functions not callable from C
Date: Thu, 26 Aug 2021 09:54:11 -0700	[thread overview]
Message-ID: <b2b0247a-39ad-097b-8fab-023ee378c806@kernel.org> (raw)
In-Reply-To: <20210823171318.2801096-8-samitolvanen@google.com>

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

  reply	other threads:[~2021-08-26 16:54 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-23 17:13 [PATCH v2 00/14] x86: Add support for Clang CFI Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 01/14] objtool: Add CONFIG_CFI_CLANG support Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 02/14] objtool: Add ASM_STACK_FRAME_NON_STANDARD Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 03/14] linkage: Add DECLARE_ASM_FUNC_SYMBOL Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 04/14] ftrace: Use an opaque type for functions not callable from C Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 05/14] lkdtm: Disable UNSET_SMEP with CFI Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 06/14] lkdtm: Use an opaque type for lkdtm_rodata_do_nothing Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 07/14] x86: Use an opaque type for functions not callable from C Sami Tolvanen
2021-08-26 16:54   ` Andy Lutomirski [this message]
2021-08-26 22:11     ` Sami Tolvanen
2021-08-26 23:23       ` Andy Lutomirski
2021-08-26 23:45         ` Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 08/14] x86/extable: Do not mark exception callback as CFI Sami Tolvanen
2021-08-26 16:56   ` Andy Lutomirski
2021-08-30 19:57     ` Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 09/14] x86/purgatory: Disable CFI Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 10/14] x86, relocs: Ignore __typeid__ relocations Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 11/14] x86, module: " Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 12/14] x86, cpu: Use LTO for cpu.c with CFI Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 13/14] x86, kprobes: Fix optprobe_template_func type mismatch Sami Tolvanen
2021-08-23 17:13 ` [PATCH v2 14/14] x86, build: Allow CONFIG_CFI_CLANG to be selected Sami Tolvanen
2021-08-23 17:16 ` [PATCH v2 00/14] x86: Add support for Clang CFI Tom Stellard
2021-08-23 17:20   ` Sami Tolvanen
2021-08-24 17:26     ` Tom Stellard
2021-08-24 17:30       ` Sami Tolvanen
2021-08-24 19:46 ` Peter Zijlstra
2021-08-25 15:49   ` Sami Tolvanen
2021-08-26 11:43     ` Peter Zijlstra
2021-08-26 21:52       ` Sami Tolvanen

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