lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Thu, 26 Aug 2021 15:11:55 -0700
From:   Sami Tolvanen <>
To:     Andy Lutomirski <>
Cc:     X86 ML <>, Kees Cook <>,
        Josh Poimboeuf <>,
        Peter Zijlstra <>,
        Nathan Chancellor <>,
        Nick Desaulniers <>,
        Sedat Dilek <>,,
        LKML <>,
        clang-built-linux <>
Subject: Re: [PATCH v2 07/14] x86: Use an opaque type for functions not
 callable from C

On Thu, Aug 26, 2021 at 9:54 AM Andy Lutomirski <> wrote:
> 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.

I considered using an incomplete struct, but that would require an
explicit '&' when we take the address of these symbols, which I
thought would be unnecessary churn. Unless you strongly prefer this
one, I'll go with option 2 and rename asm_func_t to asm_func_ptr in


Powered by blists - more mailing lists