Messages in this thread | | | Date | Thu, 26 Aug 2021 16:23:50 -0700 | From | "Andy Lutomirski" <> | Subject | Re: [PATCH v2 07/14] x86: Use an opaque type for functions not callable from C |
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On Thu, Aug 26, 2021, at 3:11 PM, Sami Tolvanen wrote: > On Thu, Aug 26, 2021 at 9:54 AM Andy Lutomirski <luto@kernel.org> 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 > v3. >
Do you have a sense for how many occurrences there are that would need an &?
> Sami >
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