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

[Borislav Petkov]

On Fri, Apr 16, 2021 at 03:06:17PM -0700, Andy Lutomirski wrote:
> On Fri, Apr 16, 2021 at 3:03 PM Borislav Petkov <bp@xxxxxxxxx> wrote:
> >
> > On Fri, Apr 16, 2021 at 02:49:23PM -0700, Sami Tolvanen wrote:
> > > __nocfi only disables CFI checking in a function, the compiler still
> > > changes function addresses to point to the CFI jump table, which is
> > > why we need function_nocfi().
> >
> > So call it __func_addr() or get_function_addr() or so, so that at least
> > it is clear what this does.
> >
> 
> This seems backwards to me.  If I do:
> 
> extern void foo(some signature);
> 
> then I would, perhaps naively, expect foo to be the actual symbol that

I'm just reading the patch:

... The function_nocfi macro always returns the address of the
+ * actual function instead.
+ */
+#define function_nocfi(x) ({                                           \
+       void *addr;                                                     \
+       asm("leaq " __stringify(x) "(%%rip), %0\n\t" : "=r" (addr));    \
+       addr;

so it does a rip-relative load into a reg which ends up with the function
address.

-- 
Regards/Gruss,
    Boris.

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