On Fri, Apr 16, 2021 at 01:38:34PM -0700, Sami Tolvanen wrote:
> With CONFIG_CFI_CLANG, the compiler replaces function addresses in
> instrumented C code with jump table addresses. This change implements
> the function_nocfi() macro, which returns the actual function address
> instead.
>
> Signed-off-by: Sami Tolvanen <samitolva...@google.com>
> ---
> arch/x86/include/asm/page.h | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/page.h
> index 7555b48803a8..5499a05c44fc 100644
> --- a/arch/x86/include/asm/page.h
> +++ b/arch/x86/include/asm/page.h
> @@ -71,6 +71,20 @@ static inline void copy_user_page(void *to, void *from,
> unsigned long vaddr,
> extern bool __virt_addr_valid(unsigned long kaddr);
> #define virt_addr_valid(kaddr) __virt_addr_valid((unsigned long)
> (kaddr))
>
> +#ifdef CONFIG_CFI_CLANG
Almost every patch is talking about this magical config symbol but it
is nowhere to be found. How do I disable it, is there a Kconfig entry
somewhere, etc, etc?
> +/*
> + * With CONFIG_CFI_CLANG, the compiler replaces function address
> + * references with the address of the function's CFI jump table
> + * entry. 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; \
> +})
Also, eww.
It seems all these newfangled things we get, mean obfuscating code even
more. What's wrong with using __nocfi instead? That's nicely out of the
way so slap that in front of functions.
Also, did you even build this on, say, 32-bit allmodconfig?
Oh well.
In file included from ./include/linux/ftrace.h:22:0,
from ./include/linux/init_task.h:9,
from init/init_task.c:2:
./include/linux/ftrace.h: In function ‘ftrace_init_nop’:
./arch/x86/include/asm/ftrace.h:9:40: error: implicit declaration of function
‘function_nocfi’ [-Werror=implicit-function-declaration]
# define MCOUNT_ADDR ((unsigned long)(function_nocfi(__fentry__)))
^
./include/linux/ftrace.h:671:35: note: in expansion of macro ‘MCOUNT_ADDR’
return ftrace_make_nop(mod, rec, MCOUNT_ADDR);
^~~~~~~~~~~
In file included from ./include/linux/ftrace.h:22:0,
from ./include/linux/perf_event.h:49,
from ./include/linux/trace_events.h:10,
from ./include/trace/syscall.h:7,
from ./include/linux/syscalls.h:85,
from init/main.c:21:
./include/linux/ftrace.h: In function ‘ftrace_init_nop’:
./arch/x86/include/asm/ftrace.h:9:40: error: implicit declaration of function
‘function_nocfi’ [-Werror=implicit-function-declaration]
# define MCOUNT_ADDR ((unsigned long)(function_nocfi(__fentry__)))
^
./include/linux/ftrace.h:671:35: note: in expansion of macro ‘MCOUNT_ADDR’
return ftrace_make_nop(mod, rec, MCOUNT_ADDR);
^~~~~~~~~~~
In file included from ./include/linux/ftrace.h:22:0,
from ./include/linux/perf_event.h:49,
from ./include/linux/trace_events.h:10,
from ./include/trace/syscall.h:7,
from ./include/linux/syscalls.h:85,
from init/initramfs.c:10:
./include/linux/ftrace.h: In function ‘ftrace_init_nop’:
./arch/x86/include/asm/ftrace.h:9:40: error: implicit declaration of function
‘function_nocfi’ [-Werror=implicit-function-declaration]
# define MCOUNT_ADDR ((unsigned long)(function_nocfi(__fentry__)))
^
./include/linux/ftrace.h:671:35: note: in expansion of macro ‘MCOUNT_ADDR’
return ftrace_make_nop(mod, rec, MCOUNT_ADDR);
^~~~~~~~~~~
In file included from ./include/linux/ftrace.h:22:0,
from ./include/linux/perf_event.h:49,
from arch/x86/events/core.c:15:
./include/linux/ftrace.h: In function ‘ftrace_init_nop’:
./arch/x86/include/asm/ftrace.h:9:40: error: implicit declaration of function
‘function_nocfi’ [-Werror=implicit-function-declaration]
# define MCOUNT_ADDR ((unsigned long)(function_nocfi(__fentry__)))
...
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
