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    SubjectRe: [PATCH 05/15] x86: Implement function_nocfi
    On Fri, Apr 16, 2021 at 2:18 PM Borislav Petkov <bp@alien8.de> wrote:
    >
    > 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 <samitolvanen@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?

    As I mentioned in the cover letter, this series is based on
    linux-next. I forgot to include a link to the original patch series
    that adds CONFIG_CFI_CLANG, but I'll be sure to point to it in the
    next version. Sorry about the confusion.

    > > +/*
    > > + * 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.

    __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().

    > 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]

    This is defined in linux-next, but I do see another issue, which I'll
    fix in v2. Note that CFI_CLANG itself cannot be selected on 32-bit
    x86.

    Sami

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