Re: [PATCH v6 16/25] init: lto: fix PREL32 relocations

Kees Cook
2020-10-15T01:19:39Z

On Mon, Oct 12, 2020 at 05:31:54PM -0700, Sami Tolvanen wrote:
> With LTO, the compiler can rename static functions to avoid global
> naming collisions. As initcall functions are typically static,
> renaming can break references to them in inline assembly. This
> change adds a global stub with a stable name for each initcall to
> fix the issue when PREL32 relocations are used.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>

This is another independent improvement... this could land before the
other portions of the series.

-Kees

> ---
> include/linux/init.h | 31 +++++++++++++++++++++++++++----
> 1 file changed, 27 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/init.h b/include/linux/init.h
> index af638cd6dd52..cea63f7e7705 100644
> --- a/include/linux/init.h
> +++ b/include/linux/init.h
> @@ -209,26 +209,49 @@ extern bool initcall_debug;
> */
> #define __initcall_section(__sec, __iid) \
> #__sec ".init.." #__iid
> +
> +/*
> + * With LTO, the compiler can rename static functions to avoid
> + * global naming collisions. We use a global stub function for
> + * initcalls to create a stable symbol name whose address can be
> + * taken in inline assembly when PREL32 relocations are used.
> + */
> +#define __initcall_stub(fn, __iid, id) \
> + __initcall_name(initstub, __iid, id)
> +
> +#define __define_initcall_stub(__stub, fn) \
> + int __init __stub(void); \
> + int __init __stub(void) \
> + { \
> + return fn(); \
> + } \
> + __ADDRESSABLE(__stub)
> #else
> #define __initcall_section(__sec, __iid) \
> #__sec ".init"
> +
> +#define __initcall_stub(fn, __iid, id) fn
> +
> +#define __define_initcall_stub(__stub, fn) \
> + __ADDRESSABLE(fn)
> #endif
>
> #ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
> -#define ____define_initcall(fn, __name, __sec) \
> - __ADDRESSABLE(fn) \
> +#define ____define_initcall(fn, __stub, __name, __sec) \
> + __define_initcall_stub(__stub, fn) \
> asm(".section \"" __sec "\", \"a\" \n" \
> __stringify(__name) ": \n" \
> - ".long " #fn " - . \n" \
> + ".long " __stringify(__stub) " - . \n" \
> ".previous \n");
> #else
> -#define ____define_initcall(fn, __name, __sec) \
> +#define ____define_initcall(fn, __unused, __name, __sec) \
> static initcall_t __name __used \
> __attribute__((__section__(__sec))) = fn;
> #endif
>
> #define __unique_initcall(fn, id, __sec, __iid) \
> ____define_initcall(fn, \
> + __initcall_stub(fn, __iid, id), \
> __initcall_name(initcall, __iid, id), \
> __initcall_section(__sec, __iid))
>
> --
> 2.28.0.1011.ga647a8990f-goog
>

--
Kees Cook