Re: [PATCH 1/7] x86/entry: Introduce __entry_text for entry code written in C

[Nick Desaulniers]

On Wed, May 11, 2022 at 12:27 AM Lai Jiangshan <jiangshanlai@xxxxxxxxx> wrote:
>
> From: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>
>
> Some entry code will be implemented in C files.
> Introduce __entry_text to set them in .entry.text section.
>
> The new __entry_text disables instrumentation like noinstr, so
> __noinstr_section() is added for noinstr and the new __entry_text.
>
> Note, entry code can not access to %gs before the %gs base is switched
> to kernel %gs base, so stack protector can not be used on the C entry
> code.  But __entry_text doesn't disable stack protector since some
> compilers might not support function level granular attribute to
> disable stack protector.  It will be disabled in C file level.
>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Reviewed-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Suggested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/idtentry.h | 3 +++
>  include/linux/compiler_types.h  | 8 +++++---
>  2 files changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
> index 72184b0b2219..acc4c99f801c 100644
> --- a/arch/x86/include/asm/idtentry.h
> +++ b/arch/x86/include/asm/idtentry.h
> @@ -13,6 +13,9 @@
>
>  #include <asm/irq_stack.h>
>
> +/* Entry code written in C. */
> +#define __entry_text __noinstr_section(".entry.text")
> +
>  /**
>   * DECLARE_IDTENTRY - Declare functions for simple IDT entry points
>   *                   No error code pushed by hardware
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 1c2c33ae1b37..8c7e81efe9bf 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -225,9 +225,11 @@ struct ftrace_likely_data {
>  #endif
>
>  /* Section for code which can't be instrumented at all */
> -#define noinstr                                                                \
> -       noinline notrace __attribute((__section__(".noinstr.text")))    \
> -       __no_kcsan __no_sanitize_address __no_profile __no_sanitize_coverage
> +#define __noinstr_section(section)                             \
> +       noinline notrace __section(section) __no_profile        \
> +       __no_kcsan __no_sanitize_address __no_sanitize_coverage
> +
> +#define noinstr __noinstr_section(".noinstr.text")

I haven't looked at the rest of the series, but isn't `noinstr` used
in a bunch of places? Wont this commit break all those uses or at
least make it break bisection?

Also, my suggestion was simply not to open code
__attribute__((section(""))). Does this work?

```
diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index 7924f27f5c8b..10ec7039e17d 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -13,6 +13,9 @@

 #include <asm/irq_stack.h>

+/* Entry code written in C. Override the section used by noinstr. */
+#define __entry_text noinstr __section(".entry.text")
+
 /**
  * DECLARE_IDTENTRY - Declare functions for simple IDT entry points
  *                   No error code pushed by hardware
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 1c2c33ae1b37..ce623099eb21 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -226,8 +226,8 @@ struct ftrace_likely_data {

 /* Section for code which can't be instrumented at all */
 #define noinstr
         \
-       noinline notrace __attribute((__section__(".noinstr.text")))    \
-       __no_kcsan __no_sanitize_address __no_profile __no_sanitize_coverage
+       noinline notrace __section(".noinstr.text") __no_kcsan          \
+       __no_sanitize_address __no_profile __no_sanitize_coverage

 #endif /* __KERNEL__ */
```
-- 
Thanks,
~Nick Desaulniers