Re: [PATCH] compiler.h: Clarify comment about the need for barrier_data()

[Nick Desaulniers <ndesaulniers@google.com>]

On Thu, Oct 15, 2020 at 11:13 AM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> Be clear about @ptr vs the variable that @ptr points to, and add some
> more details as to why the special barrier_data() macro is required.
>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>

Thanks for this distinct cleanup.
Acked-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>  include/linux/compiler.h | 33 ++++++++++++++++++++++-----------
>  1 file changed, 22 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 93035d7fee0d..d8cee7c8968d 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -86,17 +86,28 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>
>  #ifndef barrier_data
>  /*
> - * This version is i.e. to prevent dead stores elimination on @ptr
> - * where gcc and llvm may behave differently when otherwise using
> - * normal barrier(): while gcc behavior gets along with a normal
> - * barrier(), llvm needs an explicit input variable to be assumed
> - * clobbered. The issue is as follows: while the inline asm might
> - * access any memory it wants, the compiler could have fit all of
> - * @ptr into memory registers instead, and since @ptr never escaped
> - * from that, it proved that the inline asm wasn't touching any of
> - * it. This version works well with both compilers, i.e. we're telling
> - * the compiler that the inline asm absolutely may see the contents
> - * of @ptr. See also: https://llvm.org/bugs/show_bug.cgi?id=15495
> + * This version is to prevent dead stores elimination on @ptr where gcc and
> + * llvm may behave differently when otherwise using normal barrier(): while gcc
> + * behavior gets along with a normal barrier(), llvm needs an explicit input
> + * variable to be assumed clobbered.
> + *
> + * Its primary use is in implementing memzero_explicit(), which is used for
> + * clearing temporary data that may contain secrets.
> + *
> + * The issue is as follows: while the inline asm might access any memory it
> + * wants, the compiler could have fit all of the variable that @ptr points to
> + * into registers instead, and if @ptr never escaped from the function, it
> + * proved that the inline asm wasn't touching any of it. gcc only eliminates
> + * dead stores if the variable was actually allocated in registers, but llvm
> + * reasons that the variable _could_ have been in registers, so the inline asm
> + * can't reliably access it anyway, and eliminates dead stores even if the
> + * variable is actually in memory.
> + *
> + * This version works well with both compilers, i.e. we're telling the compiler
> + * that the inline asm absolutely may see the contents of the variable pointed
> + * to by @ptr.
> + *
> + * See also: https://llvm.org/bugs/show_bug.cgi?id=15495#c5
>   */
>  # define barrier_data(ptr) __asm__ __volatile__("": :"r"(ptr) :"memory")
>  #endif
> --
> 2.26.2
>


-- 
Thanks,
~Nick Desaulniers