On Thu, Oct 15, 2020 at 11:13 AM Arvind Sankar <nived...@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 <nived...@alum.mit.edu>
Thanks for this distinct cleanup. Acked-by: Nick Desaulniers <ndesaulni...@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
