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

[Arvind Sankar <nivedita@alum.mit.edu>]

On Thu, Oct 15, 2020 at 09:09:11PM +0000, David Laight wrote:
> From: Arvind Sankar
> > Sent: 15 October 2020 19:14
> > 
> > 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>
> > ---
> >  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.
> 
> I think I'd just say something like:
> 
> Although the compiler must assume a "memory" clobber may affect all
> memory, local variables (on stack) cannot actually be visible to the
> asm unless their address has been passed to an external function.
> So the compiler may assume such variables cannot be affected by
> a normal asm volatile(::"memory") barrier().
> Passing the address of the local variables to the asm barrier
> is enough to tell the compiler that the asm can 'see' the variables
> (and spill anything held in registers to the stack) so that
> the "memory" clobber has the expected effect.
> 
> This is necessary to get llvm to do a memset() of on-stack data
> at the end of a function to clear memory that contains secrets.
> 
> 	David

I think it's helpful to have the more detailed explanation about
register variables -- at first glance, it's a bit mystifying as to why
the compiler would think that the asm can't access the stack. Spilling
registers to the stack is actually an undesirable side-effect of the
workaround.