Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))

[Arnd Bergmann]

On Thu, Dec 12, 2019 at 9:50 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Dec 12, 2019 at 11:34 AM Will Deacon <will@xxxxxxxxxx> wrote:
> > The root of my concern in all of this, and what started me looking at it in
> > the first place, is the interaction with 'typeof()'. Inheriting 'volatile'
> > for a pointer means that local variables in macros declared using typeof()
> > suddenly start generating *hideous* code, particularly when pointless stack
> > spills get stackprotector all excited.
>
> Yeah, removing volatile can be a bit annoying.
>
> For the particular case of the bitops, though, it's not an issue.
> Since you know the type there, you can just cast it.
>
> And if we had the rule that READ_ONCE() was an arithmetic type, you could do
>
>     typeof(0+(*p)) __var;
>
> since you might as well get the integer promotion anyway (on the
> non-volatile result).
>
> But that doesn't work with structures or unions, of course.
>
> I'm not entirely sure we have READ_ONCE() with a struct. I do know we
> have it with 64-bit entities on 32-bit machines, but that's ok with
> the "0+" trick.

I'll have my randconfig builder look for instances, so far I found one,
see below. My feeling is that it would be better to enforce at least
the size being a 1/2/4/8, to avoid cases where someone thinks
the access is atomic, but it falls back on a memcpy.

      Arnd

diff --git a/drivers/xen/time.c b/drivers/xen/time.c
index 0968859c29d0..adb492c0aa34 100644
--- a/drivers/xen/time.c
+++ b/drivers/xen/time.c
@@ -64,7 +64,7 @@ static void xen_get_runstate_snapshot_cpu_delta(
        do {
                state_time = get64(&state->state_entry_time);
                rmb();  /* Hypervisor might update data. */
-               *res = READ_ONCE(*state);
+               memcpy(res, state, sizeof(*res));
                rmb();  /* Hypervisor might update data. */
        } while (get64(&state->state_entry_time) != state_time ||
                 (state_time & XEN_RUNSTATE_UPDATE));
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 5e88e7e33abe..f4ae360efdba 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -179,6 +179,8 @@ void ftrace_likely_update(struct
ftrace_likely_data *f, int val,

 #include <uapi/linux/types.h>

+extern void __broken_access_once(void *, const void *, unsigned long);
+
 #define __READ_ONCE_SIZE                                               \
 ({                                                                     \
        switch (size) {                                                 \
@@ -187,9 +189,7 @@ void ftrace_likely_update(struct
ftrace_likely_data *f, int val,
        case 4: *(__u32 *)res = *(volatile __u32 *)p; break;            \
        case 8: *(__u64 *)res = *(volatile __u64 *)p; break;            \
        default:                                                        \
-               barrier();                                              \
-               __builtin_memcpy((void *)res, (const void *)p, size);   \
-               barrier();                                              \
+               __broken_access_once((void *)res, (const void *)p,
size);       \
        }                                                               \
 })

@@ -225,9 +225,7 @@ static __always_inline void
__write_once_size(volatile void *p, void *res, int s
        case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
        case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
        default:
-               barrier();
-               __builtin_memcpy((void *)p, (const void *)res, size);
-               barrier();
+               __broken_access_once((void *)p, (const void *)res, size);
        }
 }