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

From: Segher Boessenkool <[email-masked]>
Date: Thu, 12 Dec 2019 09:10:51 -0600

Hi,

On Thu, Dec 12, 2019 at 04:42:13PM +1100, Michael Ellerman wrote:
> Some of the generic versions don't generate good code compared to our
> versions, but that's because READ_ONCE() is triggering stack protector
> to be enabled.

The *big* difference is the generic code has a special path that does not
do an atomic access at all.  Either that is a good idea or not, but we
probably should not change the behaviour here, not without benchmarking
anyway.

> For example, comparing an out-of-line copy of the generic and ppc
> versions of test_and_set_bit_lock():

(With what GCC version, and what exact flags?)

(A stand-alone testcase would be nice too, btw).

(Michael gave me one, thanks!)

> If you squint, the generated code for the actual logic is pretty similar, but
> the stack protector gunk makes a big mess.

And with stack protector it cannot shrink-wrap the exit, one of the bigger
performance costs of the stack protector.  The extra branch in the generic
code isn't fun either (but maybe it is good for performance?

> It's particularly bad here
> because the ppc version doesn't even need a stack frame.

You are hit by this:

  if (... || (RECORD_OR_UNION_TYPE_P (var_type)
              && record_or_union_type_has_array_p (var_type)) ...)

(in the GCC code, stack_protect_decl_p (), cfgexpand.c)

for the variable __u from

#define __READ_ONCE(x, check)                                           \
({                                                                      \
        union { typeof(x) __val; char __c[1]; } __u;                    \
        __read_once_size(&(x), __u.__c, sizeof(x));                     \
        smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \
        __u.__val;                                                      \
})

This is all optimised away later, but at the point this decision is made
GCC does not know that.

> So READ_ONCE() + STACKPROTECTOR_STRONG is problematic. The root cause is
> presumably that READ_ONCE() does an access to an on-stack variable,
> which triggers the heuristics in the compiler that the stack needs
> protecting.

Not exactly, but the problem is READ_ONCE alright.

> It seems like a compiler "mis-feature" that a constant-sized access to the stack
> triggers the stack protector logic, especially when the access is eventually
> optimised away. But I guess that's probably what we get for doing tricks like
> READ_ONCE() in the first place :/

__c is an array.  That is all that matters.  I don't think it is very
reasonable to fault GCC for this.

> I tried going back to the version of READ_ONCE() that doesn't use a
> union, ie. effectively reverting dd36929720f4 ("kernel: make READ_ONCE()
> valid on const arguments") to get:
> 
> #define READ_ONCE(x)							\
> 	({ typeof(x) __val; __read_once_size(&x, &__val, sizeof(__val)); __val; })

With that, it is that the address of __val is taken:

  ...
  || TREE_ADDRESSABLE (var)
  ...

> But it makes no difference, the stack protector stuff still triggers. So
> I guess it's simply taking the address of a stack variable that triggers
> it.

Not in the earlier testcase.  Btw, there is no such thing as a "stack
variable" at that point in the compiler: it just is a local var.

> There seems to be a function attribute to enable stack protector for a
> function, but not one to disable it:
>   https://gcc.gnu.org/onlinedocs/gcc-9.2.0/gcc/Common-Function-Attributes.html#index-stack_005fprotect-function-attribute

Yes.

> That may not be a good solution even if it did exist, because it would
> potentially disable stack protector in places where we do want it
> enabled.

Right, I don't think we want that, such an attribute invites people to
write dangerous code.  (You already can just put the functions that you
want to be unsafe in a separate source file...  It sounds even sillier
that way, heh).

Segher