Re: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo

[Akira Yokosawa]

On Tue, 26 Feb 2019 16:04:50 +0100, Peter Zijlstra wrote:
> On Tue, Feb 26, 2019 at 06:28:45AM -0800, Paul E. McKenney wrote:
> 
>> Yes, this all is a bit on the insane side from a kernel viewpoint.
>> But the paper you found does not impose this; it has instead been there
>> for about 20 years, back before C and C++ admitted to the existence
>> of concurrency.  But of course compilers are getting more aggressive,
>> and yes, some of the problems show up in single-threaded code.
> 
> But that paper is from last year!! It has Peter Sewell on, I'm sure he's
> heard of concurrency.
> 
>> The usual response is "then cast the pointers to intptr_t!" but of
>> course that breaks type checking.
> 
> I tried laundering the pointer through intptr_t, but I can't seem to
> unbreak it.
> 
> 
> root@ivb-ep:~/tmp# gcc-8 -O2 -fno-strict-aliasing  -o ptr ptr.c ; ./ptr
> p=0x55aacdc80034 q=0x55aacdc80034
> x=1 y=2 *p=11 *q=2
> root@ivb-ep:~/tmp# cat ptr.c
> #include <stdio.h>
> #include <string.h>
> #include <stdint.h>
> int y = 2, x = 1;
> int main (int argc, char **argv) {
> 	intptr_t P = (intptr_t)&x;
> 	intptr_t Q = (intptr_t)&y;
> 	P += sizeof(int);
> 	int *q = &y;
> 	printf("p=%p q=%p\n", (int*)P, (int*)Q);
> 	if (P == Q) {
> 		int *p = (int *)P;
> 		*p = 11;
> 		printf("x=%d y=%d *p=%d *q=%d\n", x, y, *p, *q);
> 	}
> }
> 

So, I'm looking at the macro RELOC_HIDE() defined in include/linux/compiler-gcc.h.

It says:

--------
/*
 * This macro obfuscates arithmetic on a variable address so that gcc
 * shouldn't recognize the original var, and make assumptions about it.
 *
 * This is needed because the C standard makes it undefined to do
 * pointer arithmetic on "objects" outside their boundaries and the
 * gcc optimizers assume this is the case. In particular they
 * assume such arithmetic does not wrap.
 *
   [...]
 */
#define RELOC_HIDE(ptr, off)						\
({									\
	unsigned long __ptr;						\
	__asm__ ("" : "=r"(__ptr) : "0"(ptr));				\
	(typeof(ptr)) (__ptr + (off));					\
})
--------

Looks like this macro has existed ever since the origin of Linus' git repo.

And the optimization "bug" discussed in this thread can be suppressed by
this macro.

For example,

$ gcc -O2 -o reloc_hide reloc_hide.c; ./reloc_hide
x=1 y=11 *p=11 *q=11
$ cat reloc_hide.c
#include <stdio.h>
#include <stdint.h>

#define RELOC_HIDE(ptr, off)						\
({									\
	uintptr_t __ptr;						\
	__asm__ ("" : "=r"(__ptr) : "0"(ptr));				\
	(typeof(ptr)) (__ptr + (off));					\
})

int y = 2, x = 1;
int main (int argc, char **argv) {
	int *p = RELOC_HIDE(&x, sizeof(*p));
	int *q = RELOC_HIDE(&y, 0);
	if (p == q) {
		*p = 11;
		printf("x=%d y=%d *p=%d *q=%d\n", x, y, *p, *q);
	}
}

Note that "uintptr_t" is used in this version of RELOC_HIDE() for user-land
code.

Am I the only one who was not aware of this gcc-specific macro?

        Thanks, Akira