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

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