[PATCH] Define vsyscall cache as blob to make clearer that user space shouldn't use it

Signed-off-by: Andi Kleen <ak@suse.de>

arch/x86_64/kernel/vsyscall.c

@@ -155,8 +155,8 @@ vgetcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *tcache)
 	   We do this here because otherwise user space would do it on
 	   its own in a likely inferior way (no access to jiffies).
 	   If you don't like it pass NULL. */
-	if (tcache && tcache->t0 == (j = __jiffies)) {
-		p = tcache->t1;
+	if (tcache && tcache->blob[0] == (j = __jiffies)) {
+		p = tcache->blob[1];
 	} else if (__vgetcpu_mode == VGETCPU_RDTSCP) {
 		/* Load per CPU data from RDTSCP */
 		rdtscp(dummy, dummy, p);
@@ -165,8 +165,8 @@ vgetcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *tcache)
 		asm("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG));
 	}
 	if (tcache) {
-		tcache->t0 = j;
-		tcache->t1 = p;
+		tcache->blob[0] = j;
+		tcache->blob[1] = p;
 	}
 	if (cpu)
 		*cpu = p & 0xfff;

include/linux/getcpu.h

@@ -1,16 +1,18 @@
 #ifndef _LINUX_GETCPU_H
 #define _LINUX_GETCPU_H 1
 
-/* Cache for getcpu() to speed it up. Results might be upto a jiffie
+/* Cache for getcpu() to speed it up. Results might be a short time
    out of date, but will be faster.
+
    User programs should not refer to the contents of this structure.
-   It is only a cache for vgetcpu(). It might change in future kernels.
+   I repeat they should not refer to it. If they do they will break
+   in future kernels.
+
+   It is only a private cache for vgetcpu(). It will change in future kernels.
    The user program must store this information per thread (__thread)
    If you want 100% accurate information pass NULL instead. */
 struct getcpu_cache {
-	unsigned long t0;
-	unsigned long t1;
-	unsigned long res[4];
+	unsigned long blob[128 / sizeof(long)];
 };
 
 #endif

kernel/sys.c

@@ -2083,12 +2083,12 @@ asmlinkage long sys_getcpu(unsigned __user *cpup, unsigned __user *nodep,
 		 * padding
 		 */
 		unsigned long t0, t1;
-		get_user(t0, &cache->t0);
-		get_user(t1, &cache->t1);
+		get_user(t0, &cache->blob[0]);
+		get_user(t1, &cache->blob[1]);
 		t0++;
 		t1++;
-		put_user(t0, &cache->t0);
-		put_user(t1, &cache->t1);
+		put_user(t0, &cache->blob[0]);
+		put_user(t1, &cache->blob[1]);
 	}
 	return err ? -EFAULT : 0;
 }