arm64: compat: Avoid sending SIGILL for unallocated syscall numbers

The ARM Linux kernel handles the EABI syscall numbers as follows:

  0           - NR_SYSCALLS-1	: Invoke syscall via syscall table
  NR_SYSCALLS - 0xeffff		: -ENOSYS (to be allocated in future)
  0xf0000     - 0xf07ff		: Private syscall or -ENOSYS if not allocated
  > 0xf07ff			: SIGILL

Our compat code gets this wrong and ends up sending SIGILL in response
to all syscalls greater than NR_SYSCALLS which have a value greater
than 0x7ff in the bottom 16 bits.

Fix this by defining the end of the ARM private syscall region and
checking the syscall number against that directly. Update the comment
while we're at it.

Cc: <stable@vger.kernel.org>
Cc: Dave Martin <Dave.Martin@arm.com>
Reported-by: Pi-Hsun Shih <pihsun@chromium.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/unistd.h

@@ -40,8 +40,9 @@
  * The following SVCs are ARM private.
  */
 #define __ARM_NR_COMPAT_BASE		0x0f0000
-#define __ARM_NR_compat_cacheflush	(__ARM_NR_COMPAT_BASE+2)
-#define __ARM_NR_compat_set_tls		(__ARM_NR_COMPAT_BASE+5)
+#define __ARM_NR_compat_cacheflush	(__ARM_NR_COMPAT_BASE + 2)
+#define __ARM_NR_compat_set_tls		(__ARM_NR_COMPAT_BASE + 5)
+#define __ARM_NR_COMPAT_END		(__ARM_NR_COMPAT_BASE + 0x800)
 
 #define __NR_compat_syscalls		399
 #endif

arch/arm64/kernel/sys_compat.c

@@ -102,12 +102,12 @@ long compat_arm_syscall(struct pt_regs *regs)
 
 	default:
 		/*
-		 * Calls 9f00xx..9f07ff are defined to return -ENOSYS
+		 * Calls 0xf0xxx..0xf07ff are defined to return -ENOSYS
 		 * if not implemented, rather than raising SIGILL. This
 		 * way the calling program can gracefully determine whether
 		 * a feature is supported.
 		 */
-		if ((no & 0xffff) <= 0x7ff)
+		if (no < __ARM_NR_COMPAT_END)
 			return -ENOSYS;
 		break;
 	}