powerpc: Change syscall_get_nr() to return int

The documentation for syscall_get_nr() in asm-generic says:

 Note this returns int even on 64-bit machines. Only 32 bits of
 system call number can be meaningful. If the actual arch value
 is 64 bits, this truncates to 32 bits so 0xffffffff means -1.

However our implementation was never updated to reflect this.

Generally it's not important, but there is once case where it matters.

For seccomp filter with SECCOMP_RET_TRACE, the tracer will set
regs->gpr[0] to -1 to reject the syscall. When the task is a compat
task, this means we end up with 0xffffffff in r0 because ptrace will
zero extend the 32-bit value.

If syscall_get_nr() returns an unsigned long, then a 64-bit kernel will
see a positive value in r0 and will incorrectly allow the syscall
through seccomp.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Kees Cook <keescook@chromium.org>

arch/powerpc/include/asm/syscall.h

@@ -22,10 +22,15 @@
 extern const unsigned long sys_call_table[];
 #endif /* CONFIG_FTRACE_SYSCALLS */
 
-static inline long syscall_get_nr(struct task_struct *task,
-				  struct pt_regs *regs)
+static inline int syscall_get_nr(struct task_struct *task, struct pt_regs *regs)
 {
-	return TRAP(regs) == 0xc00 ? regs->gpr[0] : -1L;
+	/*
+	 * Note that we are returning an int here. That means 0xffffffff, ie.
+	 * 32-bit negative 1, will be interpreted as -1 on a 64-bit kernel.
+	 * This is important for seccomp so that compat tasks can set r0 = -1
+	 * to reject the syscall.
+	 */
+	return TRAP(regs) == 0xc00 ? regs->gpr[0] : -1;
 }
 
 static inline void syscall_rollback(struct task_struct *task,