uml: improve PTRACE_SYSEMU checking

Make the PTRACE_SYSEMU checking more robust. It will make sure that system
call numbers are reported correctly. If there is a problem, it will disable
PTRACE_SYSEMU use and use PTRACE_SYSCALL instead.

Thanks to Balaji G for helping reproduce this problem.

Signed-off-by: Jeff Dike <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/um/os-Linux/start_up.c

···
 		int exit_with = WEXITSTATUS(status);
 		if (exit_with == 2)
 			non_fatal("check_ptrace : child exited with status 2. "
-				  "Serious trouble happening! Try updating "
-				  "your host skas patch!\nDisabling SYSEMU "
-				  "support.");
+				  "\nDisabling SYSEMU support.\n");
 		non_fatal("check_ptrace : child exited with exitcode %d, while "
 			  "expecting %d; status 0x%x\n", exit_with,
 			  exitcode, status);
···
 static void __init check_sysemu(void)
 {
 	void *stack;
+	unsigned long regs[MAX_REG_NR];
 	int pid, n, status, count=0;
 
 	non_fatal("Checking syscall emulation patch for ptrace...");
···
 		fatal("check_sysemu : expected SIGTRAP, got status = %d",
 		      status);
 
-	n = ptrace(PTRACE_POKEUSR, pid, PT_SYSCALL_RET_OFFSET,
-		   os_getpid());
-	if(n < 0)
-		fatal_perror("check_sysemu : failed to modify system call "
-			     "return");
+	if(ptrace(PTRACE_GETREGS, pid, 0, regs) < 0)
+		fatal_perror("check_sysemu : PTRACE_GETREGS failed");
+	if(PT_SYSCALL_NR(regs) != __NR_getpid){
+		non_fatal("check_sysemu got system call number %d, "
+			  "expected %d...", PT_SYSCALL_NR(regs), __NR_getpid);
+		goto fail;
+	}
+
+	n = ptrace(PTRACE_POKEUSR, pid, PT_SYSCALL_RET_OFFSET, os_getpid());
+	if(n < 0){
+		non_fatal("check_sysemu : failed to modify system call "
+			  "return");
+		goto fail;
+	}
 
 	if (stop_ptraced_child(pid, stack, 0, 0) < 0)
 		goto fail_stopped;