um: Fix ptrace GETREGS/SETREGS bugs

This fix two related bugs:
* PTRACE_GETREGS doesn't get the right orig_ax (syscall) value
* PTRACE_SETREGS can't set the orig_ax value (erased by initial value)

Get rid of the now useless and error-prone get_syscall().

Fix inconsistent behavior in the ptrace implementation for i386 when
updating orig_eax automatically update the syscall number as well. This
is now updated in handle_syscall().

Signed-off-by: Mickaël Salaün <[e-mail]>
Cc: Jeff Dike <[e-mail]>
Cc: Richard Weinberger <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Will Drewry <[e-mail]>
Cc: Thomas Meyer <[e-mail]>
Cc: Nicolas Iooss <[e-mail]>
Cc: Anton Ivanov <[e-mail]>
Cc: Meredydd Luff <[e-mail]>
Cc: David Drysdale <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>
Acked-by: Kees Cook <[e-mail]>

arch/um/include/shared/os.h

···
 				 void *arg);
 extern void halt_skas(void);
 extern void reboot_skas(void);
-extern int get_syscall(struct uml_pt_regs *regs);
 
 /* irq.c */
 extern int os_waiting_for_events(struct irq_fd *active_fds);

arch/um/kernel/skas/syscall.c

···
 #include <linux/ptrace.h>
 #include <kern_util.h>
 #include <sysdep/ptrace.h>
+#include <sysdep/ptrace_user.h>
 #include <sysdep/syscalls.h>
-#include <os.h>
 
 void handle_syscall(struct uml_pt_regs *r)
 {
 	struct pt_regs *regs = container_of(r, struct pt_regs, regs);
-	long result;
 	int syscall;
 
-	if (syscall_trace_enter(regs)) {
-		result = -ENOSYS;
+	/* Initialize the syscall number and default return value. */
+	UPT_SYSCALL_NR(r) = PT_SYSCALL_NR(r->gp);
+	PT_REGS_SET_SYSCALL_RETURN(regs, -ENOSYS);
+
+	if (syscall_trace_enter(regs))
 		goto out;
-	}
 
-	syscall = get_syscall(r);
+	/* Update the syscall number after orig_ax has potentially been updated
+	 * with ptrace.
+	 */
+	UPT_SYSCALL_NR(r) = PT_SYSCALL_NR(r->gp);
+	syscall = UPT_SYSCALL_NR(r);
 
-	if ((syscall > __NR_syscall_max) || syscall < 0)
-		result = -ENOSYS;
-	else
-		result = EXECUTE_SYSCALL(syscall, regs);
+	if (syscall >= 0 && syscall <= __NR_syscall_max)
+		PT_REGS_SET_SYSCALL_RETURN(regs,
+				EXECUTE_SYSCALL(syscall, regs));
 
 out:
-	PT_REGS_SET_SYSCALL_RETURN(regs, result);
-
 	syscall_trace_leave(regs);
 }

arch/um/os-Linux/skas/process.c

···
 	handle_syscall(regs);
 }
 
-int get_syscall(struct uml_pt_regs *regs)
-{
-	UPT_SYSCALL_NR(regs) = PT_SYSCALL_NR(regs->gp);
-
-	return UPT_SYSCALL_NR(regs);
-}
-
 extern char __syscall_stub_start[];
 
 static int userspace_tramp(void *stack)

arch/x86/um/ptrace_32.c

···
 	[EFL] = HOST_EFLAGS,
 	[UESP] = HOST_SP,
 	[SS] = HOST_SS,
+	[ORIG_EAX] = HOST_ORIG_AX,
 };
 
 int putreg(struct task_struct *child, int regno, unsigned long value)
···
 	case EAX:
 	case EIP:
 	case UESP:
+	case ORIG_EAX:
 		break;
 	case FS:
 		if (value && (value & 3) != 3)
···
 	case EFL:
 		value &= FLAG_MASK;
 		child->thread.regs.regs.gp[HOST_EFLAGS] |= value;
-		return 0;
-	case ORIG_EAX:
-		child->thread.regs.regs.syscall = value;
 		return 0;
 	default :
 		panic("Bad register in putreg() : %d\n", regno);
···
 
 	regno >>= 2;
 	switch (regno) {
-	case ORIG_EAX:
-		return child->thread.regs.regs.syscall;
 	case FS:
 	case GS:
 	case DS:
···
 	case EDI:
 	case EBP:
 	case EFL:
+	case ORIG_EAX:
 		break;
 	default:
 		panic("Bad register in getreg() : %d\n", regno);