x86_64: make ptrace always sign-extend orig_ax to 64 bits

This makes 64-bit ptrace calls setting the 64-bit orig_ax field for a
32-bit task sign-extend the low 32 bits up to 64. This matches what a
64-bit debugger expects when tracing a 32-bit task.

This follows on my "x86_64 ia32 syscall restart fix". This didn't
matter until that was fixed.

The debugger ignores or zeros the high half of every register slot it
sets (including the orig_rax pseudo-register) uniformly. It expects
that the setting of the low 32 bits always has the same meaning as a
32-bit debugger setting those same 32 bits with native 32-bit
facilities.

This never arose before because the syscall restart check never
matched any -ERESTART* values due to lack of sign extension. Before
that fix, even 32-bit ptrace setting orig_eax to -1 failed to trigger
the restart check anyway. So this was never noticed as a regression
of 64-bit debuggers vs 32-bit debuggers on the same 64-bit kernel.

Signed-off-by: Roland McGrath <[e-mail]>
[ Changed to just do the sign-extension unconditionally on x86-64,
since orig_ax is always just a small integer and doesn't need
the full 64-bit range ]
Signed-off-by: Linus Torvalds <[e-mail]>

arch/x86/kernel/ptrace.c

···
 		return set_flags(child, value);
 
 #ifdef CONFIG_X86_64
+	/*
+	 * Orig_ax is really just a flag with small positive and
+	 * negative values, so make sure to always sign-extend it
+	 * from 32 bits so that it works correctly regardless of
+	 * whether we come from a 32-bit environment or not.
+	 */
+	case offsetof(struct user_regs_struct, orig_ax):
+		value = (long) (s32) value;
+		break;
+
 	case offsetof(struct user_regs_struct,fs_base):
 		if (value >= TASK_SIZE_OF(child))
 			return -EIO;