[PATCH] x86_64 ptrace orig_ax on ia32 task

[Roland McGrath <roland@redhat.com>]

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 <roland@redhat.com>
---
 arch/x86/kernel/ptrace.c |   14 ++++++++++++++
 1 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index f8eed1b..92b44e1 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -322,6 +322,20 @@ static int putreg(struct task_struct *child,
 	case offsetof(struct user_regs_struct, flags):
 		return set_flags(child, value);
 
+#ifdef CONFIG_IA32_EMULATION
+	case offsetof(struct user_regs_struct, orig_ax):
+		/*
+		 * For a 32-bit task, setting only the low 32 bits and
+		 * leaving the high bits untouched (all 0) has the same
+		 * effect as setting those bits via 32-bit ptrace would.
+		 * This means sign-extending an orig_eax of -1, which
+		 * here is an orig_rax of (u32)-1.
+		 */
+		if (test_tsk_thread_flag(child, TIF_IA32))
+			value = (long) (s32) value;
+		break;
+#endif
+
 #ifdef CONFIG_X86_64
 	case offsetof(struct user_regs_struct,fs_base):
 		if (value >= TASK_SIZE_OF(child))