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Subject[PATCH] x86_64 ptrace orig_ax on ia32 task

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

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 <>
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);

+ 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;
#ifdef CONFIG_X86_64
case offsetof(struct user_regs_struct,fs_base):
if (value >= TASK_SIZE_OF(child))

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