x86: ia32 syscall restart fix

The code to restart syscalls after signals depends on checking for a
negative orig_ax, and for particular negative -ERESTART* values in ax.
These fields are 64 bits and for a 32-bit task they get zero-extended.
The syscall restart behavior is lost, a regression from a native 32-bit
kernel and from 64-bit tasks' behavior.

This patch fixes the problem by doing sign-extension where it matters.

For orig_ax, the only time the value should be -1 but winds up as
0x0ffffffff is via a 32-bit ptrace call. So the patch changes ptrace to
sign-extend the 32-bit orig_eax value when it's stored; it doesn't
change the checks on orig_ax, though it uses the new current_syscall()
inline to better document the subtle importance of the used of
signedness there.

The ax value is stored a lot of ways and it seems hard to get them all
sign-extended at their origins. So for that, we use the
current_syscall_ret() to sign-extend it only for 32-bit tasks at the
time of the -ERESTART* comparisons.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 8 insertions, 1 deletions

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 8f64abe699f..d5904eef1d3 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -1055,10 +1055,17 @@ static int putreg32(struct task_struct *child, unsigned regno, u32 value)
 	R32(esi, si);
 	R32(ebp, bp);
 	R32(eax, ax);
-	R32(orig_eax, orig_ax);
 	R32(eip, ip);
 	R32(esp, sp);
 
+	case offsetof(struct user32, regs.orig_eax):
+		/*
+		 * Sign-extend the value so that orig_eax = -1
+		 * causes (long)orig_ax < 0 tests to fire correctly.
+		 */
+		regs->orig_ax = (long) (s32) value;
+		break;
+
 	case offsetof(struct user32, regs.eflags):
 		return set_flags(child, value);