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>

2 files changed, 41 insertions, 6 deletions

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 8f64abe699fd..d5904eef1d31 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);
 
diff --git a/arch/x86/kernel/signal_64.c b/arch/x86/kernel/signal_64.c
index 56b72fb67f9b..1c83e5124c65 100644
--- a/arch/x86/kernel/signal_64.c
+++ b/arch/x86/kernel/signal_64.c
@@ -311,6 +311,35 @@ give_sigsegv:
 }
 
 /*
+ * Return -1L or the syscall number that @regs is executing.
+ */
+static long current_syscall(struct pt_regs *regs)
+{
+        /*
+         * We always sign-extend a -1 value being set here,
+         * so this is always either -1L or a syscall number.
+         */
+        return regs->orig_ax;
+}
+
+/*
+ * Return a value that is -EFOO if the system call in @regs->orig_ax
+ * returned an error.  This only works for @regs from @current.
+ */
+static long current_syscall_ret(struct pt_regs *regs)
+{
+#ifdef CONFIG_IA32_EMULATION
+        if (test_thread_flag(TIF_IA32))
+                /*
+                 * Sign-extend the value so (int)-EFOO becomes (long)-EFOO
+                 * and will match correctly in comparisons.
+                 */
+                return (int) regs->ax;
+#endif
+        return regs->ax;
+}
+
+/*
  * OK, we're invoking a handler
  */     
 
@@ -327,9 +356,9 @@ handle_signal(unsigned long sig, siginfo_t *info, struct k_sigaction *ka,
 #endif
 
         /* Are we from a system call? */
-        if ((long)regs->orig_ax >= 0) {
+        if (current_syscall(regs) >= 0) {
                 /* If so, check system call restarting.. */
-                switch (regs->ax) {
+                switch (current_syscall_ret(regs)) {
                         case -ERESTART_RESTARTBLOCK:
                         case -ERESTARTNOHAND:
                                 regs->ax = -EINTR;
@@ -426,10 +455,9 @@ static void do_signal(struct pt_regs *regs)
         }
 
         /* Did we come from a system call? */
-        if ((long)regs->orig_ax >= 0) {
+        if (current_syscall(regs) >= 0) {
                 /* Restart the system call - no handlers present */
-                long res = regs->ax;
-                switch (res) {
+                switch (current_syscall_ret(regs)) {
                 case -ERESTARTNOHAND:
                 case -ERESTARTSYS:
                 case -ERESTARTNOINTR: