x86 single_step: TIF_FORCED_TF

This changes the single-step support to use a new thread_info flag
TIF_FORCED_TF instead of the PT_DTRACE flag in task_struct.ptrace.
This keeps arch implementation uses out of this non-arch field.

This changes the ptrace access to eflags to mask TF and maintain
the TIF_FORCED_TF flag directly if userland sets TF, instead of
relying on ptrace_signal_deliver.  The 64-bit and 32-bit kernels
are harmonized on this same behavior.  The ptrace_signal_deliver
approach works now, but this change makes the low-level register
access code reliable when called from different contexts than a
ptrace stop, which will be possible in the future.

The 64-bit do_debug exception handler is also changed not to clear TF
from user-mode registers.  This matches the 32-bit kernel's behavior.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

8 files changed, 55 insertions, 48 deletions

diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 4d66a56280d3..d9905c9d0fd5 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -817,9 +817,6 @@ asmlinkage int sys_execve(struct pt_regs regs)
                         (char __user * __user *) regs.edx,
                         &regs);
         if (error == 0) {
-                task_lock(current);
-                current->ptrace &= ~PT_DTRACE;
-                task_unlock(current);
                 /* Make sure we don't return using sysenter.. */
                 set_thread_flag(TIF_IRET);
         }
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index ccc9d68d5a58..f7356e5517f6 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -709,11 +709,6 @@ long sys_execve(char __user *name, char __user * __user *argv,
         if (IS_ERR(filename)) 
                 return error;
         error = do_execve(filename, argv, envp, &regs); 
-        if (error == 0) {
-                task_lock(current);
-                current->ptrace &= ~PT_DTRACE;
-                task_unlock(current);
-        }
         putname(filename);
         return error;
 }
diff --git a/arch/x86/kernel/ptrace_32.c b/arch/x86/kernel/ptrace_32.c
index b73960885c3f..bc7fd802dcc7 100644
--- a/arch/x86/kernel/ptrace_32.c
+++ b/arch/x86/kernel/ptrace_32.c
@@ -104,6 +104,15 @@ static int putreg(struct task_struct *child,
                         break;
                 case EFL:
                         value &= FLAG_MASK;
+                        /*
+                         * If the user value contains TF, mark that
+                         * it was not "us" (the debugger) that set it.
+                         * If not, make sure it stays set if we had.
+                         */
+                        if (value & X86_EFLAGS_TF)
+                                clear_tsk_thread_flag(child, TIF_FORCED_TF);
+                        else if (test_tsk_thread_flag(child, TIF_FORCED_TF))
+                                value |= X86_EFLAGS_TF;
                         value |= get_stack_long(child, EFL_OFFSET) & ~FLAG_MASK;
                         break;
         }
@@ -119,6 +128,14 @@ static unsigned long getreg(struct task_struct *child,
         unsigned long retval = ~0UL;
 
         switch (regno >> 2) {
+                case EFL:
+                        /*
+                         * If the debugger set TF, hide it from the readout.
+                         */
+                        retval = get_stack_long(child, EFL_OFFSET);
+                        if (test_tsk_thread_flag(child, TIF_FORCED_TF))
+                                retval &= ~X86_EFLAGS_TF;
+                        break;
                 case GS:
                         retval = child->thread.gs;
                         break;
diff --git a/arch/x86/kernel/ptrace_64.c b/arch/x86/kernel/ptrace_64.c
index 4abfbced9b26..035d53e99c57 100644
--- a/arch/x86/kernel/ptrace_64.c
+++ b/arch/x86/kernel/ptrace_64.c
@@ -143,6 +143,15 @@ static int putreg(struct task_struct *child,
                         return 0;
                 case offsetof(struct user_regs_struct, eflags):
                         value &= FLAG_MASK;
+                        /*
+                         * If the user value contains TF, mark that
+                         * it was not "us" (the debugger) that set it.
+                         * If not, make sure it stays set if we had.
+                         */
+                        if (value & X86_EFLAGS_TF)
+                                clear_tsk_thread_flag(child, TIF_FORCED_TF);
+                        else if (test_tsk_thread_flag(child, TIF_FORCED_TF))
+                                value |= X86_EFLAGS_TF;
                         tmp = get_stack_long(child, EFL_OFFSET); 
                         tmp &= ~FLAG_MASK; 
                         value |= tmp;
@@ -189,6 +198,17 @@ static unsigned long getreg(struct task_struct *child, unsigned long regno)
                         if (child->thread.gsindex != GS_TLS_SEL)
                                 return 0;
                         return get_desc_base(&child->thread.tls_array[GS_TLS]);
+                case offsetof(struct user_regs_struct, eflags):
+                        /*
+                         * If the debugger set TF, hide it from the readout.
+                         */
+                        regno = regno - sizeof(struct pt_regs);
+                        val = get_stack_long(child, regno);
+                        if (test_tsk_thread_flag(child, TIF_IA32))
+                                val &= 0xffffffff;
+                        if (test_tsk_thread_flag(child, TIF_FORCED_TF))
+                                val &= ~X86_EFLAGS_TF;
+                        return val;
                 default:
                         regno = regno - sizeof(struct pt_regs);
                         val = get_stack_long(child, regno);
diff --git a/arch/x86/kernel/signal_32.c b/arch/x86/kernel/signal_32.c
index 1ac53e9a0859..0a7c812212c9 100644
--- a/arch/x86/kernel/signal_32.c
+++ b/arch/x86/kernel/signal_32.c
@@ -545,14 +545,12 @@ handle_signal(unsigned long sig, siginfo_t *info, struct k_sigaction *ka,
         }
 
         /*
-         * If TF is set due to a debugger (PT_DTRACE), clear the TF flag so
-         * that register information in the sigcontext is correct.
+         * If TF is set due to a debugger (TIF_FORCED_TF), clear the TF
+         * flag so that register information in the sigcontext is correct.
          */
-        if (unlikely(regs->eflags & TF_MASK)
-            && likely(current->ptrace & PT_DTRACE)) {
-                current->ptrace &= ~PT_DTRACE;
-                regs->eflags &= ~TF_MASK;
-        }
+        if (unlikely(regs->eflags & X86_EFLAGS_TF) &&
+            likely(test_and_clear_thread_flag(TIF_FORCED_TF)))
+                regs->eflags &= ~X86_EFLAGS_TF;
 
         /* Set up the stack frame */
         if (ka->sa.sa_flags & SA_SIGINFO)
diff --git a/arch/x86/kernel/signal_64.c b/arch/x86/kernel/signal_64.c
index 38d806467c0f..ab0178ebe00a 100644
--- a/arch/x86/kernel/signal_64.c
+++ b/arch/x86/kernel/signal_64.c
@@ -349,16 +349,12 @@ handle_signal(unsigned long sig, siginfo_t *info, struct k_sigaction *ka,
         }
 
         /*
-         * If TF is set due to a debugger (PT_DTRACE), clear the TF
-         * flag so that register information in the sigcontext is
-         * correct.
+         * If TF is set due to a debugger (TIF_FORCED_TF), clear the TF
+         * flag so that register information in the sigcontext is correct.
          */
-        if (unlikely(regs->eflags & TF_MASK)) {
-                if (likely(current->ptrace & PT_DTRACE)) {
-                        current->ptrace &= ~PT_DTRACE;
-                        regs->eflags &= ~TF_MASK;
-                }
-        }
+        if (unlikely(regs->eflags & X86_EFLAGS_TF) &&
+            likely(test_and_clear_thread_flag(TIF_FORCED_TF)))
+                regs->eflags &= ~X86_EFLAGS_TF;
 
 #ifdef CONFIG_IA32_EMULATION
         if (test_thread_flag(TIF_IA32)) {
diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 6732272e3479..243bff650ca5 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -135,7 +135,7 @@ void user_enable_single_step(struct task_struct *child)
         if (is_setting_trap_flag(child, regs))
                 return;
 
-        child->ptrace |= PT_DTRACE;
+        set_tsk_thread_flag(child, TIF_FORCED_TF);
 }
 
 void user_disable_single_step(struct task_struct *child)
@@ -144,9 +144,6 @@ void user_disable_single_step(struct task_struct *child)
         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 
         /* But touch TF only if it was set by us.. */
-        if (child->ptrace & PT_DTRACE) {
-                struct pt_regs *regs = task_pt_regs(child);
-                regs->eflags &= ~X86_EFLAGS_TF;
-                child->ptrace &= ~PT_DTRACE;
-        }
+        if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))
+                task_pt_regs(child)->eflags &= ~X86_EFLAGS_TF;
 }
diff --git a/arch/x86/kernel/traps_64.c b/arch/x86/kernel/traps_64.c
index aa248d754533..874aca397b02 100644
--- a/arch/x86/kernel/traps_64.c
+++ b/arch/x86/kernel/traps_64.c
@@ -865,27 +865,14 @@ asmlinkage void __kprobes do_debug(struct pt_regs * regs,
 
         tsk->thread.debugreg6 = condition;
 
-        /* Mask out spurious TF errors due to lazy TF clearing */
+
+        /*
+         * Single-stepping through TF: make sure we ignore any events in
+         * kernel space (but re-enable TF when returning to user mode).
+         */
         if (condition & DR_STEP) {
-                /*
-                 * The TF error should be masked out only if the current
-                 * process is not traced and if the TRAP flag has been set
-                 * previously by a tracing process (condition detected by
-                 * the PT_DTRACE flag); remember that the i386 TRAP flag
-                 * can be modified by the process itself in user mode,
-                 * allowing programs to debug themselves without the ptrace()
-                 * interface.
-                 */
                 if (!user_mode(regs))
                        goto clear_TF_reenable;
-                /*
-                 * Was the TF flag set by a debugger? If so, clear it now,
-                 * so that register information is correct.
-                 */
-                if (tsk->ptrace & PT_DTRACE) {
-                        regs->eflags &= ~TF_MASK;
-                        tsk->ptrace &= ~PT_DTRACE;
-                }
         }
 
         /* Ok, finally something we can handle */