x86 ptrace: user-sets-TF nits

This closes some arcane holes in single-step handling that can arise
only when user programs set TF directly (via popf or sigreturn) and
then use vDSO (syscall/sysenter) system call entry.  In those entry
paths, the clear_TF_reenable case hits and we must check TIF_SINGLESTEP
to be sure our bookkeeping stays correct wrt the user's view of TF.

Signed-off-by: Roland McGrath <roland@redhat.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index 0d2cb363ea75..e8b9863ef8c4 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -108,6 +108,19 @@ static int enable_single_step(struct task_struct *child)
         unsigned long oflags;
 
         /*
+         * If we stepped into a sysenter/syscall insn, it trapped in
+         * kernel mode; do_debug() cleared TF and set TIF_SINGLESTEP.
+         * If user-mode had set TF itself, then it's still clear from
+         * do_debug() and we need to set it again to restore the user
+         * state so we don't wrongly set TIF_FORCED_TF below.
+         * If enable_single_step() was used last and that is what
+         * set TIF_SINGLESTEP, then both TF and TIF_FORCED_TF are
+         * already set and our bookkeeping is fine.
+         */
+        if (unlikely(test_tsk_thread_flag(child, TIF_SINGLESTEP)))
+                regs->flags |= X86_EFLAGS_TF;
+
+        /*
          * Always set TIF_SINGLESTEP - this guarantees that
          * we single-step system calls etc..  This will also
          * cause us to set TF when returning to user mode.