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, 10 insertions, 0 deletions

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 34e77b16a42a..e37dccce85db 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -1416,6 +1416,16 @@ asmregparm long syscall_trace_enter(struct pt_regs *regs)
 {
         long ret = 0;
 
+        /*
+         * 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.  If we entered on the slow path, TF was already set.
+         */
+        if (test_thread_flag(TIF_SINGLESTEP))
+                regs->flags |= X86_EFLAGS_TF;
+
         /* do the secure computing check first */
         secure_computing(regs->orig_ax);