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Commit 6718d0d6 authored by Roland McGrath's avatar Roland McGrath
Browse files

x86 ptrace: block-step fix 

The enable_single_step() logic bails out early if TF is already set.
That skips some of the bookkeeping that keeps things straight.
This makes PTRACE_SINGLEBLOCK break the behavior of a user task
that was already setting TF itself in user mode.

Fix the bookkeeping to notice the old TF setting as it should.

Test case at: http://sources.redhat.com/cgi-bin/cvsweb.cgi/~checkout~/tests/ptrace-tests/tests/step-jump-cont-strict.c?cvsroot=systemtap



Signed-off-by: default avatarRoland McGrath <roland@redhat.com>
parent a3cf8593

arch/x86/kernel/step.c

+16 −6
Original line number Diff line number Diff line
@@ -105,6 +105,7 @@ static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs) 
static int enable_single_step(struct task_struct *child)

{

	struct pt_regs *regs = task_pt_regs(child);

	unsigned long oflags;



	/*

	 * Always set TIF_SINGLESTEP - this guarantees that
@@ -113,11 +114,7 @@ static int enable_single_step(struct task_struct *child) 
	 */

	set_tsk_thread_flag(child, TIF_SINGLESTEP);



	/*

	 * If TF was already set, don't do anything else

	 */

	if (regs->flags & X86_EFLAGS_TF)

		return 0;

	oflags = regs->flags;



	/* Set TF on the kernel stack.. */

	regs->flags |= X86_EFLAGS_TF;
@@ -126,9 +123,22 @@ static int enable_single_step(struct task_struct *child) 
	 * ..but if TF is changed by the instruction we will trace,

	 * don't mark it as being "us" that set it, so that we

	 * won't clear it by hand later.

	 *

	 * Note that if we don't actually execute the popf because

	 * of a signal arriving right now or suchlike, we will lose

	 * track of the fact that it really was "us" that set it.

	 */

	if (is_setting_trap_flag(child, regs))

	if (is_setting_trap_flag(child, regs)) {

		clear_tsk_thread_flag(child, TIF_FORCED_TF);

		return 0;

	}



	/*

	 * If TF was already set, check whether it was us who set it.

	 * If not, we should never attempt a block step.

	 */

	if (oflags & X86_EFLAGS_TF)

		return test_tsk_thread_flag(child, TIF_FORCED_TF);



	set_tsk_thread_flag(child, TIF_FORCED_TF);