linux-kernel - [PATCH] Reading the VDSO area

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Date:	Tue, 17 Jul 2007 12:23:26 -0400
From:	John Blackwood <john.blackwood@...r.com>
To:	linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...source.com>
CC:	Andi Kleen <ak@...e.de>, Roland McGrath <roland@...hat.com>,
	bugsy@...r.com
Subject: [PATCH] Reading the VDSO area - i386

Hi Jeremy,

I was doing some tests that attempt to read the VDSO area of a
task through either the /proc/pid/mem or ptrace(PTRACE_PEEKTEXT,
...) interfaces, and it seems that when the CONFIG_COMPAT_VDSO kernel
parameter is enabled, we can no longer successfully read the VDSO area
on i386 kernels.

I believe that debuggers such as gdb will attempt to sometimes walkback
through the vsyscall area, and not being able to read the vsyscall/vdso
area would thus cause debuggers problems.

So assuming that this change in behavior was not intentional, I've
provided my stab (just an idea) at a fix.  With this change below,
the code in places such as get_user_pages() can now successfully call
in_gate_area() and then subsequently call get_gate_vma(), which already
properly returns the correct info.

Thanks for taking the time to read over this.

---
/userland/johnb/s/os/kernel/linux-2.6.22/arch/i386/kernel/sysenter.c
2007-07-17 08:38:48.000000000 -0400
+++ new/arch/i386/kernel/./sysenter.c	2007-07-17 11:48:28.000000000 -0400
@@ -336,6 +336,14 @@ struct vm_area_struct *get_gate_vma(stru

  int in_gate_area(struct task_struct *task, unsigned long addr)
  {
+	struct mm_struct *mm = task->mm;
+
+	/* Check to see if this task was created in compat vdso mode
+	 * and if the address is within the gate_vma area.
+	 */
+	if (mm && mm->context.vdso == (void *)VDSO_HIGH_BASE &&
+			addr >= gate_vma.vm_start && addr <= gate_vma.vm_end)
+		return 1;
  	return 0;
  }

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