Subject: Re: [PATCH v2 2.6.38-rc8-tip 3/20]  3: uprobes: Breakground page
 replacement.
From: Steven Rostedt <rostedt@goodmis.org>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
        Linux-mm <linux-mm@kvack.org>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Christoph Hellwig <hch@infradead.org>,
        Andi Kleen <andi@firstfloor.org>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Oleg Nesterov <oleg@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
        Jim Keniston <jkenisto@linux.vnet.ibm.com>,
        Roland McGrath <roland@hack.frob.com>,
        SystemTap <systemtap@sources.redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
In-Reply-To: <20110314133433.27435.49566.sendpatchset@localhost6.localdomain6>
References: <20110314133403.27435.7901.sendpatchset@localhost6.localdomain6>
	 <20110314133433.27435.49566.sendpatchset@localhost6.localdomain6>
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Date: Mon, 14 Mar 2011 11:38:57 -0400
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On Mon, 2011-03-14 at 19:04 +0530, Srikar Dronamraju wrote:
> +/*
> + * Called with tsk->mm->mmap_sem held (either for read or write and
> + * with a reference to tsk->mm.
> + */
> +static int write_opcode(struct task_struct *tsk, struct uprobe * uprobe,
> +                       unsigned long vaddr, uprobe_opcode_t opcode)
> +{
> +       struct page *old_page, *new_page;
> +       void *vaddr_old, *vaddr_new;
> +       struct vm_area_struct *vma;
> +       spinlock_t *ptl;
> +       pte_t *orig_pte;
> +       unsigned long addr;
> +       int ret = -EINVAL;
> +
> +       /* Read the page with vaddr into memory */
> +       ret = get_user_pages(tsk, tsk->mm, vaddr, 1, 1, 1, &old_page, &vma);
> +       if (ret <= 0)
> +               return -EINVAL;
> +       ret = -EINVAL;
> +
> +       /*
> +        * check if the page we are interested is read-only mapped
> +        * Since we are interested in text pages, Our pages of interest
> +        * should be mapped read-only.
> +        */
> +       if ((vma->vm_flags & (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)) ==
> +                                               (VM_READ|VM_EXEC))
> +               goto put_out;
> + 

I'm confused by the above comment and code. You state we are only
interested text pages mapped read-only, but then if the page is mapped
read/exec we exit out? It is fine if it is anything but READ/EXEC.

I'm also curious to why we can't modify text code that is also mapped as
read/write.

-- Steve


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