Messages in this thread | | | Date | Mon, 14 Mar 2011 22:54:39 +0530 | From | Srikar Dronamraju <> | Subject | Re: [PATCH v2 2.6.38-rc8-tip 3/20] 3: uprobes: Breakground page replacement. |
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* Steven Rostedt <rostedt@goodmis.org> [2011-03-14 11:38:57]:
> 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.
You are right, it should have been if ((vma->vm_flags & (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)) != (VM_READ|VM_EXEC)) goto put_out;
Your comment applied for read_opcode function too. Will correct in the next version of the patchset.
However in the next patch, where we replace the above with valid_vma and that does the right thing.
> > I'm also curious to why we can't modify text code that is also mapped as > read/write. >
If text code is mapped read/write then on memory pressure the page gets written to the disk. Hence breakpoints inserted may end up being in the disk copy modifying the actual copy.
-- Thanks and Regards Srikar
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