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    SubjectRe: [PATCH v3 2.6.39-rc1-tip 4/26] 4: uprobes: Breakground page replacement.
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    On Fri, 2011-04-01 at 20:03 +0530, Srikar Dronamraju wrote:

    > +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;
    > +
    > + /* 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;

    Why not return the actual gup() error?

    > + ret = -EINVAL;
    > +
    > + /*
    > + * We are interested in text pages only. Our pages of interest
    > + * should be mapped for read and execute only. We desist from
    > + * adding probes in write mapped pages since the breakpoints
    > + * might end up in the file copy.
    > + */
    > + if ((vma->vm_flags & (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)) !=
    > + (VM_READ|VM_EXEC))
    > + goto put_out;

    Note how you return -EINVAL here when we're attempting to poke at the
    wrong kind of mapping.

    > + /* Allocate a page */
    > + new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, vaddr);
    > + if (!new_page) {
    > + ret = -ENOMEM;
    > + goto put_out;
    > + }
    > +
    > + /*
    > + * lock page will serialize against do_wp_page()'s
    > + * PageAnon() handling
    > + */
    > + lock_page(old_page);
    > + /* copy the page now that we've got it stable */
    > + vaddr_old = kmap_atomic(old_page, KM_USER0);
    > + vaddr_new = kmap_atomic(new_page, KM_USER1);
    > +
    > + memcpy(vaddr_new, vaddr_old, PAGE_SIZE);
    > + /* poke the new insn in, ASSUMES we don't cross page boundary */

    Why not test this assertion with a VM_BUG_ON() or something.

    > + addr = vaddr;
    > + vaddr &= ~PAGE_MASK;
    > + memcpy(vaddr_new + vaddr, &opcode, uprobe_opcode_sz);
    > +
    > + kunmap_atomic(vaddr_new, KM_USER1);
    > + kunmap_atomic(vaddr_old, KM_USER0);

    The use of KM_foo is obsolete and un-needed.

    > + orig_pte = page_check_address(old_page, tsk->mm, addr, &ptl, 0);
    > + if (!orig_pte)
    > + goto unlock_out;
    > + pte_unmap_unlock(orig_pte, ptl);
    > +
    > + lock_page(new_page);
    > + ret = anon_vma_prepare(vma);
    > + if (!ret)
    > + ret = replace_page(vma, old_page, new_page, *orig_pte);
    > +
    > + unlock_page(new_page);
    > + if (ret != 0)
    > + page_cache_release(new_page);
    > +unlock_out:
    > + unlock_page(old_page);
    > +
    > +put_out:
    > + put_page(old_page); /* we did a get_page in the beginning */
    > + return ret;
    > +}
    > +
    > +/**
    > + * read_opcode - read the opcode at a given virtual address.
    > + * @tsk: the probed task.
    > + * @vaddr: the virtual address to read the opcode.
    > + * @opcode: location to store the read opcode.
    > + *
    > + * Called with tsk->mm->mmap_sem held (for read and with a reference to
    > + * tsk->mm.
    > + *
    > + * For task @tsk, read the opcode at @vaddr and store it in @opcode.
    > + * Return 0 (success) or a negative errno.
    > + */
    > +int __weak read_opcode(struct task_struct *tsk, unsigned long vaddr,
    > + uprobe_opcode_t *opcode)
    > +{
    > + struct vm_area_struct *vma;
    > + struct page *page;
    > + void *vaddr_new;
    > + int ret;
    > +
    > + ret = get_user_pages(tsk, tsk->mm, vaddr, 1, 0, 0, &page, &vma);
    > + if (ret <= 0)
    > + return -EFAULT;

    Again, why not return the gup() error proper?

    > + ret = -EFAULT;
    > +
    > + /*
    > + * We are interested in text pages only. Our pages of interest
    > + * should be mapped for read and execute only. We desist from
    > + * adding probes in write mapped pages since the breakpoints
    > + * might end up in the file copy.
    > + */
    > + if ((vma->vm_flags & (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)) !=
    > + (VM_READ|VM_EXEC))
    > + goto put_out;

    But now you return -EFAULT if we peek at the wrong kind of mapping,
    which is inconsistent with the -EINVAL of write_opcode().

    > + lock_page(page);
    > + vaddr_new = kmap_atomic(page, KM_USER0);
    > + vaddr &= ~PAGE_MASK;
    > + memcpy(opcode, vaddr_new + vaddr, uprobe_opcode_sz);
    > + kunmap_atomic(vaddr_new, KM_USER0);

    Again, loose the KM_foo.

    > + unlock_page(page);
    > + ret = 0;
    > +
    > +put_out:
    > + put_page(page); /* we did a get_page in the beginning */
    > + return ret;
    > +}


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