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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The use of KM_foo is obsolete and un-needed.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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