Re: [PATCH v2 2.6.38-rc8-tip 3/20] 3: uprobes: Breakground pagereplacement.

[Thomas Gleixner]

On Mon, 14 Mar 2011, Srikar Dronamraju wrote:
> +/*
> + * Called with tsk->mm->mmap_sem held (either for read or write and
> + * with a reference to tsk->mm

Hmm, why is holding it for read sufficient?
.
> + */
> +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;

That initialization is pointless.

> +	/* 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;

IIRC then text pages are (VM_READ|VM_EXEC)

> +	/* 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 */

And what makes sure that we don't ?

> +	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);
> +
> +	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);
> +	if (!anon_vma_prepare(vma))

Why don't you get the error code of anon_vma_prepare()?

> +		/* flip pages, do_wp_page() will fail pte_same() and bail */

-ENOPARSE

> +		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 store the opcode.
> + * @opcode: location to store the read opcode.
> + *
> + * For task @tsk, read the opcode at @vaddr and store it in @opcode.
> + * Return 0 (success) or a negative errno.

Wants to called with mmap_sem held as well, right ?

> + */
> +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;
> +	ret = -EFAULT;
> +
> +	/*
> +	 * 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;

Same as above

> +	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);
> +	unlock_page(page);
> +	ret =  uprobe_opcode_sz;

  ret = 0 ?? At least, that's what the comment above says.

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