[libvirt] [PATCH v3] buf: Fix possible infinite loop in EscapeString, VSnprintf

[Eric Blake]

On 09/09/2010 08:09 AM, Cole Robinson wrote:
> The current code will go into an infinite loop if the printf generated
> string is>= 1000, AND exactly 1 character smaller than the amount of free
> space in the buffer. When this happens, we are dropped into the loop body,
> but nothing will actually change, because count == (buf->size - buf->use - 1),
> and virBufferGrow returns unchanged if count<  (buf->size - buf->use)
>
> Fix this by removing the '- 1' bit from 'size'. The *nprintf functions handle
> the NULL byte for us anyways, so we shouldn't need to manually accommodate
> for it.
>
> Here's a bug where we are actually hitting this issue:
> https://bugzilla.redhat.com/show_bug.cgi?id=602772
>
> v2: Eric's improvements: while ->  if (), remove extra va_list variable,
>      make sure we report buffer error if snprintf fails
>
> v3: Add tests/virbuftest which reproduces the infinite loop before this
>      patch, works correctly after

I confirm that the only change in v3 is the addition of a test.

> +static int testBufInfiniteLoop(const void *data ATTRIBUTE_UNUSED)
> +{
> +    virBuffer bufinit = VIR_BUFFER_INITIALIZER;
> +    virBufferPtr buf =&bufinit;
> +    char *addstr = NULL, *bufret = NULL;
> +    int ret = -1;
> +    const struct testInfo *info = data;
> +
> +    virBufferAddChar(buf, 'a');
> +    if (buf->a != 1002 || buf->b != 1) {
> +        TEST_ERROR("Buf was not expected size, size=%d use=%d\n",
> +                   buf->a, buf->b);

This seems a bit fragile; it is inspecting the innards of virBuffer. 
Maybe add a comment here that any change to the innards of virBuffer may 
warrant a change to this test at the same time.  But having the test, 
even if it is fragile, is a good thing, so...

ACK.

We could add further tests to this file to cover more lines of code, 
such as: printing a string that needs escapes added, and actually 
verifying that the escaped string matches expectations; passing a bad 
format string (such as "%$2s" to trigger EINVAL, 
"%*d%*d",1<<31,1,1<<31,1 to trigger EOVERFLOW) and verifying that error 
gets set.  But those can be in a separate patch; let's get the bug fix 
in now before 0.8.4.

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org