Re: [Qemu-devel] [PATCH v2 2/5] 9p: disallow the NUL character in all strings

[Eric Blake <eblake@redhat.com>]

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On 08/26/2016 10:07 AM, Greg Kurz wrote:
> According to the 9P spec at http://man.cat-v.org/plan_9/5/intro :
> 
> Data items of larger or variable lengths are represented by a
> two-byte field specifying a count, n, followed by n bytes of
> data.  Text strings are represented this way, with the text
> itself stored as a UTF-8 encoded sequence of Unicode charac-
> ters (see utf(6)). Text strings in 9P messages are not NUL-
> terminated: n counts the bytes of UTF-8 data, which include
> no final zero byte.  The NUL character is illegal in all
> text strings in 9P, and is therefore excluded from file
> names, user names, and so on.
> 
> With this patch, if a 9P client sends a text string containing a NUL
> character, the request will fail and the client is returned EINVAL.
> 
> The checking is done in v9fs_iov_vunmarshal() because it is a convenient
> place to check all client originated strings.
> 
> Suggested-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>  fsdev/9p-iov-marshal.c |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fsdev/9p-iov-marshal.c b/fsdev/9p-iov-marshal.c
> index 663cad542900..9bcdc370231d 100644
> --- a/fsdev/9p-iov-marshal.c
> +++ b/fsdev/9p-iov-marshal.c
> @@ -127,7 +127,12 @@ ssize_t v9fs_iov_vunmarshal(struct iovec *out_sg, int out_num, size_t offset,
>                                       str->size);
>                  if (copied > 0) {
>                      str->data[str->size] = 0;
> -                } else {
> +                    /* 9P forbids NUL characters in all text strings */
> +                    if (strlen(str->data) != str->size) {

If this were glibc, we could micro-optimize and do:

if (rawmemchr(str->data, 0) != str->data + str->size)

so that strlen() doesn't have to visit the tail end of the string if a
NUL is present early.  But your code is just fine as-is, and doesn't
have to worry about rawmemchr() being present.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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