Re: [Qemu-devel] [PATCH v2 1/5] 9p: forbid illegal path names

[Eric Blake]

On 08/26/2016 10:07 AM, Greg Kurz wrote:
> Empty path components don't make sense and may cause undefined behavior,
> depending on the backend.
> 
> Also, the walk request described in the 9P spec [1] clearly shows that
> the client is supposed to send individual path components: the official
> linux client never sends portions of path containing the / character for
> example.
> 
> Moreover, the 9P spec [2] also states that a system can decide to restrict
> the set of supported characters used in path components, with an explicit
> mention "to remove slashes from name components".
> 
> This patch introduces a new name_is_illegal() helper that checks the
> names sent by the client are not empty and don't contain unwanted chars.
> Since 9pfs is only supported on linux hosts, only the / character is
> checked at the moment. When support for other hosts (AKA. win32) is added,
> other chars may need to be blacklisted as well.
> 
> If a client sends an illegal path component, the request will fail and
> EINVAL is returned to the client.
> 
> [1] http://man.cat-v.org/plan_9/5/walk
> [2] http://man.cat-v.org/plan_9/5/intro
> 
> Suggested-by: Peter Maydell <address@hidden>
> Signed-off-by: Greg Kurz <address@hidden>
> ---
>  hw/9pfs/9p.c |   56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 56 insertions(+)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index b6b02b46a9da..dba11773699b 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -1256,6 +1256,11 @@ static int v9fs_walk_marshal(V9fsPDU *pdu, uint16_t 
> nwnames, V9fsQID *qids)
>      return offset;
>  }
>  
> +static bool name_is_illegal(const char *name)
> +{
> +    return name == NULL || strchr(name, '/') != NULL;

Is anyone actually passing NULL?  And the commit message says you are
blacklisting the empty string "", but that is not what you did here.
Depending on whether callers can even pass NULL, you may be able to just
s/name == NULL/!*name/

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

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