Re: [Qemu-devel] [PATCH] v9fs_walk: As per 9p2000 RFC, MAXWELEM >= nwnames >= 0.

["Venkateswararao Jujjuri (JV)" <jvrao@linux.vnet.ibm.com>]

On 3/21/2011 2:16 PM, Stefan Hajnoczi wrote:
> On Mon, Mar 21, 2011 at 6:31 PM, Harsh Prateek Bora
> <harsh@linux.vnet.ibm.com> wrote:
>> The nwnames field in TWALK message is assumed to be >=0 and <= MAXWELEM
>> which is defined as macro P9_MAXWELEM (16) in virtio-9p.h as per 9p2000 RFC.
>> Appropriate changes are required in V9fsWalkState and v9fs_walk.
>>
>> Signed-off-by: Harsh Prateek Bora <harsh@linux.vnet.ibm.com>
>> ---
>>  hw/9pfs/virtio-9p.c |    5 ++++-
>>  hw/9pfs/virtio-9p.h |    2 +-
>>  2 files changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/hw/9pfs/virtio-9p.c b/hw/9pfs/virtio-9p.c
>> index 9b44bd0..b782a19 100644
>> --- a/hw/9pfs/virtio-9p.c
>> +++ b/hw/9pfs/virtio-9p.c
>> @@ -1805,7 +1805,7 @@ static void v9fs_walk(V9fsState *s, V9fsPDU *pdu)
>>     vs->offset += pdu_unmarshal(vs->pdu, vs->offset, "ddw", &fid,
>>                                             &newfid, &vs->nwnames);
>>
>> -    if (vs->nwnames) {
>> +    if (vs->nwnames <= P9_MAXWELEM) {
>>         vs->wnames = qemu_mallocz(sizeof(vs->wnames[0]) * vs->nwnames);
>>
>>         vs->qids = qemu_mallocz(sizeof(vs->qids[0]) * vs->nwnames);
>> @@ -1814,6 +1814,9 @@ static void v9fs_walk(V9fsState *s, V9fsPDU *pdu)
>>             vs->offset += pdu_unmarshal(vs->pdu, vs->offset, "s",
>>                                             &vs->wnames[i]);
>>         }
>> +    } else {
>> +        err = -EINVAL;
vs->nwnames  = 0; will take care of v9fs_walk_complete() issue Stefan brought up.
>> +        goto out;
>>     }
> 
> v9fs_walk_complete() will attempt to free wnames, qids, and the wnames
> strings.  Freeing the strings will crash because we're indexing into
> an array based off a NULL pointer.
> 
> It would be very handy to have a PDU-level test suite.  You could then
> construct a PDU with an invalid nwnames field and exercise this code
> path.  Perhaps a debug ioctl in Linux v9fs that allows a userspace
> tool to pass PDUs through will do?

Ioctl way was ruled out in the past discussion. But yes we have in plan on to
introduce
a test suite of this kind. Malahal (who is on vacation) will start this work
from early April.

Thanks,
JV

> 
> Stefan