Re: [Qemu-devel] [PATCH 1/2] block: make bdrv_find_backing_image compare canonical filenames

[Kevin Wolf]

Am 13.10.2012 17:25, schrieb Jeff Cody:
> On 10/12/2012 05:52 PM, Eric Blake wrote:
>> On 10/09/2012 11:56 PM, Jeff Cody wrote:
>>> Currently, bdrv_find_backing_image compares bs->backing_file with
>>> what is passed in as a backing_file name.  Mismatches may occur,
>>> however, when bs->backing_file and backing_file are both not
>>> absolute or relative.
>>>
>>> Use path_combine() to make sure any relative backing filenames are
>>> relative to the current image filename being searched, and then use
>>> realpath() to make all comparisons based on absolute filenames.
>>>
>>> This also changes bdrv_find_backing_image to no longer be recursive,
>>> but iterative.
>>>
>>> Signed-off-by: Jeff Cody <address@hidden>
>>> ---
>>>  block.c | 38 ++++++++++++++++++++++++++++++++------
>>>  1 file changed, 32 insertions(+), 6 deletions(-)
>>>
>>
>>> +    for (curr_bs = bs; curr_bs->backing_hd; curr_bs = curr_bs->backing_hd) 
>>> {
>>> +        /* If not an absolute filename path, make it relative to the 
>>> current
>>> +         * image's filename path */
>>> +        path_combine(filename_tmp, sizeof(filename_tmp),
>>> +                     curr_bs->filename, backing_file);
>>
>> I just realized that it is possible to set up a qcow2 file that wraps a
>> network protocol as its backing source, such as 'nbd:...'.  In this
>> case, what does path_combine() do to that user string?
>>
>>> +
>>> +        /* We are going to compare absolute pathnames */
>>> +        if (!realpath(filename_tmp, filename_full)) {
>>> +            continue;
>>> +        }
>>
>> and realpath() certainly won't like it (most likely, it won't exist in
>> the file system, but on the off chance that it does, that file is much
>> different than the real protocol that we are using as the backing source).
>>
>> I'm afraid you may need a followup patch that handles the case of a
>> non-file backing protocol, and insist on an exact match in that case
>> without trying any normalization.
>>
> 
> Yes, it will need a follow-up patch.  Check for protocol
> (path_has_protocol()), and if true just compare unmodified
> backing_file names, other wise use path_combine + realpath().

These cases would regress, so it doesn't just need a follow-up patch,
but a respin of this series.

Kevin