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

[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().