Re: [Qemu-devel] Prevent overriding the input file with the output file when using qemu-img

[Stefan Hajnoczi]

On Tue, Jan 23, 2018 at 08:48:15AM -0600, Eric Blake wrote:
> On 01/22/2018 10:40 PM, River Chiang wrote:
> >     Signed-off-by: River Chiang <address@hidden>
> > 
> > ---------------------------------- qemu-img.c
> > ----------------------------------
> > index 68b375f998..5ce594ea00 100644
> > @@ -2098,6 +2098,9 @@ static int img_convert(int argc, char **argv)
> >      if (s.src_num < 1) {
> >          error_report("Must specify image file name");
> >          goto fail_getopt;
> > +    } else if (!strcmp(argv[optind], out_filename)) {
> > +        error_report("Override the input file with the output file");
> > +        goto fail_getopt;
> 
> Comparing names is too prone to false negatives.  'foo' and './foo' are
> the same file, but your test won't catch it.  Better might be checking
> if stat() reports the same dev/inode pair for the two files.
> 
> By the way, your patch is not in proper 'git send-email' format, which
> makes it hard to test whether it even applies.  More patch submission
> hints at http://wiki.qemu.org/Contribute/SubmitAPatch

stat(2) cannot be used since the "filenames" may not be a local file,
(nbd://, iscsi://, etc).

strcmp(3) is also not a full solution, for the reasons you mentioned.

Even file locking probably isn't a full solution.  What happens when
input and output files are nbd:// URIs?

Attempting to prevent the user from harming themselves is very hard to
do.  It's better not to second-guess the user than to have some magic
that doesn't always work (the user cannot rely on it anyway).

Stefan

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