Re: [Qemu-devel] [PATCH] block: fix backing file segfault

[Kevin Wolf]

Am 10.01.2014 um 18:27 hat Max Reitz geschrieben:
> On 09.01.2014 11:59, Kevin Wolf wrote:
> >[ CCing Max, who was recently active in this area, for another opinion ]
> >
> >Am 08.01.2014 um 20:43 hat Peter Feiner geschrieben:
> >>When a backing file is opened such that (1) a protocol is directly
> >>used as the block driver and (2) the block driver has bdrv_file_open,
> >>bdrv_open_backing_file segfaults. The problem arises because
> >>bdrv_open_common returns without setting bd->backing_hd->file.
> >>
> >>To effect (1), you seem to have to use the -F flag in qemu-img. There
> >>are several block drivers that satisfy (2), such as "file" and "nbd".
> >>Here are some concrete examples:
> >>
> >>     #!/bin/bash
> >>
> >>     echo Test file format
> >>     ./qemu-img create -f file base.file 1m
> >>     ./qemu-img create -f qcow2 -F file -o backing_file=base.file\
> >>         file-overlay.qcow2
> >>     ./qemu-img convert -O raw file-overlay.qcow2 file-convert.raw
> >>
> >>     echo Test nbd format
> >>     SOCK=$PWD/nbd.sock
> >>     ./qemu-img create -f raw base.raw 1m
> >>     ./qemu-nbd -t -k $SOCK base.raw &
> >>     trap "kill $!" EXIT
> >>     while ! test -e $SOCK; do sleep 1; done
> >>     ./qemu-img create -f qcow2 -F nbd -o backing_file=nbd:unix:$SOCK\
> >>         nbd-overlay.qcow2
> >>     ./qemu-img convert -O raw nbd-overlay.qcow2 nbd-convert.raw
> >>
> >>Without this patch, the two qemu-img convert commands segfault.
> >>
> >>This is a regression that was introduced in v1.7 by
> >>dbecebddfa4932d1c83915bcb9b5ba5984eb91be.
> >>
> >>Signed-off-by: Peter Feiner <address@hidden>
> >>---
> >>  block.c |    5 +++--
> >>  1 file changed, 3 insertions(+), 2 deletions(-)
> >>
> >>diff --git a/block.c b/block.c
> >>index 64e7d22..a4a172d 100644
> >>--- a/block.c
> >>+++ b/block.c
> >>@@ -1016,8 +1016,9 @@ int bdrv_open_backing_file(BlockDriverState *bs, 
> >>QDict *options, Error **errp)
> >>          error_free(local_err);
> >>          return ret;
> >>      }
> >>-    pstrcpy(bs->backing_file, sizeof(bs->backing_file),
> >>-            bs->backing_hd->file->filename);
> >>+    if (bs->backing_hd->file)
> >>+        pstrcpy(bs->backing_file, sizeof(bs->backing_file),
> >>+                bs->backing_hd->file->filename);
> >>      return 0;
> >>  }
> >I think if there is no bs->backing_hd->file, we should get the filename
> >from bs->backing_hd->filename instead of leaving it empty.
> >
> >In fact, can we always do that or does bs->backing_hd normally lack the
> >filename? If so, perhaps that is what we need to fix, so we can always
> >directly use bs->backing_hd->filename here.
> 
> bs->backing_hd->filename would be set by the bdrv_open_common() in
> bdrv_open(), the filename is read from file->filename (if file !=
> NULL; in this case, that would be bs->backing_hd->file->filename) or
> from the configuration option "filename".
> 
> The latter configuration option is not used by
> bdrv_open_backing_file(), as far as I can see. However,
> bs->backing_hd->file->filename is exactly the field the old code
> uses, therefore, using bs->backing_hd->filename directly should not
> break anything.
> 
> However, the patch does something different: If file is NULL, it
> leaves bs->backing_file unchanged; whereas using
> bs->backing_hd->filename would in this case result in the value of
> the "filename" option. I think leaving bs->backing_file unchanged is
> probably better, unless it is "" and the "filename" option is set.
> 
> If we want bs->backing_hd->filename to always point to a valid
> filename, we'd probably have to copy to contents of bs->backing_file
> there at some point in time, if it is not valid. But this is exactly
> a point in code where bs->backing_file is updated, so there'd be no
> gain if we instead updated bs->backing_hd->filename if necessary and
> then copied that to bs->backing_file, as long as there is no other
> place in the code where bs->backing_hd->filename always has to be a
> valid filename.
> 
> Thus, I think the patch is okay, but I'd probably prefer "if
> (bs->backing_hd->filename[0]) pstrcpy(...,
> bs->backing_hd->filename);" - although that should not differ from
> the given patch, unless the "filename" option is set for the
> backing_hd.

Ok, if you're happy with it, I'll apply it. Can I put your Reviewed-by
there?

In the long run, we need to get rid of all this copying anyway. I'm
imagining a BlockDriver function that returns a file name to reproduce
the same setup, and a removal of bs->backing_file and bs->file_name.

For some drivers, the returned filename would be a URL or some other
string that that particular driver can parse.

While doing that, we might also consider a fake protocol that handles
filenames like 'json:{"driver":"qcow2","lazy-refcounts":"on",...}',
because for some drivers this might be the only thing that comes close
to a filename as it is a single string at least...

Kevin