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Date: Fri, 15 Nov 2013 17:42:22 +0100
From: Max Reitz <mreitz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] HMP: snapshot_blkdev can not consider
 //root/sn1 and /root/sn1 as the same file
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To: lijun <junmuzi@gmail.com>, qemu-devel@nongnu.org
Cc: lcapitulino@redhat.com

On 15.11.2013 17:31, lijun wrote:
> From: Jun Li <junmuzi@gmail.com>
>
> Hi all,
>
> snapshot_blkdev can not consider //root/sn1 and /root/sn1 as the same
> file. when file /root/sn1 is the base file, do snapshot using file
> //root/sn1, qemu consider it as a new file. So this will rewrite the
> base file.

Actually, the same problem can occur anyway if you have a path with a
couple of =93.=94 and =93..=94 in it =96 or even just a hardlink. Thus, t=
o be
completely safe, we'd have to check whether the snapshot file (if it
already exists) has a different inode number and/or is located on a
different filesystem.

> Signed-off-by: Jun Li <junmuzi@gmail.com>
>
> --- a/hmp.c 2013-11-15 23:15:46.733361130 +0800
> +++ b/hmp.c 2013-11-16 00:20:23.972248509 +0800
> @@ -957,10 +957,12 @@ void hmp_snapshot_blkdev(Monitor *mon, c
> {
> const char *device =3D qdict_get_str(qdict, "device");
> const char *filename =3D qdict_get_try_str(qdict, "snapshot-file");
> + const char *p =3D filename;
> const char *format =3D qdict_get_try_str(qdict, "format");
> int reuse =3D qdict_get_try_bool(qdict, "reuse", 0);
> enum NewImageMode mode;
> Error *errp =3D NULL;
> + int count =3D 1;
>
> if (!filename) {
> /* In the future, if 'snapshot-file' is not specified, the snapshot
> @@ -970,6 +972,18 @@ void hmp_snapshot_blkdev(Monitor *mon, c
> return;
> }
>
> + /* Delete duplicate '/' in filename. */
> + while (*p !=3D '\0') {
> + if (*p =3D=3D '/') {
> + while (*(p + count++) =3D=3D '/') {
> + /* do null. */
> + }
> + strcpy((char *)(p + 1), (char *)(p + count - 1));

Casting a const char * to a char * seems very bogus to me. Using
g_strdup or something before is probably a better idea. Also, don't use
strcpy but memmove (since both strings overlap).

All in all, using realpath() is probably the better idea anyway (it'd
also resolve the . and .. problem (and symlinks), though it still won't
prevent hardlinks from messing things up).

Max

> + count =3D 1;
> + }
> + p++;
> + }
> +
> mode =3D reuse ? NEW_IMAGE_MODE_EXISTING : NEW_IMAGE_MODE_ABSOLUTE_PATH=
S;
> qmp_blockdev_snapshot_sync(device, filename, !!format, format,
> true, mode, &errp);
>