| Message ID | 20190307165303.15382-1-kwolf@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | qcow2 spec: Describe string header extensions | expand |
On 3/7/19 10:53 AM, Kevin Wolf wrote: > Be more specific about the string representation in header extensions. > > Suggested-by: Stefan Hajnoczi <stefanha@redhat.com> > Signed-off-by: Kevin Wolf <kwolf@redhat.com> > --- > docs/interop/qcow2.txt | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > Based-on: <20190227172256.30368-1-kwolf@redhat.com> > > > +== String header extensions == > + > +Some header extensions (such as the backing file format name and the external > +data file name) are just a single string. In this case, the header extension > +length is the string length and the string is not '\0' terminated. (The header > +extension padding can make it look like a string is '\0' terminated, but > +neither is padding always necessary nor is there a guarantee that zero bytes > +are used for padding.) We didn't require 0 padding? (goes and re-reads) - oops, yes that's correct. It makes it harder to extend a struct by making use of that padding if you can't guarantee what the padding had to be prior to the extension, and means that you have to consider whether there are any potential security risks of the padding being used as a side channel to leak information while still being a well-formed file. But changing the standard to require zero padding is different than documenting existing practice, so your patch is correct. Reviewed-by: Eric Blake <eblake@redhat.com>
On Thu, Mar 07, 2019 at 05:53:03PM +0100, Kevin Wolf wrote: > Be more specific about the string representation in header extensions. > > Suggested-by: Stefan Hajnoczi <stefanha@redhat.com> > Signed-off-by: Kevin Wolf <kwolf@redhat.com> > --- > docs/interop/qcow2.txt | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Am 07.03.2019 um 18:40 hat Eric Blake geschrieben: > On 3/7/19 10:53 AM, Kevin Wolf wrote: > > Be more specific about the string representation in header extensions. > > > > Suggested-by: Stefan Hajnoczi <stefanha@redhat.com> > > Signed-off-by: Kevin Wolf <kwolf@redhat.com> > > --- > > docs/interop/qcow2.txt | 14 ++++++++++++-- > > 1 file changed, 12 insertions(+), 2 deletions(-) > > > > Based-on: <20190227172256.30368-1-kwolf@redhat.com> > > > > > > > +== String header extensions == > > + > > +Some header extensions (such as the backing file format name and the external > > +data file name) are just a single string. In this case, the header extension > > +length is the string length and the string is not '\0' terminated. (The header > > +extension padding can make it look like a string is '\0' terminated, but > > +neither is padding always necessary nor is there a guarantee that zero bytes > > +are used for padding.) > > We didn't require 0 padding? (goes and re-reads) - oops, yes that's > correct. Yes. QEMU does use zeroes, but the spec doesn't mandate it and changing it would be an incompatible change. And the worst that could happen is that someone leaves a hidden message in the padding, which doesn't really hurt. > It makes it harder to extend a struct by making use of that > padding if you can't guarantee what the padding had to be prior to the > extension, and means that you have to consider whether there are any > potential security risks of the padding being used as a side channel to > leak information while still being a well-formed file. It doesn't actually make it harder because we have the length field which tells us the exact length of the valid data, so we shouldn't even try to use any of the padding. > But changing the standard to require zero padding is different than > documenting existing practice, so your patch is correct. > > Reviewed-by: Eric Blake <eblake@redhat.com> Thanks! Kevin
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt index bfb97cfde3..8c3098d8d9 100644 --- a/docs/interop/qcow2.txt +++ b/docs/interop/qcow2.txt @@ -170,11 +170,11 @@ be stored. Each extension has a structure like the following: Byte 0 - 3: Header extension type: 0x00000000 - End of the header extension area - 0xE2792ACA - Backing file format name + 0xE2792ACA - Backing file format name string 0x6803f857 - Feature name table 0x23852875 - Bitmaps extension 0x0537be77 - Full disk encryption header pointer - 0x44415441 - External data file name + 0x44415441 - External data file name string other - Unknown header extension, can be safely ignored @@ -196,6 +196,16 @@ data of compatible features that it doesn't support. Compatible features that need space for additional data can use a header extension. +== String header extensions == + +Some header extensions (such as the backing file format name and the external +data file name) are just a single string. In this case, the header extension +length is the string length and the string is not '\0' terminated. (The header +extension padding can make it look like a string is '\0' terminated, but +neither is padding always necessary nor is there a guarantee that zero bytes +are used for padding.) + + == Feature name table == The feature name table is an optional header extension that contains the name
Be more specific about the string representation in header extensions. Suggested-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com> --- docs/interop/qcow2.txt | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-)