Re: [Qemu-devel] [PATCH V2 5/8] block/qcow2: read and write the compress format extension

[Kevin Wolf <kwolf@redhat.com>]

Am 10.07.2017 um 15:29 hat Peter Lieven geschrieben:
> Am 10.07.2017 um 15:25 schrieb Kevin Wolf:
> >Am 29.06.2017 um 12:57 hat Peter Lieven geschrieben:
> >>we now read the extension on open and write it on update, but
> >>do not yet use it.
> >>
> >>Signed-off-by: Peter Lieven <pl@kamp.de>
> >>---
> >>  block/qcow2.c | 100 ++++++++++++++++++++++++++++++++++++++++++++++++++--------
> >>  block/qcow2.h |  23 +++++++++++---
> >>  2 files changed, 104 insertions(+), 19 deletions(-)
> >>
> >>diff --git a/block/qcow2.c b/block/qcow2.c
> >>index 308121a..39a8afc 100644
> >>--- a/block/qcow2.c
> >>+++ b/block/qcow2.c
> >>@@ -63,6 +63,7 @@ typedef struct {
> >>  #define  QCOW2_EXT_MAGIC_END 0
> >>  #define  QCOW2_EXT_MAGIC_BACKING_FORMAT 0xE2792ACA
> >>  #define  QCOW2_EXT_MAGIC_FEATURE_TABLE 0x6803f857
> >>+#define  QCOW2_EXT_MAGIC_COMPRESS_FORMAT 0xC03183A3
> >>  static int qcow2_probe(const uint8_t *buf, int buf_size, const char *filename)
> >>  {
> >>@@ -76,6 +77,26 @@ static int qcow2_probe(const uint8_t *buf, int buf_size, const char *filename)
> >>          return 0;
> >>  }
> >>+static int qcow2_compress_format_from_name(char *fmt)
> >>+{
> >>+    if (!fmt || !fmt[0]) {
> >>+        return QCOW2_COMPRESS_ZLIB_COMPAT;
> >>+    } else if (g_str_equal(fmt, "zlib")) {
> >>+        return QCOW2_COMPRESS_ZLIB;
> >>+    } else {
> >>+        return -EINVAL;
> >>+    }
> >>+}
> >It might make sense to allow specifying the old compression format in
> >the compression header. But I'm not so sure about using the empty string
> >rather than a proper name for it, and even less about not documenting
> >it.
> 
> The old format is used if and only if the compression header is absent.
> It makes no sense to write a header and use the old format. The old
> settings are suboptimal and old versions can't open a qcow2 with a
> compression header anyway.

Your code allows an empty string in the header extension. If you don't
want it there, you need to reject it.

Kevin