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block-qcow2: keep backing file format in a qcow2 extension (Uri Lublin) 

Use a qcow2 extension to keep the backing file format.

By keeping the backing file format, we can:
1. Provide a way to know the backing file format without probing
   it (setting the format at creation time).
2. Enable using qcow2 format over host block devices.
   (only if the user specifically asks for it, by providing the format
   at creation time).

Also fixes a security flaw found by Daniel P. Berrange on [1]
which summarizes: "Autoprobing: just say no."

[1] http://lists.gnu.org/archive/html/qemu-devel/2008-12/msg01083.html

Signed-off-by: Uri Lublin <uril@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6909 c046a42c-6fe2-441c-8c8c-71466251a162
master
aliguori 2009-03-28 17:55:14 +00:00
parent 5eb456396d
commit f965509c9e
1 changed files with 53 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

55
block-qcow2.c
View File

@ -84,6 +84,7 @@ typedef struct {
uint32_t len;
} QCowExtension;
#define QCOW_EXT_MAGIC_END 0
#define QCOW_EXT_MAGIC_BACKING_FORMAT 0xE2792ACA
typedef struct __attribute__((packed)) QCowSnapshotHeader {
@ -235,6 +236,24 @@ static int qcow_read_extensions(BlockDriverState *bs, uint64_t start_offset,
switch (ext.magic) {
case QCOW_EXT_MAGIC_END:
return 0;
case QCOW_EXT_MAGIC_BACKING_FORMAT:
if (ext.len >= sizeof(bs->backing_format)) {
fprintf(stderr, "ERROR: ext_backing_format: len=%u too large"
" (>=%lu)\n",
ext.len, sizeof(bs->backing_format));
return 2;
}
if (bdrv_pread(s->hd, offset , bs->backing_format,
ext.len) != ext.len)
return 3;
bs->backing_format[ext.len] = '\0';
#ifdef DEBUG_EXT
printf("Qcow2: Got format extension %s\n", bs->backing_format);
#endif
offset += ((ext.len + 7) & ~7);
break;
default:
/* unknown magic -- just skip it */
offset += ((ext.len + 7) & ~7);
@ -1526,13 +1545,18 @@ static void create_refcount_update(QCowCreateState *s,
}
}
static int qcow_create(const char *filename, int64_t total_size,
const char *backing_file, int flags)
static int qcow_create2(const char *filename, int64_t total_size,
const char *backing_file, const char *backing_format,
int flags)
{
int fd, header_size, backing_filename_len, l1_size, i, shift, l2_bits;
int backing_format_len = 0;
QCowHeader header;
uint64_t tmp, offset;
QCowCreateState s1, *s = &s1;
QCowExtension ext_bf = {0, 0};
memset(s, 0, sizeof(*s));
@ -1546,6 +1570,12 @@ static int qcow_create(const char *filename, int64_t total_size,
header_size = sizeof(header);
backing_filename_len = 0;
if (backing_file) {
if (backing_format) {
ext_bf.magic = QCOW_EXT_MAGIC_BACKING_FORMAT;
backing_format_len = strlen(backing_format);
ext_bf.len = (backing_format_len + 7) & ~7;
header_size += ((sizeof(ext_bf) + ext_bf.len + 7) & ~7);
}
header.backing_file_offset = cpu_to_be64(header_size);
backing_filename_len = strlen(backing_file);
header.backing_file_size = cpu_to_be32(backing_filename_len);
@ -1590,6 +1620,19 @@ static int qcow_create(const char *filename, int64_t total_size,
/* write all the data */
write(fd, &header, sizeof(header));
if (backing_file) {
if (backing_format_len) {
char zero[16];
int d = ext_bf.len - backing_format_len;
memset(zero, 0, sizeof(zero));
cpu_to_be32s(&ext_bf.magic);
cpu_to_be32s(&ext_bf.len);
write(fd, &ext_bf, sizeof(ext_bf));
write(fd, backing_format, backing_format_len);
if (d>0) {
write(fd, zero, d);
}
}
write(fd, backing_file, backing_filename_len);
}
lseek(fd, s->l1_table_offset, SEEK_SET);
@ -1609,6 +1652,12 @@ static int qcow_create(const char *filename, int64_t total_size,
return 0;
}
static int qcow_create(const char *filename, int64_t total_size,
const char *backing_file, int flags)
{
return qcow_create2(filename, total_size, backing_file, NULL, flags);
}
static int qcow_make_empty(BlockDriverState *bs)
{
#if 0
@ -2685,4 +2734,6 @@ BlockDriver bdrv_qcow2 = {
.bdrv_snapshot_delete = qcow_snapshot_delete,
.bdrv_snapshot_list = qcow_snapshot_list,
.bdrv_get_info = qcow_get_info,
.bdrv_create2 = qcow_create2,
};