[Qemu-devel] [RFC PATCH] block: for a streaming job, fix relative base name arguments

[Jeff Cody]

When block streaming an image, if a base name is passed in that
is a relative name, but not accessible from the top-level snapshot,
then the relative name is stored incorrectly in the image file.

For instance, given a snapshot case of:

/tmp/a/base.raw
/tmp/a/snap1.qcow2
/tmp/b/snap2.qcow2

if they are all chained with relative filenames, like so:

base(bak:"") <- snap1(bak:"base.raw") <- snap2(bak:"../a/snap1.qcow2")

Then the merged top-layer will point to an inaccessible path for the
base file:

base(bak:"") <- snap2(bak:"base.raw")

This patch checks for a relative path for a basename, and fixes it
so that it is stored in the top-layer image relative to the top-layer
image:

base(bak:"") <- snap2(bak:"../a/base.raw")

Signed-off-by: Jeff Cody <address@hidden>

I submitted this as an RFC, because I had made a few assumptions that
I would like to get vetted.  Assumptions:

1.) bs->backing_hd->filename is always the same file as bs->backing_file
2.) realpath() and dirname() in QEMU behave properly across OS platforms

---
 block/stream.c |   79 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 78 insertions(+), 1 deletions(-)

diff --git a/block/stream.c b/block/stream.c
index 5c939c7..b82555b 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -13,6 +13,9 @@
 
 #include "trace.h"
 #include "block_int.h"
+#include <libgen.h>
+#include <string.h>
+#include <limits.h>
 
 enum {
     /*
@@ -163,6 +166,69 @@ static int coroutine_fn is_allocated_base(BlockDriverState 
*top,
     return 1;
 }
 
+/* Fixes the filename for the proposed backing file, so that
+ * A) if it is relative, it points to the relative path accessible
+ *    from the current bs->filename, OR
+ * B) returns 'backing' if 'backing' is already an absolute path
+ *
+ * Returns NULL if no relative or absolute path can be found.
+ */
+static char *path_find_relative(char *current, char *backing)
+{
+    char *src;
+    char *dest;
+    char *src_tmp;
+    char *src_dir;
+    char *rel_backing = NULL;
+    char relpath[PATH_MAX] = {0};
+    int offset = 0;
+
+
+    src = realpath(current, NULL);
+    if (src == NULL) {
+        goto exit;
+    }
+
+    dest = realpath(backing, NULL);
+    if (dest == NULL) {
+        free(src);
+        goto exit;
+    }
+
+    src_tmp = g_strdup(src);
+
+    if (!strcmp(backing, dest)) {
+        rel_backing = g_strdup(backing);
+        goto free_and_exit;
+    }
+
+    src_dir = dirname(src_tmp);
+    g_strlcpy(src_tmp, src_dir, strlen(src));
+
+    for (;;) {
+        if (!strncmp(src_tmp, dest, strlen(src_tmp))) {
+            offset = strlen(src_tmp);
+            offset = strlen(src_tmp) > 1 ? offset+1 : offset;
+            if (offset < strlen(dest)) {
+                rel_backing = g_strconcat(relpath, &dest[offset], NULL);
+            }
+            break;
+        } else if (strlen(src_tmp) <= 1) {
+            break;
+        }
+        src_dir = dirname(src_tmp);
+        g_strlcpy(src_tmp, src_dir, strlen(src));
+        g_strlcat(relpath, "../", sizeof(relpath));
+    }
+
+free_and_exit:
+    g_free(src_tmp);
+    free(src);
+    free(dest);
+exit:
+    return rel_backing;
+}
+
 static void coroutine_fn stream_run(void *opaque)
 {
     StreamBlockJob *s = opaque;
@@ -172,6 +238,7 @@ static void coroutine_fn stream_run(void *opaque)
     int ret = 0;
     int n;
     void *buf;
+    char *base_filename = NULL;
 
     s->common.len = bdrv_getlength(bs);
     if (s->common.len < 0) {
@@ -240,9 +307,19 @@ retry:
     if (sector_num == end && ret == 0) {
         const char *base_id = NULL;
         if (base) {
-            base_id = s->backing_file_id;
+            /* fix up relative paths, if any */
+            if (!path_is_absolute(s->backing_file_id)) {
+                base_filename = path_find_relative(bs->filename,
+                                                   s->base->filename);
+                base_id = base_filename;
+            } else {
+                base_id = s->backing_file_id;
+            }
         }
         ret = bdrv_change_backing_file(bs, base_id, NULL);
+        if (base_filename) {
+            g_free(base_filename);
+        }
         close_unused_images(bs, base, base_id);
     }
 
-- 
1.7.9.rc2.1.g69204