block: Add BDRV_O_NO_SHARE for blk_new_open()

Normally, blk_new_open() just shares all permissions. This was fine
originally when permissions only protected against uses in the same
process because no other part of the code would actually get to access
the block nodes opened with blk_new_open(). However, since we use it for
file locking now, unsharing permissions becomes desirable.

Add a new BDRV_O_NO_SHARE flag that is used in blk_new_open() to unshare
any permissions that can be unshared.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210422164344.283389-2-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/block-backend.c

@@ -407,15 +407,19 @@ BlockBackend *blk_new_open(const char *filename, const char *reference,
     BlockBackend *blk;
     BlockDriverState *bs;
     uint64_t perm = 0;
+    uint64_t shared = BLK_PERM_ALL;
 
-    /* blk_new_open() is mainly used in .bdrv_create implementations and the
-     * tools where sharing isn't a concern because the BDS stays private, so we
-     * just request permission according to the flags.
+    /*
+     * blk_new_open() is mainly used in .bdrv_create implementations and the
+     * tools where sharing isn't a major concern because the BDS stays private
+     * and the file is generally not supposed to be used by a second process,
+     * so we just request permission according to the flags.
      *
      * The exceptions are xen_disk and blockdev_init(); in these cases, the
      * caller of blk_new_open() doesn't make use of the permissions, but they
      * shouldn't hurt either. We can still share everything here because the
-     * guest devices will add their own blockers if they can't share. */
+     * guest devices will add their own blockers if they can't share.
+     */
     if ((flags & BDRV_O_NO_IO) == 0) {
         perm |= BLK_PERM_CONSISTENT_READ;
         if (flags & BDRV_O_RDWR) {
@@ -425,8 +429,11 @@ BlockBackend *blk_new_open(const char *filename, const char *reference,
     if (flags & BDRV_O_RESIZE) {
         perm |= BLK_PERM_RESIZE;
     }
+    if (flags & BDRV_O_NO_SHARE) {
+        shared = BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED;
+    }
 
-    blk = blk_new(qemu_get_aio_context(), perm, BLK_PERM_ALL);
+    blk = blk_new(qemu_get_aio_context(), perm, shared);
     bs = bdrv_open(filename, reference, options, flags, errp);
     if (!bs) {
         blk_unref(blk);
@@ -435,7 +442,7 @@ BlockBackend *blk_new_open(const char *filename, const char *reference,
 
     blk->root = bdrv_root_attach_child(bs, "root", &child_root,
                                        BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
-                                       perm, BLK_PERM_ALL, blk, errp);
+                                       perm, shared, blk, errp);
     if (!blk->root) {
         blk_unref(blk);
         return NULL;

include/block/block.h

@@ -102,6 +102,7 @@ typedef struct HDGeometry {
     uint32_t cylinders;
 } HDGeometry;
 
+#define BDRV_O_NO_SHARE    0x0001 /* don't share permissions */
 #define BDRV_O_RDWR        0x0002
 #define BDRV_O_RESIZE      0x0004 /* request permission for resizing the node */
 #define BDRV_O_SNAPSHOT    0x0008 /* open the file read only and save writes in a snapshot */