block: Prevent coroutine stack overflow when recursing in bdrv_open_backing_file.

In 1.7.1 qcow2_create2 reopen the file for flushing without the BDRV_O_NO_BACKING
flags.

As a consequence the code would recursively open the whole backing chain.

These three stack arrays would pile up through the recursion and lead to a coroutine
stack overflow.

Convert these array to malloced buffers in order to streamline the coroutine
footprint.

Symptoms where freezes or segfaults on production machines while taking QMP externals
snapshots. The overflow disturbed coroutine switching.

[Resolved conflicts on qemu.git/master since the patch was against v1.7.1
--Stefan]

Signed-off-by: Benoit Canet <benoit.canet@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 18 insertions, 15 deletions

diff --git a/block.c b/block.c
index fc2edd33ae..bad7fe0f47 100644
--- a/block.c
+++ b/block.c
@@ -1068,14 +1068,14 @@ fail:
  */
 int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
 {
-    char backing_filename[PATH_MAX];
-    int back_flags, ret;
+    char *backing_filename = g_malloc0(PATH_MAX);
+    int back_flags, ret = 0;
     BlockDriver *back_drv = NULL;
     Error *local_err = NULL;
 
     if (bs->backing_hd != NULL) {
         QDECREF(options);
-        return 0;
+        goto free_exit;
     }
 
     /* NULL means an empty set of options */
@@ -1088,10 +1088,9 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
         backing_filename[0] = '\0';
     } else if (bs->backing_file[0] == '\0' && qdict_size(options) == 0) {
         QDECREF(options);
-        return 0;
+        goto free_exit;
     } else {
-        bdrv_get_full_backing_filename(bs, backing_filename,
-                                       sizeof(backing_filename));
+        bdrv_get_full_backing_filename(bs, backing_filename, PATH_MAX);
     }
 
     if (bs->backing_format[0] != '\0') {
@@ -1112,7 +1111,7 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
         error_setg(errp, "Could not open backing file: %s",
                    error_get_pretty(local_err));
         error_free(local_err);
-        return ret;
+        goto free_exit;
     }
 
     if (bs->backing_hd->file) {
@@ -1123,7 +1122,9 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
     /* Recalculate the BlockLimits with the backing file */
     bdrv_refresh_limits(bs);
 
-    return 0;
+free_exit:
+    g_free(backing_filename);
+    return ret;
 }
 
 /*
@@ -1180,8 +1181,7 @@ done:
 void bdrv_append_temp_snapshot(BlockDriverState *bs, Error **errp)
 {
     /* TODO: extra byte is a hack to ensure MAX_PATH space on Windows. */
-    char tmp_filename[PATH_MAX + 1];
-
+    char *tmp_filename = g_malloc0(PATH_MAX + 1);
     int64_t total_size;
     BlockDriver *bdrv_qcow2;
     QEMUOptionParameter *create_options;
@@ -1197,15 +1197,15 @@ void bdrv_append_temp_snapshot(BlockDriverState *bs, Error **errp)
     total_size = bdrv_getlength(bs);
     if (total_size < 0) {
         error_setg_errno(errp, -total_size, "Could not get image size");
-        return;
+        goto out;
     }
     total_size &= BDRV_SECTOR_MASK;
 
     /* Create the temporary image */
-    ret = get_tmp_filename(tmp_filename, sizeof(tmp_filename));
+    ret = get_tmp_filename(tmp_filename, PATH_MAX + 1);
     if (ret < 0) {
         error_setg_errno(errp, -ret, "Could not get temporary filename");
-        return;
+        goto out;
     }
 
     bdrv_qcow2 = bdrv_find_format("qcow2");
@@ -1221,7 +1221,7 @@ void bdrv_append_temp_snapshot(BlockDriverState *bs, Error **errp)
                          "'%s': %s", tmp_filename,
                          error_get_pretty(local_err));
         error_free(local_err);
-        return;
+        goto out;
     }
 
     /* Prepare a new options QDict for the temporary file */
@@ -1238,10 +1238,13 @@ void bdrv_append_temp_snapshot(BlockDriverState *bs, Error **errp)
                     bs->open_flags & ~BDRV_O_SNAPSHOT, bdrv_qcow2, &local_err);
     if (ret < 0) {
         error_propagate(errp, local_err);
-        return;
+        goto out;
     }
 
     bdrv_append(bs_snapshot, bs);
+
+out:
+    g_free(tmp_filename);
 }
 
 /*