Re: [Qemu-devel] [PATCH V9 1/4] block/qemu-img: Refine and export infinite loop checking in collect_image_info_list()

[Jeff Cody <jcody@redhat.com>]

On Tue, Nov 26, 2013 at 01:38:30AM -0500, Xu Wang wrote:
> If there is a loop in the backing file chain, it could cause problems
> such as no response or a segfault during system boot. Hence detecting a
> backing file loop is necessary. This patch extracts the loop check from
> collect_image_info_list() in block.c into independent functions
> bdrv_backing_chain_okay() and bdrv_image_create_okay().
> 
> Signed-off-by: Xu Wang <gesaint@linux.vnet.ibm.com>
> ---
>  block.c               | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/block/block.h |  3 +++
>  qemu-img.c            | 52 ++++++++++++++++++------------------
>  3 files changed, 102 insertions(+), 26 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 382ea71..7016ce8 100644
> --- a/block.c
> +++ b/block.c
> @@ -4497,6 +4497,79 @@ bdrv_acct_done(BlockDriverState *bs, BlockAcctCookie *cookie)
>      bs->total_time_ns[cookie->type] += get_clock() - cookie->start_time_ns;
>  }
>  
> +static bool file_chain_has_loop(GHashTable *filenames, const char *filename,
> +                                BlockDriver *drv, Error **errp)
> +{
> +    BlockDriverState *bs;
> +    char fbuf[PATH_MAX];
> +    Error *local_err = NULL;
> +    int ret;
> +
> +    while (filename && (filename[0] != '\0')) {
> +        if (g_hash_table_lookup_extended(filenames, filename, NULL, NULL)) {
> +            error_setg(errp, "Backing file '%s' creates an infinite loop.",
> +                       filename);
> +            return true;
> +        }
> +        g_hash_table_insert(filenames, (gpointer)filename, NULL);
> +
> +        bs = bdrv_new("image");
> +        ret = bdrv_open(bs, filename, NULL,
> +                        BDRV_O_CACHE_WB | BDRV_O_NO_BACKING, drv, &local_err);
> +        if (ret < 0) {
> +            error_setg(errp, "Could not open '%s': %s", filename,
> +                       error_get_pretty(local_err));
> +            return true;

This leaks *bs at this point.

> +        }
> +
> +        bdrv_get_backing_filename(bs, fbuf, sizeof(fbuf));
> +        filename = fbuf;
> +        drv = NULL;
> +
> +        bdrv_unref(bs);
> +    }
> +
> +    return false;
> +}
> +
> +/**
> + * Check backing file chain if there is a loop in it.
> + *
> + * @filename: topmost image filename of backing file chain.
> + * @drv: topmost image driver(may be NULL to autodetect).
> + * @new_filename: if a new image to be created and takes @filename as backing
> + *                file, the new chain should be checked before creating.
> + *
> + * Returns: true for backing chain okay, false for loop happened.
> + */
> +bool bdrv_backing_chain_okay(const char *filename, BlockDriver *drv,
> +                             const char *new_filename, Error **errp)
> +{
> +    GHashTable *filenames;
> +
> +    filenames = g_hash_table_new_full(g_str_hash, g_str_equal, NULL, NULL);
> +
> +    if (filename == NULL || filename[0] == '\0') {
> +        goto exit;
> +    }
> +
> +    if (new_filename && new_filename[0] != '\0') {
> +        g_hash_table_insert(filenames, (gpointer)new_filename, NULL);
> +    }
> +
> +    if (file_chain_has_loop(filenames, filename, drv, errp)) {
> +        goto err;
> +    }
> +
> +exit:
> +    g_hash_table_destroy(filenames);
> +    return true;
> +
> +err:
> +    g_hash_table_destroy(filenames);
> +    return false;
> +}
> +

Minor nit, but it would be nice to have a single cleanup path.  E.g.,
something like this:

bool ret;

    ...

exit:
    g_hash_table_destroy(filenames);
    return ret;

Then just set ret as appropriate in each 'if' statement in the function, and
jump to 'exit'.


>  void bdrv_img_create(const char *filename, const char *fmt,
>                       const char *base_filename, const char *base_fmt,
>                       char *options, uint64_t img_size, int flags,
> diff --git a/include/block/block.h b/include/block/block.h
> index 3560deb..f5e84dc 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -378,6 +378,9 @@ int bdrv_save_vmstate(BlockDriverState *bs, const uint8_t *buf,
>  int bdrv_load_vmstate(BlockDriverState *bs, uint8_t *buf,
>                        int64_t pos, int size);
>  
> +bool bdrv_backing_chain_okay(const char *filename, BlockDriver *drv,
> +                             const char *new_filename, Error **errp);
> +
>  void bdrv_img_create(const char *filename, const char *fmt,
>                       const char *base_filename, const char *base_fmt,
>                       char *options, uint64_t img_size, int flags,
> diff --git a/qemu-img.c b/qemu-img.c
> index b6b5644..1f38267 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -281,6 +281,14 @@ static BlockDriverState *bdrv_new_open(const char *filename,
>          drv = NULL;
>      }
>  
> +    /* check backing file loop if the whole chain need to be opened */
> +    if (!(flags & BDRV_O_NO_BACKING) &&
> +        !bdrv_backing_chain_okay(filename, drv, NULL, &local_err)) {
> +        error_report("bdrv_new_open: Open %s failed: %s", filename,
> +                     error_get_pretty(local_err));
> +        goto fail;
> +    }
> +
>      ret = bdrv_open(bs, filename, NULL, flags, drv, &local_err);
>      if (ret < 0) {
>          error_report("Could not open '%s': %s", filename,
> @@ -1641,11 +1649,6 @@ static void dump_human_image_info_list(ImageInfoList *list)
>      }
>  }
>  
> -static gboolean str_equal_func(gconstpointer a, gconstpointer b)
> -{
> -    return strcmp(a, b) == 0;
> -}
> -
>  /**
>   * Open an image file chain and return an ImageInfoList
>   *
> @@ -1663,30 +1666,24 @@ static ImageInfoList *collect_image_info_list(const char *filename,
>                                                bool chain)
>  {
>      ImageInfoList *head = NULL;
> +    BlockDriverState *bs;
> +    ImageInfoList *elem;
>      ImageInfoList **last = &head;
> -    GHashTable *filenames;
> +    ImageInfo *info;
>      Error *err = NULL;
> +    int flags = BDRV_O_FLAGS;
>  
> -    filenames = g_hash_table_new_full(g_str_hash, str_equal_func, NULL, NULL);
> -
> -    while (filename) {
> -        BlockDriverState *bs;
> -        ImageInfo *info;
> -        ImageInfoList *elem;
> -
> -        if (g_hash_table_lookup_extended(filenames, filename, NULL, NULL)) {
> -            error_report("Backing file '%s' creates an infinite loop.",
> -                         filename);
> -            goto err;
> -        }
> -        g_hash_table_insert(filenames, (gpointer)filename, NULL);
> +    if (!chain) {
> +        flags |= BDRV_O_NO_BACKING;
> +    }
>  
> -        bs = bdrv_new_open(filename, fmt, BDRV_O_FLAGS | BDRV_O_NO_BACKING,
> -                           false, false);
> -        if (!bs) {
> -            goto err;
> -        }
> +    bs = bdrv_new_open(filename, fmt, flags,
> +                       false, false);
> +    if (!bs) {
> +        goto err;
> +    }
>  
> +    while (filename) {
>          bdrv_query_image_info(bs, &info, &err);
>          if (error_is_set(&err)) {
>              error_report("%s", error_get_pretty(err));

Between here, and the next hunk of this patch, there is a
bdrv_unref(bs) in the original code.  This becomes important later:

> @@ -1711,14 +1708,17 @@ static ImageInfoList *collect_image_info_list(const char *filename,
>              if (info->has_backing_filename_format) {
>                  fmt = info->backing_filename_format;
>              }
> +
> +            if (filename) {
> +                bs = bdrv_find_backing_image(bs, filename);
> +            }

This function will now most likely blow up on qemu-img info --backing-chain,
if the image file has any backing files. 

Edit: just checked; it does indeed segfault:

   # ~/deploy/bin/qemu-img info --backing-chain /tmp/snap3.qcow2
   Segmentation fault (core dumped)


This patch does not have the context to show it, however as mentioned
above, the bdrv_unref(bs) is performed, yet bs is still used
afterwards in this function.  Once bdrv_unref() has been called by a
function, the BDS that was unreferenced can no longer be used.



>          }
>      }
> -    g_hash_table_destroy(filenames);
> +
>      return head;
>  
>  err:
>      qapi_free_ImageInfoList(head);
> -    g_hash_table_destroy(filenames);
>      return NULL;
>  }
>  
> -- 
> 1.8.1.4
> 
>