Re: [Qemu-devel] [PATCH for-2.3 v2 3/6] vmdk: Clean up descriptor file reading

[Markus Armbruster]

Fam Zheng <address@hidden> writes:

> Zeroing a buffer that will be filled right after is not necessary, and
> allocating a power of two + 1 is naughty.
>
> Suggested-by: Markus Armbruster <address@hidden>
> Signed-off-by: Fam Zheng <address@hidden>
> ---
>  block/vmdk.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/block/vmdk.c b/block/vmdk.c
> index 28d22db..e863a09 100644
> --- a/block/vmdk.c
> +++ b/block/vmdk.c
> @@ -558,14 +558,15 @@ static char *vmdk_read_desc(BlockDriverState *file, 
> uint64_t desc_offset,
>      }
>  
       size = bdrv_getlength(file);
       if (size < 0) {
           error_setg_errno(errp, -size, "Could not access file");
           return NULL;
       }

>      size = MIN(size, 1 << 20);  /* avoid unbounded allocation */

Consider the case where size <= 1 << 20, i.e. this line is a no-op.

> -    buf = g_malloc0(size + 1);
> +    buf = g_malloc(size);
>  
> -    ret = bdrv_pread(file, desc_offset, buf, size);
> +    ret = bdrv_pread(file, desc_offset, buf, size - 1);

Then this reads everything except the last byte (thanks to Don for
spotting it).

>      if (ret < 0) {
>          error_setg_errno(errp, -ret, "Could not read from file");
>          g_free(buf);
>          return NULL;
>      }
> +    buf[ret] = 0;
>  
>      return buf;
>  }

I figure Don suggested this instead:

diff --git a/block/vmdk.c b/block/vmdk.c
index 2cbfd3e..b7feb15 100644
--- a/block/vmdk.c
+++ b/block/vmdk.c
@@ -556,8 +556,8 @@ static char *vmdk_read_desc(BlockDriverState *file, 
uint64_t desc_offset,
         return NULL;
     }
 
-    size = MIN(size, 1 << 20);  /* avoid unbounded allocation */
-    buf = g_malloc0(size + 1);
+    size = MIN(size, (1 << 20) - 1);  /* avoid unbounded allocation */
+    buf = g_malloc(size + 1);
 
     ret = bdrv_pread(file, desc_offset, buf, size);
     if (ret < 0) {
@@ -565,6 +565,7 @@ static char *vmdk_read_desc(BlockDriverState *file, 
uint64_t desc_offset,
         g_free(buf);
         return NULL;
     }
+    buf[ret] = 0;
 
     return buf;
 }