Re: [Qemu-devel] [PATCH] qemu-img: initialize MapEntry object

[Kevin Wolf <kwolf@redhat.com>]

Am 04.02.2016 um 00:38 hat John Snow geschrieben:
> Commit 16b0d555 introduced an issue where we are not initializing
> has_filename for the 'next' MapEntry object, which leads to interesting
> errors in Valgrind and Clang -fsanitize=undefined both.
> 
> Zero the stack object at allocation AND make sure the utility to
> populate the fields properly marks has_filename as false if applicable.
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  qemu-img.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/qemu-img.c b/qemu-img.c
> index f121980..5a85178 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -2231,6 +2231,9 @@ static int get_block_status(BlockDriverState *bs, int64_t sector_num,
>      if (file && e->has_offset) {
>          e->has_filename = true;
>          e->filename = file->filename;
> +    } else {
> +        e->has_filename = false;
> +        e->filename = NULL;
>      }
>      return 0;
>  }

I guess this fixes the bug, but wouldn't it actually be nicer to just
reinitialise the whole object? As everyone knows, I love compound
literals, so I'd make it one big assignment that zeroes everything that
isn't specified:

*e = (MapEntry) {
    ...
};

> @@ -2264,7 +2267,7 @@ static int img_map(int argc, char **argv)
>      BlockDriverState *bs;
>      const char *filename, *fmt, *output;
>      int64_t length;
> -    MapEntry curr = { .length = 0 }, next;
> +    MapEntry curr = { .length = 0 }, next = { .length = 0 };
>      int ret = 0;

At first I didn't quite understand what this was for, but I think you
tried to cover newly added fields. If you overwrite the whole struct
above, you wouldn't need to initialise it here any more.

Kevin