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

[Fam Zheng]

On Wed, 12/03 09:21, Markus Armbruster wrote:
> 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).

Yes, I was wrong.

> 
> >      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:

Yes. Thanks.

Fam

> 
> 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;
>  }