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


From: Don Koch
Subject: Re: [Qemu-devel] [PATCH for-2.3 v2 3/6] vmdk: Clean up descriptor file reading
Date: Wed, 3 Dec 2014 11:43:39 -0500

On Wed, 3 Dec 2014 17:20:27 +0800
Fam Zheng <address@hidden> wrote:

> 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:

Correct.

> 
> 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;
> >  }
> 

-d



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