Re: [PATCH 2/2] libxl: fix -Werror=stringop-truncation in libxl__prepare_sockaddr_un

[Marek Marczykowski-G??recki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Tue, Aug 18, 2020 at 08:43:56PM -0700, Elliott Mitchell wrote:
> On Wed, Aug 19, 2020 at 04:00:36AM +0200, Marek Marczykowski-G??recki wrote:
> > diff --git a/tools/libxl/libxl_utils.c b/tools/libxl/libxl_utils.c
> > index f360f5e228..b039143b8a 100644
> > --- a/tools/libxl/libxl_utils.c
> > +++ b/tools/libxl/libxl_utils.c
> 
> 
> >      }
> >      memset(un, 0, sizeof(struct sockaddr_un));
> >      un->sun_family = AF_UNIX;
> > -    strncpy(un->sun_path, path, sizeof(un->sun_path));
> > +    strncpy(un->sun_path, path, sizeof(un->sun_path) - 1);
> >      return 0;
> >  }
> 
> While the earlier lines are okay, this line introduces an error.  

Why exactly? strncpy() copies up to n characters, quoting its manual
page:

    If there is no null byte among the first n bytes of src, the string
    placed in dest will not be null-terminated

But since the whole struct is zeroed out initially, this should still
result in a null terminated string, as the last byte of that buffer will
not be touched by the strncpy.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

Attachment: signature.asc
Description: PGP signature