[systemd-devel] [PATCH] libudev-monitor: ensure proper string termination

[Zbigniew Jędrzejewski-Szmek]

On Sat, Jan 24, 2015 at 10:39:56AM +0200, Topi Miettinen wrote:
> Leave space for the terminating zero when reading and make sure
> that the last byte is zero. This also makes the check for long packets
> equivalent to code before 9c89c1ca: reject also packets with size 8192.
> ---
>  src/libudev/libudev-monitor.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/src/libudev/libudev-monitor.c b/src/libudev/libudev-monitor.c
> index 4cfb2f6..b7fc031 100644
> --- a/src/libudev/libudev-monitor.c
> +++ b/src/libudev/libudev-monitor.c
> @@ -590,7 +590,7 @@ retry:
>          if (udev_monitor == NULL)
>                  return NULL;
>          iov.iov_base = &buf;
> -        iov.iov_len = sizeof(buf);
> +        iov.iov_len = sizeof(buf) - 1; /* Leave space for terminating zero */
>          memzero(&smsg, sizeof(struct msghdr));
>          smsg.msg_iov = &iov;
>          smsg.msg_iovlen = 1;
> @@ -642,6 +642,8 @@ retry:
>          if (udev_device == NULL)
>                  return NULL;
>  
> +        buf.raw[sizeof(buf.raw) - 1] = '\0';
> +
>          if (memcmp(buf.raw, "libudev", 8) == 0) {
>                  /* udev message needs proper version magic */
>                  if (buf.nlh.magic != htonl(UDEV_MONITOR_MAGIC)) {
A buffer only needs to be terminated by a zero in certain cases: usually if it
is passed to a function which expectes that. iovecs can contain binary data,
and have an explicit size field, so they do not need to be zero-terminated.
Is there a reason why the buffer has to be zero-terminated in this case?

Zbyszek