[PATCH] add attribute nonstring

[Martin Sebor]

On 11/12/2017 01:39 PM, Paul Eggert wrote:
> [Dropping tz@iana.org as this email isn't relevant to tzdb.]
>
> Martin Sebor wrote:
>> I tried to make only minimal changes to suppress
>> the new warning to avoid excessive churn.
>
> As the main goal here is to find bugs not to pacify GCC, it's helpful to
> take these diagnostics as an opportunity to look more carefully at the
> strncpy calls. I just now looked at the other two data structures
> affected by the patch, and found some issues.
>
> First, the patch to sysdeps/gnu/bits/utmp.h seems reasonable, as the
> members ut_user etc. have used strncpy format since the 1970s and people
> who deal with this format are used to it. However, this format does not
> appear to be documented in the manual. I suggest accompanying it with a
> patch to the part of manual/users.texi that talks about ut_user etc., so
> that the __NONSTRING issue is documented there.
>
> Second, the patch to sysdeps/gnu/net/if.h is dubious. Its goal appears
> to be to pacify calls like the first call to strncpy in
> sysdeps/unix/sysv/linux/if_index.c. But that first call appears to be a
> bug in glibc, since the underlying Linux system call expects a
> null-terminated string. My guess is that
> this particular data structure should indeed be a null-terminated
> string, and should not be marked with __NONSTRING, and that we should
> instead change the glibc code to not use strncpy here. I suppose the
> code should report an error instead of creating a non-null-terminated
> array, and that it need not initialize the rest of the array to zeros.

I'm not familiar with this ioct call beyond what I've read on
the Linux netdevice man page before submitting the patch.  If
ifr_name is, in fact, supposed to be a string and I missed it
then that would actually be awesome because it would prove
the warning useful.

Martin

PS I still don't see it discussed on the Linux man page but
I did find such a requirement on an AIX 6.1 ioctl man page:
https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_61/apis/ioctl.htm

The descriptions of the if_indextoname and if_nametoindex
functions specified by RFC 3493 also talk about the name being
a nul-terminated string so it looks to me like you are correct
and the warning has found a Glibc bug.  Yay! :)