[ntp:hackers] New Coding Standard Statement - Re: Please use strlcpy(), strlcat() in NTP distribution.

todd glassey tglassey at earthlink.net
Wed Apr 6 11:55:46 UTC 2011


On 4/5/2011 3:43 PM, Dave Hart wrote:

Good idea Dave, but I want to propose we (ntp.org) create a  new 
document to accompany the codebase of the reference port which is a 
formal statement of the coding rules which are used in the reference 
port. We should then publish it as an engineering analysis document for 
compliance and audit controls.

It will seem silly initially but since you clearly are changing the 
coding standards we need a mechanism to track them separately from the 
code-base so that its easy to tell what was done when.

Todd
> I've converted nearly all of the strcpy() and strncpy() uses in NTP to use
> strlcpy(), and strcat() and strncat() to strlcat().  The big win is the
> strl... routines always null-terminate the resulting string, even when
> truncating.  strncpy() zero-fills after the string to the end of the buffer,
> which is needlessly expensive particularly with larger buffers.  strncat()
> is difficult to use correctly, as it takes not the buffer size but the
> remaining buffer after the existing string.  Most uses in NTP got this
> wrong, refclock_jjy.c being the major exception.  strlcpy() and strlcat()
> are documented in both ntp_stdlib.h and libntp/strl_obsd.c, but here's the
> cheat sheet:
>
> To avoid buffer overrun and ignore truncation, call them passing the
> sizeof(buffer) or equivalent, and ignore the return value.  The return value
> is the strlen() of the string that would have resulted, assuming sufficient
> buffer.  The return value will be strictly less than the provided size if
> the string fits, so to check for truncation:
>
> if (strlcpy(buf, src, sizeof(buf))>= sizeof(buf))
>      /* handle truncation */
>
> The remaining strcpy()/strcat() calls in lib/isc and sntp/libopts have all
> been audited and have no overrun issues.  That leaves
> ports/winnt/instsrv/instsrv.c and ntp_crypto.c as the only strcpy()/strcat()
> consumers.  I'll take care of ntp_crypto.c separately.  I'm tempted to
> remove instsrv, adapted from an ancient BIND windows port.  I never use it
> or recommend its use.
>
> http://www.gratisoft.us/todd/papers/strlcpy.html is from the mid-90s when
> strlcpy() and strlcat() were introduced.
>
> Cheers,
> Dave Hart
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