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From: Archie Cobbs <archie@whistle.com>
Message-Id: <199811240841.AAA03369@bubba.whistle.com>
Subject: Re: snprintf() in the kernel
In-Reply-To: <199811240741.SAA17412@godzilla.zeta.org.au> from Bruce Evans at "Nov 24, 98 06:41:35 pm"
To: bde@zeta.org.au (Bruce Evans)
Date: Tue, 24 Nov 1998 00:41:56 -0800 (PST)
Cc: dillon@apollo.backplane.com, freebsd-current@FreeBSD.ORG, grog@lemis.com,
        rnordier@nordier.com
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Bruce Evans writes:
> >Thanks to those who checked out the snprintf() patches. "Rev 2" is now
> >available in ftp://ftp.whistle.com/pub/archie/misc. I took out
> >a bunch of strncpy() patches and tweaked a couple of other things.
> 
> It would be better without any strncpy() patches.

Well, it's certainly easy enough to take them out.. however, some
are bug fixes...

For example, if a function takes a string argument, when can you
assume an upper bound on how long the string is?

If you "know" it can't be too long then sure, you don't need to
check.  But unless that's obvious by looking at it (with someone
else's eyes, someone who didn't write the code) then for
maintainability's sake why not just be safe...? Especially if
someone else wanted to change strings and or buffer sizes later.
Eg, on alpha a printed "long" may be longer than 10 characters or
whatever.

When it comes to maintainability, the less dependent things you
have to change (and by implication, know about) in order to make
a single change, the better..

Of the strncpy() replacements, there are three categories:

- "Possibly unterminated string", where one is required:

    netatm/spans/spans_print.c
    netatm/uni/uniarp_cache.c
    pc98/pc98/diskslice_machdep.c
    pci/pci_compat.c

- "Simplification" (eg, replacing constants like "16" with sizeof())
  with otherwise no functional effect (including strncpy()'s zero'ing
  out of the buffer):

    i386/ibcs2/ibcs2_stat.c
    i386/ibcs2/ibcs2_xenix.c
    i386/linux/linux_misc.c
    netinet/ip_divert.c

- "Gratuitous" (?)

    alpha/tc/tcds.c
    dev/dpt/dpt_control.c
    kern/subr_devstat.c
    netatm/atm_aal5.c
    netatm/atm_socket.c

Tell me which (or all) of these you don't want and I'll take them out;
however my instinct would say to keep the first two sets.

If it's performance you're thinking about, my general assumption is
that string manipulation in the kernel is uncommon, but that could
be wrong.

Thanks for reviewing :-)

-Archie

___________________________________________________________________________
Archie Cobbs   *   Whistle Communications, Inc.  *   http://www.whistle.com

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