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Date: Mon, 21 Sep 2015 16:28:36 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: out of range struct tm fields in strftime
On Sun, Sep 20, 2015 at 09:54:50PM +0200, Szabolcs Nagy wrote:
> * Szabolcs Nagy <nsz@...t70.net> [2015-09-20 18:44:35 +0200]:
> > * Rich Felker <dalias@...c.org> [2015-09-20 12:36:29 -0400]:
> > > On Sun, Sep 20, 2015 at 02:44:50PM +0200, Szabolcs Nagy wrote:
> > > > out of range tm fields should not be treated as ub
> > > > as noted in the thread
> > > > http://sourceware.org/ml/libc-alpha/2015-09/msg00546.html
> > > >
> > > > i have a patch but there might be simpler approaches
> > >
> > > Wouldn't it be less invasive to just make some small changes like
> > > putting a U on some of the constants so that the arithmetic happens as
> > > unsigned?
> > >
> >
> > that might be simpler (and probably generates better
> > code for div,mod by const)
> >
> > but for the nl_langinfo item computation the range
> > has to be limited properly (tm_wday and tm_mon are
> > affected).
>
> implemented this approach with wday%7U and using some 0U+
> static int week_num(const struct tm *tm)
> {
> - int val = (tm->tm_yday + 7 - (tm->tm_wday+6)%7) / 7;
> + int val = (tm->tm_yday + 7U - (tm->tm_wday+6U)%7) / 7;
> /* If 1 Jan is just 1-3 days past Monday,
> * the previous week is also in this year. */
> - if ((tm->tm_wday - tm->tm_yday - 2 + 371) % 7 <= 2)
> + if ((0U + tm->tm_wday - tm->tm_yday - 2 + 371) % 7 <= 2)
> val++;
> if (!val) {
> val = 52;
> /* If 31 December of prev year a Thursday,
> * or Friday of a leap year, then the
> * prev year has 53 weeks. */
> - int dec31 = (tm->tm_wday - tm->tm_yday - 1 + 7) % 7;
> + int dec31 = (0U + tm->tm_wday - tm->tm_yday - 1 + 7) % 7;
This is okay but it might (or might not) be less ugly to just reorder
the constants to avoid the 0U. Thoughts?
> @@ -57,17 +57,17 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
>
> switch (f) {
> case 'a':
> - item = ABDAY_1 + tm->tm_wday;
> + item = ABDAY_1 + tm->tm_wday%7U;
This is going to be a significant code size increase on many archs,
and possibly significant performance cost too. I wonder if there's a
better way.
> @@ -143,10 +143,10 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
> width = 1;
> goto number;
> case 'U':
> - val = (tm->tm_yday + 7 - tm->tm_wday) / 7;
> + val = (tm->tm_yday + 7U - tm->tm_wday) / 7;
These look fine.
> @@ -165,7 +165,7 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
> val = tm->tm_year % 100;
> goto number;
> case 'Y':
> - val = tm->tm_year + 1900;
> + val = tm->tm_year + 1900LL;
Also looks ok. Hopefully the compiler does not actually evaluate in
64-bit.
Rich
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