GNU bug report logs -
#7928
mktime test in configure: UB resulting in infinite loop
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Reported by: Rich Felker <dalias <at> aerifal.cx>
Date: Thu, 27 Jan 2011 06:44:02 UTC
Severity: normal
Done: Paul Eggert <eggert <at> cs.ucla.edu>
Bug is archived. No further changes may be made.
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On Thu, Jan 27, 2011 at 08:14:56AM -0700, Eric Blake wrote:
> # define TYPE_MINIMUM(t) \
> ((t) (! TYPE_SIGNED (t) \
> ? (t) 0 \
> : TYPE_SIGNED_MAGNITUDE (t) \
> ? ~ (t) 0 \
> : ~ (t) 0 << (sizeof (t) * CHAR_BIT - 1)))
> # define TYPE_MAXIMUM(t) \
> ((t) (! TYPE_SIGNED (t) \
> ? (t) -1 \
> : ~ (~ (t) 0 << (sizeof (t) * CHAR_BIT - 1))))
The last line of this macro has UB due to signed integer overflow in
the << operation. Replace it with
(( (t)1 << CHAR_BIT*sizeof(time_t)-2 ) - 1) * 2 + 1
Which for a 32-bit type would expand as:
(0x40000000 - 1) * 2 + 1
0x3fffffff *2 + 1
0x7ffffffe + 1
0x7fffffff
With no overflows.
> and no one has complained yet, so we might as well just use this same
> logic in m4/mktime.m4.
Well apparently no one complained about the overflow in coreutils
either. Perhaps later gcc versions are more forgiving; I found it
building on a system where I have to use gcc 3.2.3, which, being one
of the earlier versions to utilize the UB of signed overflow, might be
less diplomatic about it. Anyway to avoid future trouble, I would
strive to remove all signed overflow UB from the tests even if it
doesn't presently hurt anyone.
> 8 is a magic number; it would be better to use CHAR_BIT, as was done in
> intprops.h.
As you wish. This code (coreutils) is sufficiently POSIX-like-system
dependent that I thought using POSIX's requirement CHAR_BIT==8 was
reasonable.
> > If this test comes from higher-up (gnulib?) please forward my bug
> > report to the relevant upstream.
>
> Forwarded; and the patch should be applied shortly.
Thanks!
Rich
This bug report was last modified 14 years and 177 days ago.
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