This issue tracker has been migrated to GitHub, and is currently read-only.
For more information, see the GitHub FAQs in the Python's Developer Guide.

Author mark.dickinson
Recipients alexandre.vassalotti, christian.heimes, donmez, gregory.p.smith, gvanrossum, loewis, mark.dickinson, matejcik, nnorwitz, pitrou, vstinner
Date 2009-05-13.20:05:38
SpamBayes Score 5.632565e-09
Marked as misclassified No
Message-id <>
Thanks Ismail.  I'm currently using gcc-4.4 with the -ftrapv (not 
-fwrapv!) option to see how much breaks.  (Answer: quite a lot. :-[ )

I'm finding many overflow checks that look like:

	size = Py_SIZE(a) * n;
	if (n && size / n != Py_SIZE(a)) {
			"repeated bytes are too long");
		return NULL;

where size and n have type Py_ssize_t.  That particular one comes
from bytesobject.c (in py3k), but this style of check occurs
frequently throughout the source.

Do people think that all these should be fixed?  

The fix itself s reasonably straightforward:  instead of multiplying
and then checking for an overflow that's already happened (and hence
has already invoked undefined behaviour according to the standards),
get an upper bound for n *first* by dividing PY_SSIZE_T_MAX
by Py_SIZE(a) and use that to do the overflow check *before*
the multiplication.  It shouldn't be less efficient:  either way
involves an integer division, a comparison, and a multiplication.

The hard part is finding all the places that need to be fixed.
Date User Action Args
2009-05-13 20:05:42mark.dickinsonsetrecipients: + mark.dickinson, gvanrossum, loewis, nnorwitz, gregory.p.smith, pitrou, vstinner, christian.heimes, alexandre.vassalotti, donmez, matejcik
2009-05-13 20:05:41mark.dickinsonsetmessageid: <>
2009-05-13 20:05:40mark.dickinsonlinkissue1621 messages
2009-05-13 20:05:38mark.dickinsoncreate