From a28df6fa3041523f9a96cf0d3c3929f8bdc7c797 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Fri, 12 Feb 2021 16:26:47 -0500 Subject: [PATCH] Avoid divide-by-zero in regex_selectivity() with long fixed prefix. Given a regex pattern with a very long fixed prefix (approaching 500 characters), the result of pow(FIXED_CHAR_SEL, fixed_prefix_len) can underflow to zero. Typically the preceding selectivity calculation would have underflowed as well, so that we compute 0/0 and get NaN. In released branches this leads to an assertion failure later on. That doesn't happen in HEAD, for reasons I've not explored yet, but it's surely still a bug. To fix, just skip the division when the pow() result is zero, so that we'll (most likely) return a zero selectivity estimate. In the edge cases where "sel" didn't yet underflow, perhaps this isn't desirable, but I'm not sure that the case is worth spending a lot of effort on. The results of regex_selectivity_sub() are barely worth the electrons they're written on anyway :-( Per report from Alexander Lakhin. Back-patch to all supported versions. Discussion: https://postgr.es/m/6de0a0c3-ada9-cd0c-3e4e-2fa9964b41e3@gmail.com --- src/backend/utils/adt/selfuncs.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index 18f4bedb37..17640b2328 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -6482,9 +6482,18 @@ regex_selectivity(const char *patt, int pattlen, bool case_insensitive, sel *= FULL_WILDCARD_SEL; } - /* If there's a fixed prefix, discount its selectivity */ + /* + * If there's a fixed prefix, discount its selectivity. We have to be + * careful here since a very long prefix could result in pow's result + * underflowing to zero (in which case "sel" probably has as well). + */ if (fixed_prefix_len > 0) - sel /= pow(FIXED_CHAR_SEL, fixed_prefix_len); + { + double prefixsel = pow(FIXED_CHAR_SEL, fixed_prefix_len); + + if (prefixsel > 0.0) + sel /= prefixsel; + } /* Make sure result stays in range */ CLAMP_PROBABILITY(sel);