Re: BUG #16176: NULL value returned by category_sql argument to crosstab() causes segmentation fault

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Joe Conway <mail(at)joeconway(dot)com>
Cc: ipluta(at)wp(dot)pl, pgsql-bugs(at)lists(dot)postgresql(dot)org
Subject: Re: BUG #16176: NULL value returned by category_sql argument to crosstab() causes segmentation fault
Date: 2019-12-21 15:08:37
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Joe Conway <mail(at)joeconway(dot)com> writes:
> It appears that in pg11 (and presumably prior) when snprintf() is called
> it is resolved (here at least )to __GI___snprintf() which comes directly
> from libc. On my desktop machine the system snprintf() deals with a null
> pointer argument without crashing. I guess this is why the crash was
> platform dependent.

Right, glibc's version of snprintf has produced "(nil)" or "(null)"
or something like that for many years. I'm not sure if that's true
among the BSDen. One place where the platform snprintf does *not*
survive this case is Windows.

> In pg12 (and presumably master), it is resolved to our own port function
> pg_snprintf(), which in turn works its way to dopr(), where strlen() is
> called on a null pointer and "<boom>".

Right. While it would only take a couple more lines of code to act like
glibc does, we intentionally adopted the stricter definition because it
seemed more likely to expose bugs. Looks like it just did.

> From what I can see, even on pg11 and prior, having a null category
> never did anything useful. And in the 16 years or so since this has been
> around, no one in my memory ever asked for that functionality, so I am
> inclined to refuse NULL category values unless someone wants to make a
> good case otherwise.

WFM, but I've never used crosstab() much so I don't have a good feeling
for significant use-cases.

regards, tom lane

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