Subject: Re: segfault in HEAD when too many nested functions call
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>,
 Andres Freund <andres@anarazel.de>
References: <b0af9eaa-130c-60d0-9e4e-7a135b1e0c76@dalibo.com>
 <12302.1500132157@sss.pgh.pa.us>
From: Julien Rouhaud <julien.rouhaud@dalibo.com>
Message-ID: <3845ed70-201a-d72e-9ee7-04c26ee30b07@dalibo.com>
Date: Sun, 16 Jul 2017 01:59:40 +0200
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On 15/07/2017 17:22, Tom Lane wrote:
> Julien Rouhaud <julien.rouhaud@dalibo.com> writes:
>> Since b8d7f053c5c2bf2a7e8734fe3327f6a8bc711755 (Andres in Cc), if you
>> write queries which result in infinite recursion (or just too many
>> nested function calls), execution ends with segfault instead of intended
>> exhausted max_stack_depth:
> 
> Yes.  We discussed this before the patch went in [1].

Ah, I totally forgot about it, sorry.

>  I wanted to put
> a stack depth check in ExecProcNode(), and still do.  Andres demurred,
> claiming that that was too much overhead, but didn't really provide a
> credible alternative.  The thread drifted off without any resolution,
> but clearly we need to do something before 10.0 final.
> 

I wanted to add an open item but I see you already did, thanks!

>> Please find attached a trivial patch to fix this.  I'm not sure
>> ExecMakeTableFunctionResult() is the best or only place that needs to
>> check the stack depth.
> 
> I don't think that that's adequate at all.
> 
> I experimented with a variant that doesn't go through
> ExecMakeTableFunctionResult:
> 
> [...]
> This manages not to crash, but I think the reason it does not is purely
> accidental: "SELECT so()" has a nontrivial targetlist so we end up running
> ExecBuildProjectionInfo on that, meaning that a fresh expression
> compilation happens at every nesting depth, and there are
> check_stack_depth calls in expression compilation.  Surely that's
> something we'd try to improve someday.  Or it could accidentally get
> broken by unrelated changes in the way plpgsql sets up queries to be
> executed.
> 
> I still think that we really need to add a check in ExecProcNode().
> Even if there's an argument to be made that every recursion would
> somewhere go through ExecMakeTableFunctionResult, very large/complex
> queries could result in substantial stack getting chewed up before
> we get to that --- and we don't have an infinite amount of stack slop.

I was pretty sceptical about checking depth in
ExecMakeTableFunctionResult() only vs ExecProcNode(), and this has
finished convincing me so I definitely agree.

-- 
Julien Rouhaud
http://dalibo.com - http://dalibo.org


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