Date: Mon, 16 Oct 2000 15:25:33 -0700
From: Alfred Perlstein <bright@wintelcom.net>
To: Igor Gavriloff <igor@nupes.cefetpr.br>
Cc: "pgsql-interfaces@postgresql.org" <pgsql-interfaces@postgresql.org>
Subject: Re: What's going wrong?
Message-ID: <20001016152532.U272@fw.wintelcom.net>
References: <39EB8A6B.E1EB3E44@nupes.cefetpr.br>
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	from igor@nupes.cefetpr.br on Mon, Oct 16, 2000 at 09:08:27PM
	-0200

* Igor Gavriloff <igor@nupes.cefetpr.br> [001016 15:16] wrote:
> Hi,
> 
>     I'm trying to write a function in C that I can load into postgres,
> that basically removes a file from the system (quite unsafe thought, but
> it'll be suitable for me here):
> 
> #include <postgres.h>
> #include <stdio.h>
> 
> bool file_remove(text *this)
> {
>         if (remove((char *)this) == 0) return (true);
>         else return (false);
> }
> 
>     I compile it and link as instructed for Solaris (where the backend
> runs), and load it with:
> 
> CREATE FUNCTION file_remove(text) RETURNS bool AS '/path/file_remove.so'
> LANGUAGE 'c';
> 
>     But it always returns false when I try to remove something. I'm sure
> that I'm trying to remove files that the the backend's user owns (have
> write access). If I try to make any changes on this code (eg.: the
> casts...), when I try to use it the backends terminates abnormally.
>     Does someone can give me a clue?

from pgsql/src/includes/c.h:

 *      BTW: when you need to copy a non-null-terminated string (like a text
 *      datum) and add a null, do not do it with StrNCpy(..., len+1).  That
 *      might seem to work, but it fetches one byte more than there is in the
 *      text object.  One fine day you'll have a SIGSEGV because there isn't
 *      another byte before the end of memory.  Don't laugh, we've had real
 *      live bug reports from real live users over exactly this mistake.
 *      Do it honestly with "memcpy(dst,src,len); dst[len] = '\0';", instead.

from postgres.h:

struct varlena
{
        int32           vl_len;
        char            vl_dat[1];
};
typedef struct varlena text;

so it seems that 'text' is not a char *, you'll probably want to
use malloc and a memcpy to make a proper C string from a 'text'
object.

had you thought of running a debugger or looking at the struct
definitions before doing such a dangerous cast?

-- 
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
"I have the heart of a child; I keep it in a jar on my desk."