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From: arisawa <arisawa@ar.aichi-u.ac.jp>
In-Reply-To: <efdc7fb80faff207dd6a1a65536f084d@felloff.net>
Date: Sun, 14 Sep 2014 13:41:00 +0900
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Message-Id: <26DF63C8-B7D9-4BEC-86EE-1DEBEDCC0482@ar.aichi-u.ac.jp>
References: <efdc7fb80faff207dd6a1a65536f084d@felloff.net>
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
Subject: Re: [9fans] segattach in telnetd
Topicbox-Message-UUID: 167b9b0a-ead9-11e9-9d60-3106f5b1d025

Hello cinap,

My experiment here.

void
main(int argc, char *argv[])
{	char *ap;
	char buf[1];
	ARGBEGIN{
	default: usage();
	}ARGEND
=09
	ap =3D segattach(0,"share",0,1024);
	print("%p\n",ap);	// ffffffff  error

	/* compare */
	ap =3D sbrk(0);
	print("sbrk %p\n",ap);	// 6cb0
	ap +=3D 2*1024*1024;
	print("%p\n",ap);	// 206cb0
	ap =3D segattach(0, "shared", ap, 1024);
	print("%p\n",ap);	// 206000

	read(0,buf,1);
}

we can find the allocated share memory:

term% cat /proc/612768/segment
Stack     defff000 dffff000    1
Text   R      1000     6000    1
Data          6000     7000    1
Bss           7000     7000    1
Shared      206000   207000    1
term%=20

note that 0x206cb0 + 0x400 > 0x20700.
if we believe that we have a memory
of range [mp,mp+1024) by executing
mp =3D share(1024)
then we will fail.

Kenji Arisawa

2014/09/14 12:17=E3=80=81cinap_lenrek@felloff.net =E3=81=AE=E3=83=A1=E3=83=
=BC=E3=83=AB=EF=BC=9A

> it should round va down and va+len up as neccesary
> to make them page aligned. but the initial passed
> range should be covered by the created segment
> in any case.