From mboxrd@z Thu Jan  1 00:00:00 1970
From: erik quanstrom <quanstro@quanstro.net>
Date: Fri, 20 May 2011 23:27:05 -0400
To: 9fans@9fans.net
Message-ID: <edcfbdc9e2482002efd4dc8adb011fa7@brasstown.quanstro.net>
In-Reply-To: <BANLkTim3p0FqDzey=gaBRDordN2BSB5j2A@mail.gmail.com>
References: <309c101f23bbb6ec6a92b2bf1c525fd7@brasstown.quanstro.net>
	<BANLkTimDdG3MnLejCBdJA+Cg-OjUOEK_0g@mail.gmail.com>
	<4be7f81375a5450a8b6536d66f756703@brasstown.quanstro.net>
	<BANLkTim3p0FqDzey=gaBRDordN2BSB5j2A@mail.gmail.com>
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Subject: Re: [9fans] a pair nec bugs
Topicbox-Message-UUID: e7a15e8e-ead6-11e9-9d60-3106f5b1d025

here's an improved version.  the previous version had a problem when
shared memory but not shared fd in this situation
	p0:
		nsec()	-> fd 3
		...
	p1:
		open()	-> fd 3
		nsec()	-> fail

one potential modification is to ditch the malloc and allocate
two privalloc entries.

one interesting bit that is not documented in the privalloc(2) man
page is that privalloc() allocations are shared and inherited when
a process forks, but the memory spaces are not.  thus a privalloc
for each new pid would be an error.

it would require a modification to kexit() to change this.  but it might
make sense, because sharing privalloc entries seems exactly the
opposite of what privalloc is supposed to be doing.

- erik

---

#include <u.h>
#include <libc.h>
#include <tos.h>

typedef	struct	Nfd	Nfd;
struct Nfd {
	int	pid;
	int	fd;
};

static	void	**nsecpriv;
#define	Fd	((Nfd*)nsecpriv[0])


static uvlong order = 0x0001020304050607ULL;

static void
be2vlong(vlong *to, uchar *f)
{
	uchar *t, *o;
	int i;

	t = (uchar*)to;
	o = (uchar*)&order;
	for(i = 0; i < sizeof order; i++)
		t[o[i]] = f[i];
}

static Nfd*
getfd(void)
{
	Nfd *p;

	if(nsecpriv != nil && Fd->pid == _tos->pid)
		return Fd;
	if(nsecpriv == nil){
		/*
		 * privalloc's allocates slots on a shared
		 * basis, even though the memory slots
		 * themselves are proc-private.
		 */
		nsecpriv = privalloc();
		if(nsecpriv == nil)
			return nil;
		*nsecpriv = p = malloc(sizeof *p);
		if(p == nil)
			return nil;
	}else
		p = *nsecpriv;
	p->fd = -1;
	return p;
}

vlong
nsec(void)
{
	uchar b[8];
	vlong t;
	Nfd *p;

	if((p = getfd()) == nil)
		return 0;
	if(p->fd == -1){
		p->fd = open("/dev/bintime", OREAD|OCEXEC);
		p->pid = _tos->pid;
	}
	if(pread(p->fd, b, sizeof b, 0) == sizeof b){
		be2vlong(&t, b);
		return t;
	}
	return 0;
}