Date: Wed, 07 Jun 2000 09:16:41 -0400 From: David Dagon <david.dagon@mindspring.com> To: Chris Williams <psion@geekspace.com> Cc: freebsd-security@FreeBSD.ORG Subject: Re: FreeBSDDEATH.c.txt (mmap dirty page no check bug) Message-ID: <393E4B39.354DCDA4@mindspring.com> References: <200006070424.e574Od303232@cwsys.cwsent.com> <200006070655.XAA97086@apollo.backplane.com> <393E3710.363AC6CF@geekspace.com>
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Chris Williams wrote:
>
> I have to wonder, with all this discussion about one tmp being cleared
> at boot and the other not...Do FreeBSD users really reboot so often that
> this is an important case? ;)
>
In the late 70s, /tmp was frequently on a small,
fast disk, often a RAM disk. It was usually used for
small, quick access jobs, such as output from the
multiple passes of cc. /usr/tmp (now /var/tmp) was
slower, but often with larger capacity, and was used
for jobs like sort, and such.
The hier(7) pages only give reboot as a key difference
between the two. But some systems may been configured
to give faster access to /tmp. (Others might just as
well be soft linked to /var/tmp, but that's up to the
sysadmin.) So expected speed of access and general
lifetime of the files are two factors that hier(7) does
not discuss.
I think you've made an excellent point. In
these days where a single-spindle holds gigs and gigs,
and the entire system resides on a single drive,
the separation between /tmp and /var/tmp seems less
important. And of course depending on a reboot as part of
system security does not seem wise.
On this matter, I reread with interest McKusick (et al.)'s
book on 4.4BSD:
Use of dedicated memory to support a RAM disk exclusively
is a poor use of resources. The system can improve overall
throughput by using the memory for the locations with high
access rates. . . .
[in efficiency of memory-to-memory copy noted for RAM storage]
. . .
The 4.4BSD system avoids these problems by building its RAM-disk
filesystem in pageable memory, insted of in dedicated memory.
The goal is to provide the speed benefits of a RAM disk without
paying the performance penalty . . . The /tmp filesystem can be
allocated a virtual address space that is larger than the
physical memory on the machine.
s. 8.4. (Yes, yes, there are differences between FreeBSD and 4.4).
It seems that the idea behind /tmp is that it's supposed to
be fast. This could mean a RAM disk, an MFS (ack!), or even just
locating /tmp on another spindle so it does not contend with
/usr traffic. Since it competes for resources, /tmp is cleaned
regularly (most often, but not exclusively, after a reboot). One
should use /tmp for transient or easily recreated information.
So, it strikes me that there are still differences between /tmp
and /var/tmp--expected speed of access (which may be a myth
these days), and the anticipated lifetime of transient files.
Maybe most sysadmins don't care to honor this distinction,
and just sym link the two. But having two systems allows for
tweaks.
--
David
dagon@cc.gatech.edu
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