Re: [uml-devel] [Pkg-uml-pkgs] How is user-mode-linux using /dev/shm/?

[Blaisorblade <blaisorblade@yahoo.it>]

On Thursday 28 September 2006 21:47, Mattia Dongili wrote:
> Thanks Petter for writing,
> I was going to followup to the sysvinit bug :)
>
> I'm Cc-ing upstream too.

> I don't think it uses shm* functions, as far as I can tell the /dev/shm
> location is sensible to $TMP, $TEMP or $TEMPDIR, thus changing one of
> them to a different location goes farther than what's reported in the
> bureport:

> $ TMPDIR=./TMP linux ubd0=rootfs_debian-sid.172.20.0.20
> ubd1=swap-172.20.0.20 eth0=tuntap,,,172.20.0.19 umid=debian mem=128
> Checking that ptrace can change system call numbers...OK
> Checking syscall emulation patch for ptrace...OK
> Checking advanced syscall emulation patch for ptrace...OK
> Checking for tmpfs mount on /dev/shm...OK
> Checking PROT_EXEC mmap in ./TMP/...OK
> Checking for the skas3 patch in the host:
>   - /proc/mm...not found
>   - PTRACE_FAULTINFO...not found
>   - PTRACE_LDT...not found
> UML running in SKAS0 mode
> Mapping memory: Cannot allocate memory

There is probably some additional bug here, it should be easy to fix.

> Don't know now, it's maybe easy to fix, there's a nice global variable
> set to "/dev/shm" in arch/um/os-Linux/mem.c and UML uses mkstemp() to
> create files.
>
> > To make some writable tmpfs available to those in need of such system,
> > and to avoid using /dev/shm/ which is reserved for the shm-functions,
> > I just uploaded sysvinit version 2.86.ds1-26.  It will mount a tmpfs
> > on /lib/init/rw/ that can be used instead.  If /lib/init/rw/.ramfs
> > exist, that mount point is a tmpfs.  I'm not sure if this last change
> > will make it into Etch or not, but I hope so, to solve any problems
> > with packages previously using /dev/shm/ as a generic tmpfs file
> > system.
>
> Well, I'd actually prefer if you could remove the noexec flag from
> /dev/shm. I understand the security reasons given in the bugreport but
> I'd prefer avoid having to deal with one more Debian-only (is it?)

It is not - it can happen on Gentoo too (actually, the issue there is simply 
that the manuals _recommend_ to use noexec for /dev/shm).

> thing given the soon to come general freeze.

As Jeff Dike explained, we just need to use an executable tmpfs mount. For a 
long time, we used /tmp and _suggested_ people to use tmpfs for it, or to use 
it elsewhere and point TMP there. And quite frankly, it would be nicer to 
make that a default (i.e. if /tmp is automatically a tmpfs mount, switching 
back there would be nice).

In 2.4 days, using tmpfs for /tmp was bad for one issue: you could not do a 
loopback mount from tmpfs, and this broke mkinitrd; but that has been solved 
since then, for 2.6 at least.
-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade
http://www.user-mode-linux.org/~blaisorblade
Chiacchiera con i tuoi amici in tempo reale! 
 http://it.yahoo.com/mail_it/foot/*http://it.messenger.yahoo.com 


-------------------------------------------------------------------------
Take Surveys. Earn Cash. Influence the Future of IT
Join SourceForge.net's Techsay panel and you'll get the chance to share your
opinions on IT & business topics through brief surveys -- and earn cash
http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV
_______________________________________________
User-mode-linux-devel mailing list
User-mode-linux-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/user-mode-linux-devel