[uml-devel] Re: pcap cross-linking

[Blaisorblade <blaisorblade@yahoo.it>]

On Sunday 18 December 2005 20:03, Antoine Martin wrote:
> Paolo,

> pcap builds and runs fine on amd64 but there is a problem when building
> with SUBARCH=i386: it uses the wrong version of libpcap.a:
> ld -r -dp -o arch/um/drivers/pcap.o arch/um/drivers/pcap_kern.o
> arch/um/drivers/pcap_user.o  -m elf_i386
> -r /usr/lib/gcc/x86_64-pc-linux-gnu/3.4.4/../../../libpcap.a

> Whereas the one it needs to link against is here:
> /emul/linux/x86/usr/lib/libpcap.a

This is IMHO a Gentoo bug - it is not setup for compilation of 32-bit binaries 
using anything else than glibc. I hit this problem with libncurses, when 
doing make menuconfig ARCH=um (now this was solved as helper programs are 
built as native again).

However, I built uml with libpcap by default for some time - possibly it was 
before me switching to 64-bit linux.

> Fixing the Makefile is beyond me (probably not that hard),

it is a bit hard - in this case it probably suffices to add -L searchpath 
(i.e. -L /emul/linux/x86/usr/lib/ ), but it's non-standard (aka only Gentoo 
works this way) - and merging fixes for every possible distro is not a good 
idea.

> sorry - so I 
> link it manually. After that it works perfectly.

> On the subject of pcap transport, I couldn't dig the email where you
> suggested a way of figuring out which libraries should be placed in the
> chroot, I tried to use the same libraries as I used on a 32-bit setup
> but it fails to bring up the interface (when it works outside chroot).

Don't know what I did suggest, but using strace -e open() (or, even better, 
ltrace -e dlopen ) would probably find the point where it's failing and the 
failed cmd line.

> I tried to guess and even added a few more, here is the chroot's /lib (I
> made /lib64 a link to /lib in chroot):
> ld-2.3.5.so           libcrack.so.2.8.0  libm.so.6

> libnss_compat-2.3.5.so  libnss_hesiod-2.3.5.so   libpam.so

Good thing, libnss_* is probably good - but don't forget /etc/nsswitch.conf 
and /etc/pam.d/* - /etc/pam.conf


> (also lib/security so I can get into the chroot)
That's for su, right? There are some tools (including "compartment") to 
combine chroot + su together.

> But that's not enough... and ldd didn't show me anything missing.

Ldd won't help you with dynamically loaded libraries.

Likely, a recursive "strings object|grep lib" will help - recursive means "do 
that also on libraries found this way".

Also, I see you're using SeLinux. I don't know anything about its library 
handling, and possibly it's going to make the story more difficult. However, 
strace/ltrace as suggested above should diagnose any problem.

-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

	

	
		
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