linux-kernel - Re: Unable to load modules from 9p filesystem with kmod 16

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Message-ID: <20140110230303.GA19369@animx.eu.org>
Date:	Fri, 10 Jan 2014 18:03:03 -0500
From:	Wakko Warner <wakko@...mx.eu.org>
To:	linux-kernel@...r.kernel.org
Subject: Re: Unable to load modules from 9p filesystem with kmod 16

Wakko Warner wrote:
> Kernel 3.12.7 from kernel.org
> With kmod-16, I'm unable to load any modules on my guest kvm machines.
> The vm is booted via direct kernel boot.  The modules are located on the
> host and is passed to the guest via the fsdev.
> 
> I have a mountpoint on the guest filesystem located at /kernel.  It is
> mounted like this:
> kernel /kernel 9p rw,sync,dirsync,relatime,trans=virtio,version=9p2000.L 0 0
> 
> /boot, /lib/modules, and /lib/firmware are tmpfs filesystems like this:
> kboot /boot tmpfs rw,relatime,size=0k,nr_inodes=8 0 0
> kfirmware /lib/firmware tmpfs rw,relatime,size=0k,nr_inodes=8 0 0
> kmodules /lib/modules tmpfs rw,relatime,size=0k,nr_inodes=8 0 0
> 
> /lib/modules/$(uname -r) is a symlink to /kernel/lib/modules/$(uname -r)
> 
> When trying to load any module, I get 
> Error: could not insert module /lib/modules/3.12.7/kernel/crypto/af_alg.ko:
> Invalid module format
> 
> This module was just one I picked at random, all modules fail the same way.
> Strace shows this:
> open("/lib/modules/3.12.7/kernel/crypto/af_alg.ko", O_RDONLY|O_CLOEXEC) = 3
> fstat(3, {st_mode=S_IFREG|0644, st_size=13822, ...}) = 0
> mmap(NULL, 13822, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f199aebd000
> syscall_313(0x3, 0x7f199aaa2de0, 0, 0x3, 0, 0x7f199b7b2010, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0) = -1 (errno 8)
> munmap(0x7f199aebd000, 13822)           = 0
> close(3)                                = 0
> 
> If I use kmod-9, it works w/o problems.  An strace of kmod-9 shows:
> open("/lib/modules/3.12.7/kernel/crypto/af_alg.ko", O_RDONLY|O_CLOEXEC) = 3
> fstat(3, {st_mode=S_IFREG|0644, st_size=13822, ...}) = 0
> mmap(NULL, 13822, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f658ba41000
> init_module(0x7f658ba41000, 13822, "")  = 0
> munmap(0x7f658ba41000, 13822)           = 0
> close(3)                                = 0
> 
> If I copy the module to /tmp, kmod-16 loads it just fine.  It appears to be
> related to the 9p filesystem.
> 
> I won't rule out a bug in kmod-16, but I was unable to figure out what the
> cause of the error was.  I tried to search through kmod's source and the
> kernel source, but I gave up because I don't understand what's going on.

I did some testing with the various kmod versions between kmod-9 and 16. 
The problem was introduced in 13.  The change that broke this was the
addition of finit_module().  Appears that the kernel cannot handle modules
that is on the 9p filesystem.  On the line that checked err == 0 (around
line libkmod-module.c:823), I added a test for ENOEXEC.  The finit_module
still fails,but it uses the old method and works.

I would say from what I've seen, the problem is in the kernel for
finit_module().  I don't know enough about the kernel to go any further.
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