netdev - Re: [PATCH 02/86] dccp: revert buggy auto-loading of dccp module

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Message-ID: <20100201122119.GC25094@hmsreliant.think-freely.org>
Date:	Mon, 1 Feb 2010 07:21:19 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	Gerrit Renker <gerrit@....abdn.ac.uk>
Cc:	davem@...emloft.net, dccp@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 02/86] dccp: revert buggy auto-loading of dccp module

On Mon, Feb 01, 2010 at 07:18:07AM +0100, Gerrit Renker wrote:
> This reverts commit (38ff3e6bb987ec583268da8eb22628293095d43b) ("dccp_probe:
> Fix module load dependencies between dccp and dccp_probe", from 15 Jan). Not
> only does it not work:
> 
> % modprobe -v dccp_probe
> kernel: [ 1431.442912] sys_init_module: 'dccp_probe'->init suspiciously \
>  		      returned 1, it should follow 0/-E convention
> kernel: [ 1431.442915] sys_init_module: loading module anyway...
> 
> 
> ... but it also causes a crash:
> 
> % rmmod dccp_probe
> kernel: [ 1777.305846] kernel BUG at /usr/src/davem-2.6/mm/slab.c:521!
> kernel: [ 1777.305852] invalid opcode: 0000 [#1] SMP
> kernel: [ 1777.305861] last sysfs file: /sys/class/power_supply/BAT0/energy_full
> kernel: [ 1777.305867] Modules linked in: dccp_probe(-) iwl3945 iwlcore [last unloaded: dccp]
> kernel: [ 1777.305883]
> kernel: [ 1777.305891] Pid: 12912, comm: rmmod Tainted: G  R        2.6.33-rc5 #6 2008URG/2008URG
> kernel: [ 1777.305899] EIP: 0060:[<c01d5e43>] EFLAGS: 00010046 CPU: 1
> kernel: [ 1777.305910] EIP is at kfree+0x73/0x150
> kernel: [ 1777.305916] EAX: c1678c00 EBX: 00000000 ECX: c01d5e15 EDX: 40080000
> kernel: [ 1777.305922] ESI: c015cb9a EDI: 080488a0 EBP: f4ffbf34 ESP: f4ffbf10
> kernel: [ 1777.305929]  DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
> kernel: [ 1777.305936] Process rmmod (pid: 12912, ti=f4ffb000 task=f61e8620 task.ti=f4ffb000)
> 
> ==> After reverting the commit:
> 
> % modprobe -v dccp_probe
> insmod /lib/modules/2.6.33-rc5/kernel/net/dccp/dccp.ko
> insmod /lib/modules/2.6.33-rc5/kernel/net/dccp/dccp_probe.ko
> 
> % lsmod
> Module                  Size  Used by
> dccp_probe              2345  0
> dccp                  120233  1 dccp_probe
> 
> Previously (during about 4 years of this module's history) there had never
> been a problem with the 'silent dependency' that the commit tried to fix:
> this dependency is deliberate and required, since dccp_probe performs probing
> of dccp connections and hence needs to know about dccp internals.
> 
> Signed-off-by: Gerrit Renker <gerrit@....abdn.ac.uk>

This doesn't make any sense.  Gerrit, you don't understand what the patch was
trying to do.  There is a silent dependency, in that this module requires the
dccp module to be loaded, but the reference to the dccp_send_probe symbol isn't
one that depmod can see.  If you don't load dccp first, dccp_probe fails, why
bother to allow that when try_then_request_module can avoid it?

The problem here is the construction of the first argument,
try_then_request_module should only return valid return codes from the first
argument, and my first argument is malformed.  register_jprobe returns zero on
success, so I need to check its return in the call for 0, in case we need to
trigger the request_module action, but in so doing ret gets the value of
(register_jprobe(&dccp_send_probe) == 0), which will always be 0 or 1.  What we
actually need to do is assign the result of register_jprobe to ret, without the
side effect of the comparison.  I've not tested it, but this should do it,
without re-breaking the silent dependency.

Signed-off-by: Neil Horman <nhorman@...driver.com>


 
 probe.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)


diff --git a/net/dccp/probe.c b/net/dccp/probe.c
index bace1d8..a8f5fdf 100644
--- a/net/dccp/probe.c
+++ b/net/dccp/probe.c
@@ -161,7 +161,8 @@ static __init int dccpprobe_init(void)
 	if (!proc_net_fops_create(&init_net, procname, S_IRUSR, &dccpprobe_fops))
 		goto err0;
 
-	ret = try_then_request_module((register_jprobe(&dccp_send_probe) == 0),
+	try_then_request_module(
+		((ret = register_jprobe(&dccp_send_probe)) == 0),
 					"dccp");
 	if (ret)
 		goto err1;
--
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