On Tue, Aug 19, 2025 at 10:52:16AM +0200, Petr Pavlu wrote:
On 8/18/25 11:34 AM, Phil Sutter wrote:
On Sun, Aug 17, 2025 at 05:54:27PM +0200, Christophe Leroy wrote:
Le 17/08/2025 à 01:33, Phil Sutter a écrit :
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Hi,
I admittedly didn't fully analyze the cause, but on my system a call to:
# insmod /lib/module/$(uname -r)/kernel/net/netfilter/nf_conntrack_ftp.ko
fails with -EEXIST (due to a previous call to 'nfct add helper ftp inet
tcp'). A call to:
# modprobe nf_conntrack_ftp
though returns 0 even though module loading fails. Is there a bug in
modprobe error status handling?
Read the man page : https://linux.die.net/man/8/modprobe
In the man page I see:
Normally, modprobe will succeed (and do nothing) if told to
insert a module which is already present or to remove a module which
isn't present.
This is not a case of already inserted module, it is not loaded before
the call to modprobe. It is the module_init callback
nf_conntrack_ftp_init() which returns -EEXIST it received from
nf_conntrack_helpers_register().
is this a real failure condition or something benign like "if it's
already registered, there's nothing to do"?
Can't user space distinguish the two causes of -EEXIST? Or in other
words, is use of -EEXIST in module_init callbacks problematic?
Unfortunately, error return codes from (f)init_module cannot be reliably
depended upon. For instance, cpufreq drivers have similar behavior of
returning -EEXIST when another cpufreq driver is already registered.
Returning this code unexpectedly can then confuse kmod, as it interprets
-EEXIST to mean "the module is already loaded" [1].
well, it's not that it can't be relied on. There's 1 exit code that is
treated specially, EEXISTS, because that error is used by the module
loading part, before the module_init call, to signify the module is
already loaded.
I have thought about this problem before. We might fix the main
problematic occurrences, but we can't really audit all the code that
module init functions can invoke. I then wonder if it would make sense
for the module loader to warn about any -EEXIST returned by a module's
init function and translate it to -EBUSY.
If it's a failure condition then yes, -EBUSY looks appropriate.
Lucas De Marchi
Ensuring the reliability of the 0 and -EEXIST return codes from
(f)init_module should help user space.
[1]
https://github.com/kmod-project/kmod/blob/695fd084a727cf76f51b129b67d5a4be1d6db32e/libkmod/libkmod-module.c#L1087
-- Petr
