[PATCH v6 02/15] module: Introduce ksys_finit_module()

[Roberto Sassu]

On Tue, 2024-11-19 at 12:10 -0800, Luis Chamberlain wrote:
> On Tue, Nov 19, 2024 at 01:14:02PM +0100, Christoph Hellwig wrote:
> > On Tue, Nov 19, 2024 at 11:49:09AM +0100, Roberto Sassu wrote:
> > > From: Roberto Sassu <roberto.sassu at huawei.com>
> > > 
> > > Introduce ksys_finit_module() to let kernel components request a kernel
> > > module without requiring running modprobe.
> > 
> > That does sound more than sketchy, even more so because the commit log
> > completely fails to explain why you'd need to do that.
> 
> I also don't think the commit log is correct, I don't see how the
> code is preventing calling modprobe, the indepotent check is intended
> to prevent duplicate module init calls which may allocate extra vmalloc
> space only to release it. You can test to see if your patch has any
> improvments by enabling MODULE_STATS and MODULE_DEBUG_AUTOLOAD_DUPS
> and check before / after results of /sys/kernel/debug/modules/stats  ,
> right now this patch and commit log is not telling me anything useful.

Maybe I misunderstood the code, but what causes modprobe to be executed
in user space is a call to request_module().

In my patch, I simply ported the code of the finit_module() system call
to _ksys_finit_module(), net the conversion from struct fd to struct
file, which is kept in the system call code.

Also, from the kernel side, I'm providing a valid address for module
arguments, and duplicating the string either with kmemdup() or
strndup_user() in load_module(), depending on where the memory belongs
to.

Again, maybe I misunderstood, but I'm not introducing any functional
change to the current behavior, the kernel side also provides a file
descriptor and module arguments as user space would do (e.g. by
executing insmod).

As for the motivation, please have a look at my response to Christian:

https://lore.kernel.org/linux-integrity/ZzzvAPetAn7CUEvx@bombadil.infradead.org/T/#ma8656b921bb5bfb60e7f10331061d462a87ce9f4


In addition, you could also see how ksys_finit_module() is used here:

https://lore.kernel.org/linux-integrity/20241119104922.2772571-8-roberto.sassu@huaweicloud.com/

Thanks

Roberto