Re: [PATCH] export symbol mount_lock

[Matthew Wilcox <willy@infradead.org>]

On Thu, Jul 13, 2017 at 11:24:01AM -0700, Jay Lan wrote:
> On 07/13/2017 12:15 AM, Christoph Hellwig wrote:
> > On Wed, Jul 12, 2017 at 02:40:11PM -0700, Jay Lan wrote:
> > > Hi all,
> > > 
> > > I was writing a livepatch patch, but compilation failed in creating the .ko
> > > because mount_lock is "undefined."
> > > 
> > > The mount_lock is defined globally in fs/namespace.c
> > > __cacheline_aligned_in_smp DEFINE_SEQLOCK(mount_lock);
> > > 
> > > and extern in fs/mount.h.
> > > extern seqlock_t mount_lock;
> > > 
> > > If there is a reason that mount_lock should not be EXPORT_SYMBOL_GPL, please
> > > advise; otherwise, I propose the patch to export this symbol.
> > Because there is no %$^$^ reason to use it from a module ever?
> Is livepatch a good reason? Besides security fixes, livepatch is a good tool
> to use for debugging and providing a temporary fix on production systems
> (until next release from vendors) I know this argument is not strong, but at
> least mount_lock is declared globally. Yes?

Something I've done in the past as a local hack when a symbol I wanted
wasn't exported is:

extern seqlock_t *mount_lock_p;
module_param(mount_lock_p, charp, 0);

and then pass the address (from /proc/kallsyms) as a module parameter
at load time.