Re: [PATCH v2 22/50] convert efivarfs

[Ard Biesheuvel <ardb@kernel.org>]

On Wed, 29 Oct 2025 at 19:08, Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Tue, Oct 28, 2025 at 10:34:51PM +0100, Ard Biesheuvel wrote:
>
> > I'll let James respond to the specifics of your suggestion, but I'll
> > just note that this code has a rather convoluted history, as we used
> > to have two separate pseudo-filesystem drivers, up until a few years
> > ago: the sysfs based 'efivars' and this efivarfs driver. Given that
> > modifications in one needed to be visible in the other, they shared a
> > linked list that shadowed the state of the underlying variable store.
> > 'efivars' was removed years ago, but it was only recently that James
> > replaced the linked list in this driver with the dentry cache as the
> > shadow mechanism.
>
> Hmm...  Another question about that code: is efivar_get_variable()
> safe outside of efivar_lock()?

Not according to its kerneldoc

/*
 * efivar_get_variable() - retrieve a variable identified by name/vendor
 *
 * Must be called with efivars_lock held.
 */

But actually, I'm not convinced this is accurate. The reason for
locking at this level is mainly to ensure that a SetVariable() call
does not interfere with an ongoing enumeration calling
GetNextVariable() in a loop. The individual EFI runtime calls are
still serialized at a lower level, given that the firmware is not
reentrant, and so holding efivars_lock does not provide anything
meaningful for a GetVariable() call.