Re: [RFC PATCH] vfs: Fix might sleep in load_unaligned_zeropad() with rcu read lock held

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Wed, Nov 26, 2025 at 07:26:40PM +0000, Al Viro wrote:
> On Wed, Nov 26, 2025 at 07:05:05PM +0000, Russell King (Oracle) wrote:
> > On Wed, Nov 26, 2025 at 06:48:20PM +0000, Al Viro wrote:
> > > It's been years since I looked at 32bit arm exception handling, so I'd need
> > > quite a bit of (re)RTF{S,M} before I'm comfortable with poking in
> > > arch/arm/mm/fault.c; better let ARM folks deal with that.  But arch/* is
> > > where it should be dealt with; as for papering over that in fs/*:
> > 
> > Don't expect that to happen. I've not looked at it for over a decade,
> > I do very little 32-bit ARM stuff anymore. Others have modified the
> > fault handling, the VM has changed, I basically no longer have the
> > knowledge. Effectively, 32-bit ARM is unmaintained now, although it
> > still has many users.
> 
> Joy...  For quick and dirty variant (on current tree), how about
> adding
> 	if (unlikely(addr > TASK_SIZE) && !user_mode(regs))
> 		goto no_context;
> 
> right after
> 
> 	if (!ttbr0_usermode_access_allowed(regs))
> 		goto no_context;
> 
> in do_page_fault() there?
> 
> NOTE: that might or might not break vdso; I don't think it would, but...

I don't understand how that helps. Wasn't the report that the filename
crosses a page boundary in userspace, but the following page is
inaccessible which causes a fault to be taken (as it always would do).
Thus, wouldn't "addr" be a userspace address (that the kernel is
accessing) and thus be below TASK_SIZE ?

I'm also confused - if we can't take a fault and handle it while
reading the filename from userspace, how are pages that have been
swapped out or evicted from the page cache read back in from storage
which invariably results in sleeping - which we can't do here because
of the RCU context (not that I've ever understood RCU, which is why
I've always referred those bugs to Paul.)

-- 
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