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 08:02:21PM +0000, Al Viro wrote:
> On Wed, Nov 26, 2025 at 07:51:54PM +0000, Russell King (Oracle) wrote:
> 
> > 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.)
> 
> No, the filename is already copied in kernel space *and* it's long enough
> to end right next to the end of page.  There's NUL before the end of page,
> at that, with '/' a couple of bytes prior.  We attempt to save on memory
> accesses, doing word-by-word fetches, starting from the beginning of
> component.  We *will* detect NUL and ignore all subsequent bytes; the
> problem is that the last 3 bytes of page might be '/', 'x' and '\0'.
> We call load_unaligned_zeropad() on page + PAGE_SIZE - 2.  And get
> a fetch that spans the end of page.
> 
> We don't care what's in the next page, if there is one mapped there
> to start with.  If there's nothing mapped, we want zeroes read from
> it, but all we really care about is having the bytes within *our*
> page read correctly - and no oops happening, obviously.
> 
> That fault is an extremely cold case on a fairly hot path.  We don't
> want to mess with disabling pagefaults, etc. - not for the sake
> of that.

I think, looking at the x86 handling, 32-bit ARM has missed a heck of
a lot of changes to the fault handling code, going all the way back to
pre-git history.

I seem to remember that I had updated it to match i386's implementation
at one point in the distant past, which is essentially what we have
today with a few tweaks. As code ages, it gets more difficult to
justify wholesale rewrites to bring it back up.

Relevant to this, looking at i386, that at some point added:

+       /*
+        * We fault-in kernel-space virtual memory on-demand. The
+        * 'reference' page table is init_mm.pgd.
+        *
+        * NOTE! We MUST NOT take any locks for this case. We may
+        * be in an interrupt or a critical region, and should
+        * only copy the information from the master page table,
+        * nothing more.
+        *
+        * This verifies that the fault happens in kernel space
+        * (error_code & 4) == 0, and that the fault was not a
+        * protection error (error_code & 1) == 0.
+        */
+       if (unlikely(address >= TASK_SIZE)) {
+               if (!(error_code & 5))
+                       goto vmalloc_fault;
+               /*
+                * Don't take the mm semaphore here. If we fixup a prefetch
+                * fault we could otherwise deadlock.
+                */
+               goto bad_area_nosemaphore;
+       }

which is after notify_die() and the test to see whether we need a
local_irq_enable(). This means we go straight to the fixing up etc
for these addresses.

In today's kernel, this has morphed into:

        /* Was the fault on kernel-controlled part of the address space? */
        if (unlikely(fault_in_kernel_space(address))) {
                do_kern_addr_fault(regs, error_code, address);
        } else {
                do_user_addr_fault(regs, error_code, address);

meaning any page fault for a kernel space address is handled entirely
separately from the normal page fault handling, and it looks like
this is entirely sensible.

Interestingly, however, I notice that x86 appears to no longer call
notify_die(DIE_PAGE_FAULT) in its page fault handling path, and I
wonder whether that's a regression on x86.

Now, for 32-bit ARM, I think I am coming to the conclusion that Al's
suggestion is probably the easiest solution. However, whether it has
side effects, I couldn't say - the 32-bit ARM fault code has been
modified by quite a few people in ways I don't yet understand, so I
can't be certain at the moment whether it would cause problems.

I think the only thing to do is to try the solution and see what
breaks. I'm not in a position to be able to do that as, having not
had reason to touch 32-bit ARM for years, I don't have a hackable
platform nearby. Maybe Xie Yuanbin can test it?

-- 
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