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SubjectRe: [GIT PULL] arm64 fixes for 5.11-rc6
On Sun, 31 Jan 2021 at 19:55, Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Fri, Jan 29, 2021 at 02:09:05PM -0800, Linus Torvalds wrote:
> > On Fri, Jan 29, 2021 at 11:03 AM Catalin Marinas
> > <catalin.marinas@arm.com> wrote:
> > >
> > > arm64 fixes:
> > >
> > > - Fix the virt_addr_valid() returning true for < PAGE_OFFSET addresses.
> >
> > That's a really odd fix.
> >
> > It went from an incorrect bitwise operation (masking) to an _odd_
> > bitwise operation (xor).
> >
> > Yes, PAGE_OFFSET has the bit pattern of all upper bits set, so "(addr
> > ^ PAGE_OFFSET)" by definition reverses the upper bits - and for a
> > valid case turns them to zero.
> >
> > But isn't the *logical* thing to do to use a subtract instead? For the
> > valid cases, the two do the same thing (clear the upper bits), but
> > just conceptually, isn't the operation that you actually want to do
> > "(addr - PAGE_OFFSET)"?
> >
> > IOW, why is it using that odd xor pattern that doesn't make much
> > sense? I believe it _works_, but it looks very strange to me.
>
> This macro used to test a single bit and it evolved into a bitmask. So,
> yes, basically what we need is:
>
> #define __is_lm_address(addr) ((u64)(addr) >= PAGE_OFFSET && \
> (u64)(addr) < PAGE_END)
>
> I wasn't sure whether the code generation with two comparisons is
> similar to the xor variant but the compiler should probably be smart
> enough to use CMP and CCMP. In the grand scheme, it probably doesn't
> even matter.
>
> Unless I miss something, I don't see any overflow issues even if we do
> (((u64)addr - PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET)).
>
> We can backport the fix already upstream and clean-up the code in
> mainline going forward (after some sanity check on the code generation).
> It would be easier to parse in the future.
>
> > Also, shouldn't _lm_to_phys() do the same? It does that "mask upper
> > bits" too that was problematic in __is_lm_address(). Again, shouldn't
> > that logically be a subtract op?
>
> Yes, that's similar and a subtract should do.
>

The original bit test was written like that because it removes the
need to reason about a potential tag in the upper bits. I tried to
preserve that behavior when removing the guaranteed 1:1 split between
the vmalloc and linear regions, by masking with PAGE_OFFSET and
comparing with PAGE_END - PAGE_OFFSET, but unfortunately, both
approaches suffer from the issue fixed by this patch, i.e., that
virt_addr_valid(0x0) erroneously returns true.

I think both proposed fixes are appropriate, but they both reintroduce
the need to consider the tag. I don't know whether or where this could
pose a problem, but it needs to be taken into account.

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