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Re: [PATCH] Arm: correct FIX_LAST
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Michal Orzel <michal.orzel@xxxxxxx>
- Date: Tue, 13 Aug 2024 11:50:11 +0200
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- Cc: Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
- Delivery-date: Tue, 13 Aug 2024 09:50:48 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 13/08/2024 10:36, Jan Beulich wrote:
>
>
> While reviewing a RISC-V patch cloning the Arm code, I noticed an
> off-by-1 here: FIX_PMAP_{BEGIN,END} being an inclusive range, FIX_LAST
> cannot be the same as FIX_PMAP_END, or else the BUG_ON() in
> virt_to_fix() would trigger if FIX_PMAP_END ended up being used.
>
> Fixes: 4f17357b52f6 ("xen/arm: add Persistent Map (PMAP) infrastructure")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Alternatively the definition of FIXADDR_TOP could be changed, if "last"
> should retain its strict meaning. Possibly a guard page also wants
> having at the end of the fixmap range, which could be effected by
> changing both #define-s at the same time.
I understand FIX_LAST as the last slot used, so in this regard it should retain
its definition.
That said, I realized that we don't even check that the highest slot is still
within the max number
of fixmap slots (today we have 512 slots). IMO we should gain a BUILD_BUG_ON to
catch it.
With:
#define FIXADDR_TOP FIXMAP_ADDR(FIX_LAST + 1)
and sth like:
BUILD_BUG_ON(FIXADDR_TOP >= BOOT_FDT_VIRT_START);
possibly in build_assertions() in setup.c, we would:
- fix off-by-1 issue
- catch out-of-slot issue
- gain a guard page
~Michal
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