linux-kernel - Re: [PATCH] asm/sections: fix memory object end check

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Message-ID: <YcGkwf0WSmfmOQXB@shell.armlinux.org.uk>
Date:   Tue, 21 Dec 2021 09:56:17 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Thierry Reding <treding@...dia.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] asm/sections: fix memory object end check

On Tue, Dec 21, 2021 at 08:06:24AM +0100, Alexander Gordeev wrote:
> Function memory_contains() checks whether a memory object is
> entirely contained within a memory region. The condition that
> checks the upper bound of the object against the upper bound
> of the region is inclusive. That does not correspond to the
> similar checks in memory_intersects() friend function, nor
> to the actual regions memory_contains() is called against.
> 
> In particular, __init_end address assumed do not belong to
> the init section itself. Similarly, on ARM __idmap_text_end
> and __entry_text_end are affected.

__init_end is exclusive as are the other symbols you mention here.
They point at the byte immediately following the area.

When testing an virt + size, the resulting address of "virt + size" is
always exclusive - this also points at the byte immediately following
the range of addresses. The preceeding byte is part of the object.

For example, if size is one, then we have a single byte, which is at
address "virt". "virt + 1" is not part of the object. Therefore, if
size is 1MiB, then "virt + 1048576" is similarly not part of the
object.

Hence, we _do_ want to test address + size <= end.

This code appears to me to be correct, and this patch _introduces_ a
bug.

-- 
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