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Re: [PATCH v2] xen/guest_access: Harden *copy_to_guest_offset() to prevent const dest operand

Hi Jan,

On 06/04/2020 12:01, Jan Beulich wrote:
On 04.04.2020 15:06, Julien Grall wrote:
From: Julien Grall <jgrall@xxxxxxxxxx>

At the moment, *copy_to_guest_offset() will allow the hypervisor to copy
data to guest handle marked const.

Thankfully, no users of the helper will do that. Rather than hoping this
can be caught during review, harden copy_to_guest_offset() so the build
will fail if such users are introduced.

There is no easy way to check whether a const is NULL in C99. The
approach used is to introduce an unused variable that is non-const and
assign the handle. If the handle were const, this would fail at build
because without an explicit cast, it is not possible to assign a const
variable to a non-const variable.

Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

I'm not convinced it is a good idea to add (recurring) comments
like you do - there are more aspects of these macros that would
be worth commenting on, and commenting on some but not all may
give the wrong impression of all subtleties being covered (also
for others).

I thought you would say that, but I don't think I am the best person to describe all the other subtetly of the code. Yet I didn't want to not comment the oddity of using a maybe_unused variable.

In any event I'd like to ask that each header gain such a
comment only once, with the other being a tiny reference to the
one "complete" instance.

I am not entirely sure how this would look like. We would need to rely on _t having the same meaning across all the headers. This is quite easy to miss during review, so my preference still sticks to multiple comments.

Although I can reduce the size of the comment to one on top of the definition of _t. Something like: "Check if the handler is not const".


Julien Grall



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