bug#40913: 24.5; Crash on open of file

[Jason Gibson <jgibson@perforce.com>]

>> Since this would seem to be a good vector for remote buffer overflow, it
>> might make sense to backport this to prior releases.
>
> There's no practical way for us to do so, since we do not intend to
> put out any new releases of Emacs before 27.  Emacs 27.1 will be
> released soon, and this problem will be fixed there.
>
> It is also worth noting that the use case where this bug can rear its
> ugly head is quite rare.  Most sequences of composed characters are
> very short, and the way we allocate the buffers for them always
> allocates more than strictly needed, which is why this bug, although
> blatant, went unnoticed for a very long time.  You just happened to
> hit a file which (being in fact just a stream of binary bytes) looked
> to Emacs as a long sequence of characters all of which should be
> composed, and that sequence overflowed the allocated buffer by many
> hundreds of bytes, thus triggering memory corruption.

Sounds good, thanks for the explanations.

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