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Re: Bindat can exhaust memory when unpacking to vector

From: Petteri Hintsanen
Subject: Re: Bindat can exhaust memory when unpacking to vector
Date: Mon, 23 Oct 2023 22:36:01 +0300
User-agent: Gnus/5.13 (Gnus v5.13)

Eli Zaretskii <eliz@gnu.org> writes:

> Hang or just slow down tremendously due to paging?  How much time did
> you wait before deciding that Emacs hung?  And how much memory do you
> have on that machine?

Sorry, I was careless with wording; obviously what I meant was "slowed
down due to paging."  I did not wait for long, maybe a minute or so.
This machine has ample memory, 16 GB RAM plus another 16 GB swap space.

> Can you argue why this should be considered a problem for Emacs to
> solve, and not a cockpit error on the part of the Lisp program that
> makes such calls?

No, not really.  Now when I think about it, it is fairly obvious that
the programmer should realize this when using such low level mechanism
as Bindat.

>> Nonetheless, I think it would be prudent to mention this potential
>> pitfall in the Emacs Lisp manual so that users become aware of it.
>> I can write a texinfo patch if you agree.
> What would we say? that unpacking vectors larger than available memory
> would cause Emacs to run out of memory?

I was thinking of some kind of note about validation, e.g.:

  Notice that unpacking data to a vector of length LEN will
  unconditionally preallocate memory with at least LEN bytes.  If LEN is
  calculated from the input, it is a good idea to validate its value
  before unpacking.

But, again, users of Bindat can be expected to know that tainted data
should be sanitized.  And what is described above is an implementation
detail.  So it does not make much sense to mention this in the manual.

Sorry for the noise.

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