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bug#27585: segfault when evaluating a file containing only backticks

From: Steve Kemp
Subject: bug#27585: segfault when evaluating a file containing only backticks
Date: Wed, 05 Jul 2017 18:55:31 +0000

> >    deagol ~ $ perl -e 'print "`" x ( 1024 * 1024  * 12);' > t.el
> >    deagol ~ $ /usr/bin/emacs --batch --script ./t.el
> >    ..
> >    Segmentation fault (core dumped)
> Here it says:
>   Re-entering top level after C stack overflow
> and doesn't crash.

  That's great.  For me it crashes on all the versions I've tested.
 Sometimes with a shorter input, others need longer, but it had seemed

> See above: the machinery to try and prevent it exists, but it doesn't
> always succeed.  And it really can't be 100% reliable.  So I'm unsure
> what did you expect, and why.

  Honestly?  I expect Emacs to not crash.  The mechanics might be
 simple, or they might be complex but as a mere user I shouldn't
 be expected to know or care about the details.  I expect evaluating
 lisp to not kill the editor - although I appreciate that executing
 unknown could code do all kinds of nasty things, from uploading
 my images to the internet, to running "rm -rf $HOME".

  The fine manual, in "54.2 When Is There a Bug", agrees.  When
 paraphrased as "segfault == bug".

> IOW: why would someone want to run such a silly "program"?

  In the real world?  Nobody.  It was just the first crash that
 came out of fuzz-testing.  Perhaps there will be more subtle
 and interesting ones to report in the future when the fuzzer
 has ran for a few more days/weeks.  Perhaps not.  Either way
 I'd regard it as a bug that should be fixed, even if it is
 not a security hole, and not something that is going to surprise
 users in practice.


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