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Re: Segfault on recursive trap/kill


From: Valentin Bajrami
Subject: Re: Segfault on recursive trap/kill
Date: Sun, 7 Oct 2018 08:52:25 +0200

Hi Mike,

As earlier expained, you are calling foo function recursively. To mitigate
this behaviour you simple set FUNCNEST=<N> foo() { foo; }; foo where N
denotes the number of nested functios to be called.


Op zo 7 okt. 2018 07:57 schreef Mike Gerwitz <address@hidden>:

> Hey, Bob!
>
> On Sat, Oct 06, 2018 at 22:44:17 -0600, Bob Proulx wrote:
> > Let me give the discussion this way and I think you will be
> > convinced. :-)
>
> Well, thanks for taking the time for such a long reply. :)
>
> > How is your example any different from a C program?  Or Perl, Python,
> > Ruby, and so forth?  All of those also allow infinite recursion and
> > the kernel will terminate them with a segfault.  Because all of those
> > also allow infinite recursion.  A program that executes an infinite
> > recursion would use infinite stack space.  But real machines have a
> > finite amount of stack available and therefore die when the stack is
> > exceeded.
>
> I expect this behavior when writing in C, certainly.  But in languages
> where the user does not deal with memory management, I'm used to a more
> graceful abort when the stack gets out of control.  A segfault means
> something to a C hacker.  It means very little to users who are
> unfamiliar with the concepts that you were describing.
>
> I don't have enough experience with Perl, Python, or Ruby to know how
> they handle stack issues.  But, out of interest, I gave it a try:
>
>   $ perl -e 'sub foo() { foo(); }; foo()'
>   Out of memory!
>
>   $ python <<< 'def foo():
>   >  foo()
>   >foo()' |& tail -n1
>   RuntimeError: maximum recursion depth exceeded
>
>   $ ruby -e 'def foo()
>   >   foo()
>   > end
>   > foo()'
>   -e:2: stack level too deep (SystemStackError)
>
> Some languages I'm more familiar with:
>
>   $ node -e '(function foo() { foo(); })()'
>   [eval]:1
>   (function foo() { foo(); })()
>                ^
>
>   RangeError: Maximum call stack size exceeded
>
>   $ php -r 'function foo() { foo(); } foo();'
>
>   Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to
>   allocate 262144 bytes) in Command line code on line 1
>
>   $ guile -e '(let x () (+ (x)))'
>   allocate_stack failed: Cannot allocate memory
>   Warning: Unwind-only `stack-overflow' exception; skipping pre-unwind
> handler.
>
>   $ emacs --batch --eval '(message (defun foo () (foo)) (foo))'
>   Lisp nesting exceeds ‘max-lisp-eval-depth’
>
> And so on.
>
> I understand that in C you usually don't manage your own stack and,
> consequently, you can't say that it falls under "memory management" in
> the sense of malloc(3) and brk(2) and such.  But C programmers are aware
> of the mechanisms behind the stack (or at least better be) and won't be
> surprised when they get a segfault in this situation.
>
> But if one of my coworkers who knows some web programming and not much
> about system programming gets a segfault, that's not a friendly
> error.  If Bash instead said something like the above languages, then
> that would be useful.
>
> When I first saw the error, I didn't know that my trap was
> recursing.  My immediate reaction was "shit, I found a bug".  Once I saw
> it was the trap, I _assumed_ it was just exhausting the stack, but I
> wanted to report it regardless, just in case; I didn't have the time to
> dig deeper, and even so, I wasn't sure if it was intended behavior to
> just let the kernel handle it.
>
>
> > This following complete C program recurses infinitely.  Or at least
> > until the stack is exhausted.  At which time it triggers a segfault
> > because it tries to use memory beyond the page mapped stack.
>
> [...]
>
> > Would you say that is a bug in the C language?  A bug in gcc that
> > compiled it?  A bug in the Unix/Linux kernel for memory management
> > that trapped the error?  The parent shell that reported the exit code
> > of the program?  Or in the program source code?  I am hoping that we
> > will all agree that it is a bug in the program source code and not
> > either gcc or the kernel. :-)
>
> I agree, yes.
>
> > Shell script code is program source code.  Infinite loops or infinite
> > recursion are bugs in the shell script source code not the interpreter
> > that is executing the code as written.
>
> I also agree.  But the context is very different.  Shell is a very,
> very high-level language.
>
> > This feels to me to be related to The Halting Problem.
>
> Knowing in advance whether there may be a problem certainly is, but we
> don't need to do that; we'd just need to detect it at runtime to provide
> a more useful error message.
>
> > Other shells are also fun to check:
> >
> >   $ dash -c 'trap "kill 0" TERM; kill 0'
> >   Segmentation fault
> >
> >   $ ash -c 'trap "kill 0" TERM; kill 0'
> >   Segmentation fault
> >
> >   $ mksh -c 'trap "kill 0" TERM; kill 0'
> >   Segmentation fault
>
> Heh, interesting!
>
> >   $ ksh93 -c 'trap "kill 0" TERM; kill 0'
> >   $ echo $?
> >   0
>
> This is not the behavior I'd want.
>
> >   $ posh -c 'trap "kill 0" TERM; kill 0'
> >   Terminated
> >   Terminated
> >   Terminated
> >   ...
> >   Terminated
> >   ^C
>
> :x
>
> > This finds what look like bugs in posh and ksh93.
>
> That's a fair assessment.
>
> >> it's just that most users assume that a segfault represents a
> >> problem with the program
> >
> > Yes.  And here it indicates a bug too.  It is indicating a bug in the
> > shell program code which sets up the infinite recursion.  Programs
> > should avoid doing that. :-)
>
> Indeed they should, but inevitably, such bugs do happen, and mine was a
> particularly common pitfall: I was trying to set the variable from
> within a subprocess.  But it wasn't in a subprocss when I originally
> wrote it.
>
> > The proper way for a program to terminate itself upon catching a
> > signal is to set the signal handler back to the default value and then
> > send the signal to itself so that it will be terminated as a result of
> > the signal and therefore the exit status will be set correctly.
>
> That's good advice.  Thank you.
>
> >   https://www.cons.org/cracauer/sigint.html
>
> Thanks.
>
> > Hope this helps!
>
> There was useful information, yes.  I hope I was able to further clarify
> my concerns as well.
>
> --
> Mike Gerwitz
>


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