[ruby-core:39285] [Ruby 1.9 - Bug #5244] Continuation causes Bus Error on Debian sparc

Issue #5244 has been updated by Jurij Smakov.


I poked at it a bit. While I don't understand fully what's going on, it looks like it's using _setjmp/longjmp, and one thing in setjmp page caught my eye: "setjmp() saves  the stack context/environment in env for later use by longjmp(3). The stack context will be  invalidated  if  the  function  which  called setjmp() returns." I think that such invalidation takes place here. Function rb_callcc (in cont.c) does the following:

rb_callcc(VALUE self)
{
    volatile int called;
    volatile VALUE val = cont_capture(&called);

    if (called) {
        return val;
    }
    else {
        return rb_yield(val);
    }
}

In cont_capture() the _setjmp is invoked via

    if (ruby_setjmp(cont->jmpbuf)) {
        volatile VALUE value;

        value = cont->value;
        if (cont->argc == -1) rb_exc_raise(value);
        cont->value = Qnil;
        *stat = 1;
        return value;
    }
    else {
        *stat = 0;
        return cont->self;
    }

So, after invoking _setjmp, cont_capture returns which, according to the man page, should invalidate the saved stack context. Later, when something in the rb_yield(val) call chain tries to do the longjmp, we arrive at the location where _setjmp was originally called (in cont_capture) with a smashed stack, resulting in a SIGBUS.

The fact that it works for other arches (and even on sparc, if I rebuild everything with -O0 instead of -O2) is somewhat surprising, but it might be that it just working "by accident" in most cases (i.e. the saved stack is preserved, even though it's not guaranteed).
----------------------------------------
Bug #5244: Continuation causes Bus Error on Debian sparc
http://redmine.ruby-lang.org/issues/5244

Author: Lucas Nussbaum
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 
ruby -v: 1.9.3


Hi,

$ ./miniruby -I./lib -I. -I.ext/common ./tool/runruby.rb --extout=.ext -rcontinuation -e 'callcc { |c| c.call }'
-e:1: [BUG] Bus Error
ruby 1.9.3dev (2011-08-26) [sparc-linux]

-- Control frame information -----------------------------------------------
c:0004 p:---- s:0009 b:0009 l:000008 d:000008 CFUNC  :callcc
c:0003 p:0009 s:0006 b:0006 l:000fcc d:001d74 EVAL   -e:1
c:0002 p:---- s:0004 b:0004 l:000003 d:000003 FINISH
c:0001 p:0000 s:0002 b:0002 l:000fcc d:000fcc TOP   

-- Ruby level backtrace information ----------------------------------------
-e:1:in `<main>'
-e:1:in `callcc'

-- C level backtrace information -------------------------------------------
Bus error


gdb says:
(gdb) run -I./lib -I. -I.ext/common ./tool/runruby.rb --extout=.ext -rcontinuation -e 'callcc { |c| c.call }'
Starting program: /home/lucas/ruby1.9.1-1.9.3~preview1+svn33077/miniruby -I./lib -I. -I.ext/common ./tool/runruby.rb --extout=.ext -rcontinuation -e 'callcc { |c| c.call }'
[Thread debugging using libthread_db enabled]
[New Thread 0xf7fc7b70 (LWP 31418)]
[Thread 0xf7fc7b70 (LWP 31418) exited]
process 31417 is executing new program: /home/lucas/ruby1.9.1-1.9.3~preview1+svn33077/ruby1.9.1
[Thread debugging using libthread_db enabled]
[New Thread 0xf79e5b70 (LWP 31419)]

Program received signal SIGBUS, Bus error.
0xf7f4d304 in cont_capture (stat=Cannot access memory at address 0x49
) at cont.c:439
439	    if (ruby_setjmp(cont->jmpbuf)) {

(gdb) print cont
Cannot access memory at address 0xfffffff9


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