JIT breaks on feather3

Mark Glines mark at glines.org
Wed Dec 3 06:27:15 UTC 2008

Tene++ has been having some issues recently with polyglotbot, the 
evalbot in #parrot IRC.  I spent some time today trying to track this down.

The symptoms of this are really weird.  The segfault seems to happen in 
Tene's parrot checkouts, when the user is running it inside of a screen 
session.  It doesn't seem to crash in my own parrot checkouts, and it 
doesn't crash in a bare (non-screen) ssh connection.

With the help of tewk++, we discovered that the segfault occurs as soon 
as some JIT code is executed.  After some more digging, it looks like 
the heap does not have the executable bit set.

Parrot's /proc/<pid>/maps contains this when run under screen:

0804f000-08437000 rw-p 0804f000 00:00 0          [heap]

And this when run outside of screen:

0804f000-08322000 rwxp 0804f000 00:00 0          [heap]

That's for the same binary, executed by the same user, from the same 
working directory, with the same arguments.

Its not clear yet what is causing the heap to not have the executable 
bit set.  It's really troublesome to reproduce this - I haven't seen it 
anywhere else, and it only occurs under a few circumstances (feather3, 
tene's homedir, running under screen) that don't really make sense 
together yet.  But in any case, whatever is causing this, it breaks JIT 

Kevin mentioned that parrot is using mprotect() to explicitly ask glibc 
to make this memory executable.  Since it isn't, that possibly makes 
this a glibc bug.  Either way, this seems like the sort of issue which 
would keep cropping up in weird ways, unless we could detect/prevent it 

So, List, does anyone happen to know why processes under screen have 
non-executable stacks?  Or possibly a bash shopt or ulimit or dropped 
privilege or somesuch?


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