Re: -fstack-check on Linux x86--Dumb question...

[Mike Stump <mrs at windriver dot com>]

> Date: Fri, 17 Mar 2000 20:14:21 -0500
> From: "George T. Talbot" <george@moberg.com>

> I've got a program which fails with a SIGSEGV when -fstack-check is
> turned on.  Would anyone have more information about how
> -fstack-check works on an Intel processor under Linux?

Sure, it works by probing the stack before using it, to ensure that
the stack is really there.  If it isn't, the compiler expects the page
to not be mapped and for the operation to fault.  The amount of stack
space is set for the main thread by the limit or ulimit command, see
you shell's documentation.  If you are running out, just up the amount
allocated, and/or add more swap.

If you're not using threads, there is almost no point in using this
flag.  Why do you bother with this flag?

> I'm specifically interested in two things:

> 1)  How does -fstack-check determine whether the stack has been overrun?

It doesn't.  It just does an access to fault the program, should you
overrun it.  The OS determines that you overran the stack by not
mapping the page just past the stack.  This compiler flag arranges to
access all stack pages in growth order without skipping pages.

> 2)  Is there a way using GDB, or some other tool of which I'm not aware,
>     to find the specific instruction and access address of the SIGSEGV?

Yes, the command is run.  You can't be using it, without having tried
this command already.  By default, gdb will stop on the instruction
that faulted (or just after it, I forget which).  `x/20i $pc' will
show you the instructions, if you wish to see them.  `where' will
similar information.

Hope this helps.