ASLR and Stack Gap != 0

[Konstantin Belousov]

On Sat, Oct 26, 2019 at 10:28:07AM -0700, Cy Schubert wrote:
> Let's try this again. This time with a subject line.
> 
> Hi,
> 
> The following little test case segfaults when aslr is enabled:
> 
> #include <stdio.h>
> #include <sys/types.h>
> #include <sys/time.h>
> #include <sys/resource.h>
> 
> int
> main(int argc, char *argv[])
> {
> 	struct rlimit   rl;
> 	int rc;
> 
> 	rl.rlim_cur = 50 * 4096;
> 	rl.rlim_max = 50 * 4096;
> 	rc = setrlimit(RLIMIT_STACK, &rl);
> 
> 	return(rc);
> }
> 
> 
> slippy# sysctl kern.elf64.aslr.enable=1
> kern.elf64.aslr.enable: 0 -> 1
> slippy# ./test 
> Segmentation fault (core dumped)
> slippy# gdb test
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> Reading symbols from test...
> (gdb) run
> Starting program: /export/home/cy/freebsd/tests/setrlimit/test 
> 
> Program received signal SIGSEGV, Segmentation fault.
> setrlimit () at setrlimit.S:4
> 4	setrlimit.S: No such file or directory.
> (gdb) bt
> #0  setrlimit () at setrlimit.S:4
> Backtrace stopped: Cannot access memory at address 0x7fffffe372e8
> (gdb) 
> 
> 
> It only occurs with aslr enabled and stack gap != 0. This isn't right.
> 
I already explained this, also you might find this written down one
more time at
https://github.com/freebsd/freebsd-quarterly/blob/master/2019q3/stack_gap.md