Re: [PATCH 31/40] target_ops/C++: Base FreeBSD target

[John Baldwin <jhb@freebsd.org>]

On Tuesday, April 17, 2018 06:07:37 PM Pedro Alves wrote:
> On 04/17/2018 05:05 PM, John Baldwin wrote:
> > On Saturday, April 14, 2018 08:09:44 PM Pedro Alves wrote:
> >> The
> >>
> >>   $architecture x NetBSD/OpenBSD/FreeBSD
> >>
> >> support matrix complicates things a bit.  There's common BSD target
> >> code, and there's common architecture-specific code shared between the
> >> different BSDs.  Current, all that is stiched together to form a final
> >> target, via the i386bsd_target, x86bsd_target, fbsd_nat_add_target
> >> functions etc.
> >>
> >> Introduces a fbsd_nat_target base/prototype target.  To be used in
> >> following patches.
> > 
> > I will do some tests of FreeBSD/amd64 first and let you know what I find.
> 
> Thank you!

I've pushed a target_ops-cxx branch to github.com/bsdjhb/gdb.git that has
some small fixups (compile fixes).  I've built the amd64, i386, arm, and
aarch64 FreeBSD native targets so far.  Simple testing of the the amd64
and i386 binaries seems to work, but I encountered a new test failure
in the testsuite for FreeBSD/amd64 that is a bit odd.  In particular,
I get a core dump running 'info set' when it tries to display the
current setting of whether ASLR is disabled.  Looking at the core of gdb:

Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x0000000000d3a7d4 in target_ops::supports_disable_randomization (
    this=0x28d4c68 <the_amd64_fbsd_nat_target>)
    at ../../gdb/target-delegates.c:2732
2732      return this->beneath->supports_disable_randomization ();

(top-gdb) where
#0  0x0000000000d3a7d4 in target_ops::supports_disable_randomization (
    this=0x28d4c68 <the_amd64_fbsd_nat_target>)
    at ../../gdb/target-delegates.c:2732
#1  0x0000000000d3a857 in find_default_supports_disable_randomization (
#2  0x0000000000d3a805 in dummy_target::supports_disable_randomization (
    this=0x8040d6060) at ../../gdb/target-delegates.c:2738
#3  0x0000000000d2edd8 in target_supports_disable_randomization ()
    at ../../gdb/target.c:2560
#4  0x0000000000b9adbc in show_disable_randomization (file=0x8047b1200, 
    from_tty=1, c=0x80476db80, value=0x7fffffffd859 "on")
    at ../../gdb/infrun.c:181
#5  0x00000000007b1178 in do_show_command (arg=0x0, from_tty=1, c=0x80476db80)
    at ../../gdb/cli/cli-setshow.c:646
#6  0x00000000007b1434 in cmd_show_list (list=0x80476db80, from_tty=1, 
    prefix=0x22a7ead "") at ../../gdb/cli/cli-setshow.c:685
...

The native target isn't currently in the target_stack so its 'beneath' is
NULL:

(top-gdb) p beneath
During symbol reading, missing name for subprogram DIE at 14709647.
$1 = (target_ops *) 0x0
(top-gdb) p target_stack
$2 = (target_ops *) 0x8040d6060
(top-gdb) p *target_stack
$3 = {_vptr$target_ops = 0x183aaf0 <vtable for dummy_target+16>, 
  beneath = 0x0, to_stratum = dummy_stratum}

From the stack trace we can see that it already bounced down to the dummy
target which calls find_default_supports_disable_randomization.  That
finds the native "run" target and invokes its method without pushing
it onto the stack.  I think before if a native target didn't support ASLR
at all it just didn't set the function pointer and no harm was done.
Now the function pointer is effectively always set but to something that
assumes 'beneath' is valid.  I'm not quite sure how you want to fix this.
The simple solution is to change the default method to return false if
beneath is NULL, but I'm not quite sure that fits in with the design this
branch is aiming for.

-- 
John Baldwin