The current ASLR stack gap feature will be removed, and with that the
need for this change, and the kern.stactop sysctl, is gone.  Moreover,
the approach taken in this revision does not provide compatibility for
old copies of libthr.so, and the revision should have also updated
__libc_map_stacks_exec().

This reverts commit 78df56ccfcb40013a3e6904bd6d39836220c3550.

Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation

(cherry picked from commit f75b1ff6e5698ac092aa33d66ea73a56f381ed99)

Use the new kern.stacktop sysctl to retrieve the address of stack top
instead of kern.usrstack. kern.usrstack does not have any knowledge
of the stack gap, so this can cause problems with thread stacks.
Using kern.stacktop sysctl should fix most of those problems.
kern.usrstack is used as a fallback when kern.stacktop cannot be read.

Rename usrstack variables to stacktop to reflect this change.

Fixes problems with firefox and thunderbird not starting with
stack gap enabled.

PR: 239873
Reviewed by: kib
Obtained from: Semihalf
Sponsored by: Stormshield
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D31898

(cherry picked from commit 78df56ccfcb40013a3e6904bd6d39836220c3550)

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using mis-identified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

People tweaking the build system or compilers tend to look into
the Makefile and not into the source.  Having some warning controls
in the Makefile and some in the source code is surprising.

Pragmas have the advantage that they leave the warnings enabled
for more code, but that advantage isn't very relevant in these cases.

Requested by:	kib
Reviewed by:	kib
MFC after:	3 days
Sponsored by:	Dell EMC
Differential Revision:	https://reviews.freebsd.org/D10832

Fix warnings about the following when WARNS=6 (which I will commit soon):

- casting away const
- no previous 'extern' declaration for non-static variable
- others as explained by #pragmas and comments
- unused parameters

The last is the only functional change.

Reviewed by:	kib
MFC after:	3 days
Sponsored by:	Dell EMC
Differential Revision:	https://reviews.freebsd.org/D10808

Sponsored by:	The FreeBSD Foundation

do not participate in the global symbols namespace, but rtld locks are
still replaced and functions are interposed.  In particular,
__pthread_map_stacks_exec is resolved to the libc version.  If a
library is loaded later, which requires adjustment of the stack
protection mode, rtld calls into libc __pthread_map_stacks_exec due to
the symbols scope.  The libc version might recurse into binder and
recursively acquire rtld bind lock, causing the hang.

Make libc __pthread_map_stacks_exec() interposed, which synchronizes
rtld locks and version of the stack exec hook when libthr loaded,
regardless of the symbol scope control or symbol resolution order.

The __pthread_map_stacks_exec() symbol is removed from the private
version in libthr since libc symbol now operates correctly in presence
of libthr.

Reported and tested by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks

the whole RLIMIT_STACK-sized region of the kernel-allocated stack as
the stack of main thread.

By default, the main thread stack is clamped at 2MB (4MB on 64bit
ABIs) and the rest is used for other threads stack allocation.  Since
there is no programmatic way to adjust the size of the main thread
stack, pthread_attr_setstacksize() is too late, the knob allows user
to manage the main stack size both for single-threaded and
multi-threaded processes with the rlimit.

Reported by:	"Ivan A. Kosarev" <ivan@ivan-labs.com>
Tested by:	dim
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days

threading, fall back to libc method of performing
__pthread_map_stacks_exec() job.

Reported and tested by:	Mykola Dzham <i levsha me>