This prctl(2) option can be used to set the "child subreaper" attribute
of the calling process.  This allows a process to take on the role of
'init', which means the process will inherit descendant processes when
their immediate parent terminates.

This will be used in an upcoming commit that uses a double fork(2) +
unshare(2) to create a new PID namespace.  The parent from the second
fork will terminate leaving the child process to be inherited by 'init'.
Aside from it being better to maintain the parent/child relationships
between the various unit processes, without setting this you need to ^C
twice to fully quit unit when running in the foreground after the double
fork.

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Due to the need to replace our use of clone/__NR_clone on Linux with
fork(2)/unshare(2) for enabling Linux namespaces(7) to keep the
pthreads(7) API working.  Let's rename NXT_HAVE_CLONE to
NXT_HAVE_LINUX_NS, i.e name it after the feature, not how it's
implemented, then in future if we change how we do namespaces again we
don't have to rename this.

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

This prctl(2) option is checked for in auto/isolation, unfortunately due
to a typo this feature has never been enabled.

In the auto/isolation script the feature name was down as
NXT_HAVE_PR_SET_NO_NEW_PRIVS0, which means we end up with the following
in build/nxt_auto_config.h

  #ifndef NXT_HAVE_PR_SET_NO_NEW_PRIVS0
  #define NXT_HAVE_PR_SET_NO_NEW_PRIVS0  1
  #endif

Whereas everywhere else is checking for NXT_HAVE_PR_SET_NO_NEW_PRIVS.

This also guards the inclusion of sys/prctl.h in src/nxt_process.c which
is required by a subsequent commit.

Fixes: e2b53e1 ("Added "rootfs" feature.")
Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Future releases of GCC are planning to remove[0] default support for
some old features that were removed from C99 but GCC still accepts.

We can test for these changes by using the following -Werror=
directives

  -Werror=implicit-int
  -Werror=implicit-function-declaration
  -Werror=int-conversion
  -Werror=strict-prototypes
  -Werror=old-style-definition

Doing so revealed an issue with the auto/ tests in that the test
programs always define main as

  int main()

rather than

  int main(void)

which results in a bunch of errors like

build/autotest.c:3:23: error: function declaration isn't a prototype [-Werror=strict-prototypes]
    3 |                   int main() {
      |                       ^~~~
build/autotest.c: In function 'main':
build/autotest.c:3:23: error: old-style function definition [-Werror=old-style-definition]

The fix was easy, it only required fixing the main prototype with

  find -type f -exec sed -i 's/int main() {/int main(void) {/g' {} \;

Regardless of these upcoming GCC changes, this is probably a good thing
to do anyway for correctness.

[0]: https://fedoraproject.org/wiki/Changes/PortingToModernC

Link: <https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/message/CJXKTLXJUPZ4F2C2VQOTNMEA5JAUPMBD/>
Link: <https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/message/6SGHPHPAXKCVJ6PUZ57WVDQ5TDBVIRMF/>
Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Some non-Linux systems implement pivot_root(2), even if they
don't document that.  An example is MacOS:

$ grepc pivot_root / 2>/dev/null
.../sys/sysproto.h:3012:
int pivot_root(struct proc *, struct pivot_root_args *, int *);

Since the prototype of the syscall differs from that of Linux, we
can't use that syscall.  Let's make sure the test only detects
pivot_root(2) under Linux.  Also, rename the feature macro to make
clear that it's only about Linux's pivot_root(2).

This closes #737 issue on GitHub.

With NXT_HAVE_PIVOT_ROOT, we had issues in MacOS.  Headers should
normally be included unconditionally, except of course if they
don't exist.

This fixes part of the #737 issue on GitHub.

User-space programs should use the SYS_*form, as documented in
syscall(2).  That also adds compatibility to non-Linux systems.

The setuid/setgid syscalls requires root capabilities but if the kernel
supports unprivileged user namespace then the child process has the full
set of capabilities in the new namespace, then we can allow setting "user"
and "group" in such cases (this is a common security use case).

Tests were added to ensure user gets meaningful error messages for
uid/gid mapping misconfigurations.