MacOS implements pivot_root(), but differently from Linux

[alejandro-colomar]

The recent commit 5015b05 improved portability of syscalls by using SYS_xxx instead of __NR_xxx.

This had a few unexpected side effects:

• Since only Linux documents the existence of pivot_root(2), we thought that its existence guaranteed the existence of a header that only exists in GNU/Linux systems: <mntent.h>. Now that the configure script detects the existence of SYS_pivot_root in MacOS, the code tries to include <mntent.h>.

• The implementation of MacOS's pivot_root() is very different from the one in Linux. Even worse: it's undocumented; there's no pivot_root(2) manual page in MacOS. See details for MacOS's pivot_root() below.

The easy solution for the first problem is testing for the existence of the header before including it. Just a minor problem.

The second one is a more important problem. We should make sure that we only use pivot_root(2) in Linux. So we should wrap uses of the syscall with #if (NXT_LINUX) or similar. NXT_HAVE_PIVOT_ROOT is not enough. Maybe we should rename it to NXT_HAVE_LINUX_PIVOT_ROOT, and fix the test accordingly.

---

pivot_root() in MacOS:

$ grepc -k SYS_pivot_root / 2>/dev/null
/Library/Developer/CommandLineTools/SDKs/MacOSX11.1.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/syscall.h:577:#define	SYS_pivot_root     537
/Library/Developer/CommandLineTools/SDKs/MacOSX11.1.sdk/usr/include/sys/syscall.h:577:#define	SYS_pivot_root     537
/Library/Developer/CommandLineTools/SDKs/MacOSX11.3.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/syscall.h:577:#define	SYS_pivot_root     537
/Library/Developer/CommandLineTools/SDKs/MacOSX11.3.sdk/usr/include/sys/syscall.h:577:#define	SYS_pivot_root     537
/Library/Developer/CommandLineTools/SDKs/MacOSX12.1.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/syscall.h:577:#define	SYS_pivot_root     537
/Library/Developer/CommandLineTools/SDKs/MacOSX12.1.sdk/usr/include/sys/syscall.h:577:#define	SYS_pivot_root     537
/System/Volumes/Data/Library/Developer/CommandLineTools/SDKs/MacOSX11.1.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/syscall.h:577:#define	SYS_pivot_root     537
/System/Volumes/Data/Library/Developer/CommandLineTools/SDKs/MacOSX11.1.sdk/usr/include/sys/syscall.h:577:#define	SYS_pivot_root     537
/System/Volumes/Data/Library/Developer/CommandLineTools/SDKs/MacOSX11.3.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/syscall.h:577:#define	SYS_pivot_root     537
/System/Volumes/Data/Library/Developer/CommandLineTools/SDKs/MacOSX11.3.sdk/usr/include/sys/syscall.h:577:#define	SYS_pivot_root     537
/System/Volumes/Data/Library/Developer/CommandLineTools/SDKs/MacOSX12.1.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/syscall.h:577:#define	SYS_pivot_root     537
/System/Volumes/Data/Library/Developer/CommandLineTools/SDKs/MacOSX12.1.sdk/usr/include/sys/syscall.h:577:#define	SYS_pivot_root     537

$ grepc -k pivot_root / 2>/dev/null
/Library/Developer/CommandLineTools/SDKs/MacOSX11.1.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/sysproto.h:2977:int pivot_root(struct proc *, struct pivot_root_args *, int *);
/Library/Developer/CommandLineTools/SDKs/MacOSX11.3.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/sysproto.h:2990:int pivot_root(struct proc *, struct pivot_root_args *, int *);
/Library/Developer/CommandLineTools/SDKs/MacOSX12.1.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/sysproto.h:3012:int pivot_root(struct proc *, struct pivot_root_args *, int *);
/System/Volumes/Data/Library/Developer/CommandLineTools/SDKs/MacOSX11.1.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/sysproto.h:2977:int pivot_root(struct proc *, struct pivot_root_args *, int *);
/System/Volumes/Data/Library/Developer/CommandLineTools/SDKs/MacOSX11.3.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/sysproto.h:2990:int pivot_root(struct proc *, struct pivot_root_args *, int *);
/System/Volumes/Data/Library/Developer/CommandLineTools/SDKs/MacOSX12.1.sdk/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/sys/sysproto.h:3012:int pivot_root(struct proc *, struct pivot_root_args *, int *);

exists but is undocumented:

$ man -k pivot_root
pivot_root: nothing appropriate
$ man -K pivot_root
$ echo $?
1

And most importantly, it differs from Linux's pivot_root(2):

SYNOPSIS
       #include <sys/syscall.h>      /* Definition of SYS_* constants */
       #include <unistd.h>

       int syscall(SYS_pivot_root, const char *new_root, const char *put_old);

       Note: glibc provides no wrapper for pivot_root(), necessitating the use
       of syscall(2).

[alejandro-colomar]

Since the fix will involve changing NXT_HAVE_PIVOT_ROOT, it'll conflict with the headers fixup (in ReviewBoard now), so I'll wait to write the fix about pivot_root(2).