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Date:   Sat, 25 Feb 2017 19:54:47 -0500
From:   Brian Ashworth <bosrsf04@...il.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Petr Mladek <pmladek@...e.com>, Helge Deller <deller@....de>,
        Rik van Riel <riel@...hat.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Parav Pandit <pandit.parav@...il.com>,
        "seokhoon . yoon" <iamyooon@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...el.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     josh@...htriplett.org, brkkurek192@...il.com, conorcurry@...il.com,
        fanofbond138@...il.com, Brian Ashworth <bosrsf04@...il.com>
Subject: [PATCH 1/1] fs: Allows for the pivot_root syscall to be optional

The pivot_root syscall is not needed on systems that do not use
any intermediate filesystem. Allowing for pivot_root to be
ommitted from the kernel will aid in the tinification efforts.

Without CONFIG_PIVOT_ROOT_SYSCALL set
add/remove: 0/2 grow/shrink: 1/0 up/down: 45/-707 (-662)
function                                     old     new   delta
attach_recursive_mnt                         349     394     +45
attach_mnt                                    71       -     -71
sys_pivot_root                               636       -    -636
Total: Before=1899893, After=1899231, chg -0.03%

Signed-off-by: Brian Ashworth <bosrsf04@...il.com>
---
 fs/namespace.c  |  2 ++
 init/Kconfig    | 10 ++++++++++
 kernel/sys_ni.c |  1 +
 3 files changed, 13 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index 487ba30bb5c6..5e24a08bfb36 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3058,6 +3058,7 @@ bool path_is_under(const struct path *path1, const struct path *path2)
 }
 EXPORT_SYMBOL(path_is_under);
 
+#ifdef CONFIG_PIVOT_ROOT_SYSCALL
 /*
  * pivot_root Semantics:
  * Moves the root file system of the current process to the directory put_old,
@@ -3180,6 +3181,7 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
 out0:
 	return error;
 }
+#endif  /* CONFIG_PIVOT_ROOT_SYSCALL */
 
 static void __init init_mount_tree(void)
 {
diff --git a/init/Kconfig b/init/Kconfig
index 8c39615165b7..4ea9ab25ec30 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1450,6 +1450,16 @@ config SYSCTL_SYSCALL
 
 	  If unsure say N here.
 
+config PIVOT_ROOT_SYSCALL
+	bool "Pivot_root syscall support" if EXPERT
+	default y
+	help
+	  pivot_root is a system call that allows the root to be moved and
+	  replaced by another root. This is needed for intermediate file
+	  systems such as initrd.
+
+	  If unsure say Y here.
+
 config POSIX_TIMERS
 	bool "Posix Clocks & timers" if EXPERT
 	default y
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 8acef8576ce9..7bd207571f87 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -178,6 +178,7 @@ cond_syscall(sys_setfsgid);
 cond_syscall(sys_capget);
 cond_syscall(sys_capset);
 cond_syscall(sys_copy_file_range);
+cond_syscall(sys_pivot_root);
 
 /* arch-specific weak syscall entries */
 cond_syscall(sys_pciconfig_read);
-- 
2.11.1

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