linux-kernel - [PATCH 3/3] fs: Allows for the pivot

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Date:   Fri, 24 Feb 2017 19:21:51 -0500
From:   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 3/3] fs: Allows for the pivot_root syscall to be omitted

From: Brian Ashworth <bosrsf04@...il.com>

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: 0/1 grow/shrink: 0/0 up/down: 0/-614 (-614)
function                                     old     new   delta
sys_pivot_root                               614       -    -614
Total: Before=1899577, After=1898963, chg -0.03%

Signed-off-by: Brian Ashworth <bosrsf04@...il.com>
---
 fs/Makefile     |  4 +++-
 init/Kconfig    | 10 ++++++++++
 kernel/sys_ni.c |  1 +
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/fs/Makefile b/fs/Makefile
index 34cb58c4127d..1a35bc90b0d5 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -11,7 +11,7 @@ obj-y :=	open.o read_write.o file_table.o super.o \
 		attr.o bad_inode.o file.o filesystems.o namespace.o \
 		seq_file.o xattr.o libfs.o fs-writeback.o \
 		pnode.o splice.o sync.o utimes.o \
-		stack.o fs_struct.o statfs.o fs_pin.o nsfs.o pivot_root.o
+		stack.o fs_struct.o statfs.o fs_pin.o nsfs.o
 
 ifeq ($(CONFIG_BLOCK),y)
 obj-y +=	buffer.o block_dev.o direct-io.o mpage.o
@@ -19,6 +19,8 @@ else
 obj-y +=	no-block.o
 endif
 
+obj-$(CONFIG_PIVOT_ROOT_SYSCALL) += pivot_root.o
+
 obj-$(CONFIG_PROC_FS) += proc_namespace.o
 
 obj-y				+= notify/
diff --git a/init/Kconfig b/init/Kconfig
index 4dd8bd232a1d..33ed0282765f 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1462,6 +1462,16 @@ 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