Re: [PATCH 31/34] vfs: syscall: Add fspick() to select a superblock for reconfiguration [ver #12]

Provide an fspick() system call that can be used to pick an existing
mountpoint into an fs_context which can thereafter be used to reconfigure a
superblock (equivalent of the superblock side of -o remount).

This looks like:

	int fd = fspick(AT_FDCWD, "/mnt",
			FSPICK_CLOEXEC | FSPICK_NO_AUTOMOUNT);
	fsconfig(fd, FSCONFIG_SET_FLAG, "intr", NULL, 0);
	fsconfig(fd, FSCONFIG_SET_FLAG, "noac", NULL, 0);
	fsconfig(fd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);

At the point of fspick being called, the file descriptor referring to the
filesystem context is in exactly the same state as the one that was created
by fsopen() after fsmount() has been successfully called.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-api@vger.kernel.org
---

  arch/x86/entry/syscalls/syscall_32.tbl |    1 +
  arch/x86/entry/syscalls/syscall_64.tbl |    1 +
  fs/fsopen.c                            |   54 ++++++++++++++++++++++++++++++++
  include/linux/syscalls.h               |    1 +
  include/uapi/linux/fs.h                |    5 +++
  5 files changed, 62 insertions(+)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index c78b68256f8a..d1eb6c815790 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl

@@ -403,3 +403,4 @@
  389	i386	fsopen			sys_fsopen			__ia32_sys_fsopen
  390	i386	fsconfig		sys_fsconfig			__ia32_sys_fsconfig
  391	i386	fsmount			sys_fsmount			__ia32_sys_fsmount
+392	i386	fspick			sys_fspick			__ia32_sys_fspick

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index d44ead5d4368..d3ab703c02bb 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl

@@ -348,6 +348,7 @@
  337	common	fsopen			__x64_sys_fsopen
  338	common	fsconfig		__x64_sys_fsconfig
  339	common	fsmount			__x64_sys_fsmount
+340	common	fspick			__x64_sys_fspick
  
  #
  # x32-specific system call numbers start at 512 to avoid cache impact

diff --git a/fs/fsopen.c b/fs/fsopen.c
index 5955a6b65596..9ead9220e2cb 100644
--- a/fs/fsopen.c
+++ b/fs/fsopen.c

@@ -155,6 +155,60 @@ SYSCALL_DEFINE2(fsopen, const char __user *, _fs_name, unsigned int, flags)
  	return ret;
  }
  
+/*
+ * Pick a superblock into a context for reconfiguration.
+ */
+SYSCALL_DEFINE3(fspick, int, dfd, const char __user *, path, unsigned int, flags)
+{
+	struct fs_context *fc;
+	struct path target;
+	unsigned int lookup_flags;
+	int ret;
+
+	if (!ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN))
+		return -EPERM;