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vfs: syscall: Add fspick() to select a superblock for reconfiguration

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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
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
David Howells 2018-11-01 23:36:23 +00:00 • committed by Al Viro
parent 93766fbd26
commit cf3cba4a42
5 changed files with 68 additions and 1 deletions
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57
fs/fsopen.c
View file

@ -156,6 +156,63 @@ err_fc:
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;
if ((flags & ~(FSPICK_CLOEXEC |
FSPICK_SYMLINK_NOFOLLOW |
FSPICK_NO_AUTOMOUNT |
FSPICK_EMPTY_PATH)) != 0)
return -EINVAL;
lookup_flags = LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT;
if (flags & FSPICK_SYMLINK_NOFOLLOW)
lookup_flags &= ~LOOKUP_FOLLOW;
if (flags & FSPICK_NO_AUTOMOUNT)
lookup_flags &= ~LOOKUP_AUTOMOUNT;
if (flags & FSPICK_EMPTY_PATH)
lookup_flags |= LOOKUP_EMPTY;
ret = user_path_at(dfd, path, lookup_flags, &target);
if (ret < 0)
goto err;
ret = -EINVAL;
if (target.mnt->mnt_root != target.dentry)
goto err_path;
fc = fs_context_for_reconfigure(target.dentry, 0, 0);
if (IS_ERR(fc)) {
ret = PTR_ERR(fc);
goto err_path;
}
fc->phase = FS_CONTEXT_RECONF_PARAMS;
ret = fscontext_alloc_log(fc);
if (ret < 0)
goto err_fc;
path_put(&target);
return fscontext_create_fd(fc, flags & FSPICK_CLOEXEC ? O_CLOEXEC : 0);
err_fc:
put_fs_context(fc);
err_path:
path_put(&target);
err:
return ret;
}
/*
* Check the state and apply the configuration. Note that this function is
* allowed to 'steal' the value by setting param->xxx to NULL before returning.