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fs: add fchroot()

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Add a file descriptor based counterpart to chroot(2). This has been
overdue for a long time. It is the natural companion to fchdir() and
avoids re-resolving a path that the caller already holds a file
descriptor to. No TOCTOU between resolving the target and changing the
root. It composes with modern fd-based APIs meaning it works with O_PATH
file descriptors and file descriptors to detached mount trees created
via open_tree(OPEN_TREE_CLONE).

The permission model is identical to chroot(2). The caller must have
CAP_SYS_CHROOT in its user namespace, must pass MAY_EXEC | MAY_CHDIR
permission checks on the target directory, and LSMs are consulted via
the same security_path_chroot() hook.

The system call takes a flags argument for future extensibility which
must currently be zero.

Link: https://patch.msgid.link/20260724-work-failfs-v2-3-485dabbae185@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Christian Brauner 2026-07-24 15:41:19 +02:00
parent cdf930a009
commit 20370a5f5d
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GPG Key ID: 91C61BC06578DCA2
2 changed files with 30 additions and 0 deletions
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29
fs/open.c
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@ -618,6 +618,35 @@ SYSCALL_DEFINE1(chroot, const char __user *, filename)
return error;
}
SYSCALL_DEFINE2(fchroot, int, fd, unsigned int, flags)
{
int error;
if (flags)
return -EINVAL;
CLASS(fd_raw, f)(fd);
if (fd_empty(f))
return -EBADF;
if (!d_can_lookup(fd_file(f)->f_path.dentry))
return -ENOTDIR;
error = file_permission(fd_file(f), MAY_EXEC | MAY_CHDIR);
if (error)
return error;
if (!ns_capable(current_user_ns(), CAP_SYS_CHROOT))
return -EPERM;
error = security_path_chroot(&fd_file(f)->f_path);
if (error)
return error;
set_fs_root(current->fs, &fd_file(f)->f_path);
return 0;
}
int chmod_common(const struct path *path, umode_t mode)
{
struct inode *inode = path->dentry->d_inode;

1
include/linux/syscalls.h
View File

@ -457,6 +457,7 @@ asmlinkage long sys_faccessat2(int dfd, const char __user *filename, int mode,
asmlinkage long sys_chdir(const char __user *filename);
asmlinkage long sys_fchdir(unsigned int fd);
asmlinkage long sys_chroot(const char __user *filename);
asmlinkage long sys_fchroot(int fd, unsigned int flags);
asmlinkage long sys_fchmod(unsigned int fd, umode_t mode);
asmlinkage long sys_fchmodat(int dfd, const char __user *filename,
umode_t mode);