Add pivot_root syscall. (#1116)

It is enabled only in Linux, and with the feature `fs` (like `chroot`).

Author: ayosec

src/backend/libc/process/syscalls.rs

@@ -725,6 +725,17 @@ pub(crate) fn pidfd_getfd(
     }
 }
 
+#[cfg(target_os = "linux")]
+pub(crate) fn pivot_root(new_root: &CStr, put_old: &CStr) -> io::Result<()> {
+    syscall! {
+        fn pivot_root(
+            new_root: *const c::c_char,
+            put_old: *const c::c_char
+        ) via SYS_pivot_root -> c::c_int
+    }
+    unsafe { ret(pivot_root(c_str(new_root), c_str(put_old))) }
+}
+
 #[cfg(all(feature = "alloc", not(target_os = "wasi")))]
 pub(crate) fn getgroups(buf: &mut [Gid]) -> io::Result<usize> {
     let len = buf.len().try_into().map_err(|_| io::Errno::NOMEM)?;

src/backend/linux_raw/process/syscalls.rs

@@ -601,6 +601,12 @@ pub(crate) fn pidfd_send_signal(fd: BorrowedFd<'_>, sig: Signal) -> io::Result<(
     }
 }
 
+#[cfg(feature = "fs")]
+#[inline]
+pub(crate) fn pivot_root(new_root: &CStr, put_old: &CStr) -> io::Result<()> {
+    unsafe { ret(syscall_readonly!(__NR_pivot_root, new_root, put_old)) }
+}
+
 #[cfg(feature = "alloc")]
 #[inline]
 pub(crate) fn getgroups(buf: &mut [Gid]) -> io::Result<usize> {

src/process/mod.rs

@@ -17,6 +17,8 @@ mod membarrier;
 mod pidfd;
 #[cfg(target_os = "linux")]
 mod pidfd_getfd;
+#[cfg(target_os = "linux")]
+mod pivot_root;
 #[cfg(linux_kernel)]
 mod prctl;
 #[cfg(not(any(target_os = "fuchsia", target_os = "vita", target_os = "wasi")))]
@@ -57,6 +59,8 @@ pub use membarrier::*;
 pub use pidfd::*;
 #[cfg(target_os = "linux")]
 pub use pidfd_getfd::*;
+#[cfg(target_os = "linux")]
+pub use pivot_root::*;
 #[cfg(linux_kernel)]
 pub use prctl::*;
 #[cfg(not(any(target_os = "fuchsia", target_os = "vita", target_os = "wasi")))]

src/process/pivot_root.rs

@@ -0,0 +1,18 @@
+#[cfg(feature = "fs")]
+#[cfg_attr(doc_cfg, doc(cfg(feature = "fs")))]
+use crate::{backend, io, path};
+
+/// `pivot_root(new_root, put_old)`—Change the root mount.
+///
+/// # References
+///  - [Linux]
+///
+/// [Linux]: https://man7.org/linux/man-pages/man2/pivot_root.2.html
+#[cfg(feature = "fs")]
+#[cfg_attr(doc_cfg, doc(cfg(feature = "fs")))]
+#[inline]
+pub fn pivot_root<P: path::Arg, Q: path::Arg>(new_root: P, put_old: Q) -> io::Result<()> {
+    new_root.into_with_c_str(|new_root| {
+        put_old.into_with_c_str(|put_old| backend::process::syscalls::pivot_root(new_root, put_old))
+    })
+}