sys::mman: adding few madvise flags. (#2559)

* sys::mman: adding few madvise flags.

- `MADV_PAGEOUT` which reclaim the address range, swapping it out
if the page is anonymous or written back to disk if it's backed up
by a file.
- `MADV_COLD` which deactivate the address range but as a hint.
- `MADV_WIPEONFORK` after `fork()`, the address range is wiped out
to avoid sharing sensible data between processes.
- `MADV_KEEPONFORK` disables `MADV_WIPEONFORK` workflow.

* Update changelog/2559.added.md

---------

Co-authored-by: SteveLauC <[email protected]>

Author: devnexen

changelog/2559.added.md

@@ -0,0 +1 @@
+Add `sys::mman::MmapAdvise` `MADV_PAGEOUT`, `MADV_COLD`, `MADV_WIPEONFORK`, `MADV_KEEPONFORK` for Linux and Android targets

src/sys/mman.rs

@@ -316,6 +316,18 @@ libc_enum! {
         #[cfg(apple_targets)]
         #[allow(missing_docs)]
         MADV_CAN_REUSE,
+        /// Reclaim the address range when applicable.
+        #[cfg(linux_android)]
+        MADV_PAGEOUT,
+        /// Deactivate the address range when applicable.
+        #[cfg(linux_android)]
+        MADV_COLD,
+        /// After fork, the adress range is zero filled.
+        #[cfg(linux_android)]
+        MADV_WIPEONFORK,
+        /// Undo `MADV_WIPEONFORK` when it applied.
+        #[cfg(linux_android)]
+        MADV_KEEPONFORK,
     }
 }
 

test/sys/test_mman.rs

@@ -156,3 +156,45 @@ fn test_mremap_dontunmap() {
         std::slice::from_raw_parts_mut(mem.cast().as_ptr(), 10 * ONE_K)
     };
 }
+
+#[test]
+#[cfg(target_os = "linux")]
+fn test_madv_wipeonfork() {
+    use nix::libc::size_t;
+    use nix::sys::mman::{madvise, MmapAdvise};
+    use nix::unistd::{fork, ForkResult};
+    use std::num::NonZeroUsize;
+
+    const ONE_K: size_t = 1024;
+    let ten_one_k = NonZeroUsize::new(10 * ONE_K).unwrap();
+    let slice: &mut [u8] = unsafe {
+        let mem = mmap_anonymous(
+            None,
+            ten_one_k,
+            ProtFlags::PROT_READ | ProtFlags::PROT_WRITE,
+            MapFlags::MAP_PRIVATE,
+        )
+        .unwrap();
+        madvise(mem, ONE_K, MmapAdvise::MADV_WIPEONFORK)
+            .expect("madvise failed");
+        std::slice::from_raw_parts_mut(mem.as_ptr().cast(), ONE_K)
+    };
+    slice[ONE_K - 1] = 0xFF;
+    let _m = crate::FORK_MTX.lock();
+
+    unsafe {
+        let res = fork().expect("fork failed");
+        match res {
+            ForkResult::Child => {
+                // that s the whole point of MADV_WIPEONFORK
+                assert_eq!(slice[ONE_K - 1], 0x00);
+                libc::_exit(0);
+            }
+            ForkResult::Parent { child } => {
+                nix::sys::signal::kill(child, nix::sys::signal::SIGTERM)
+                    .unwrap();
+                let _ = nix::sys::wait::wait().unwrap();
+            }
+        }
+    }
+}