Add mmap_anonymous function (#2127)

* Add `mmap_anonymous` function

* Add changelog note

* Add changelog note for mmap change

Author: newpavlov

changelog/2127.added.md

@@ -0,0 +1 @@
+Added `mmap_anonymous` function

changelog/2127.changed.md

@@ -0,0 +1 @@
+`mmap` function now accepts `F` instead of `Option<F>`

src/sys/mman.rs

@@ -411,7 +411,9 @@ pub fn munlockall() -> Result<()> {
     unsafe { Errno::result(libc::munlockall()) }.map(drop)
 }
 
-/// allocate memory, or map files or devices into memory
+/// Allocate memory, or map files or devices into memory
+///
+/// For anonymous mappings (`MAP_ANON`/`MAP_ANONYMOUS`), see [mmap_anonymous].
 ///
 /// # Safety
 ///
@@ -423,13 +425,12 @@ pub unsafe fn mmap<F: AsFd>(
     length: NonZeroUsize,
     prot: ProtFlags,
     flags: MapFlags,
-    f: Option<F>,
+    f: F,
     offset: off_t,
 ) -> Result<*mut c_void> {
-    let ptr =
-        addr.map_or(std::ptr::null_mut(), |a| usize::from(a) as *mut c_void);
+    let ptr = addr.map_or(std::ptr::null_mut(), |a| a.get() as *mut c_void);
 
-    let fd = f.map(|f| f.as_fd().as_raw_fd()).unwrap_or(-1);
+    let fd = f.as_fd().as_raw_fd();
     let ret =
         libc::mmap(ptr, length.into(), prot.bits(), flags.bits(), fd, offset);
 
@@ -440,6 +441,34 @@ pub unsafe fn mmap<F: AsFd>(
     }
 }
 
+/// Create an anonymous memory mapping.
+///
+/// This function is a wrapper around [`mmap`]:
+/// `mmap(ptr, len, prot, MAP_ANONYMOUS | flags, -1, 0)`.
+///
+/// # Safety
+///
+/// See the [`mmap(2)`] man page for detailed requirements.
+///
+/// [`mmap(2)`]: https://man7.org/linux/man-pages/man2/mmap.2.html
+pub unsafe fn mmap_anonymous(
+    addr: Option<NonZeroUsize>,
+    length: NonZeroUsize,
+    prot: ProtFlags,
+    flags: MapFlags,
+) -> Result<*mut c_void> {
+    let ptr = addr.map_or(std::ptr::null_mut(), |a| a.get() as *mut c_void);
+
+    let flags = MapFlags::MAP_ANONYMOUS | flags;
+    let ret = libc::mmap(ptr, length.into(), prot.bits(), flags.bits(), -1, 0);
+
+    if ret == libc::MAP_FAILED {
+        Err(Errno::last())
+    } else {
+        Ok(ret)
+    }
+}
+
 /// Expands (or shrinks) an existing memory mapping, potentially moving it at
 /// the same time.
 ///
@@ -519,14 +548,14 @@ pub unsafe fn madvise(
 ///
 /// ```
 /// # use nix::libc::size_t;
-/// # use nix::sys::mman::{mmap, mprotect, MapFlags, ProtFlags};
+/// # use nix::sys::mman::{mmap_anonymous, mprotect, MapFlags, ProtFlags};
 /// # use std::ptr;
 /// # use std::os::unix::io::BorrowedFd;
 /// const ONE_K: size_t = 1024;
 /// let one_k_non_zero = std::num::NonZeroUsize::new(ONE_K).unwrap();
 /// let mut slice: &mut [u8] = unsafe {
-///     let mem = mmap::<BorrowedFd>(None, one_k_non_zero, ProtFlags::PROT_NONE,
-///                    MapFlags::MAP_ANON | MapFlags::MAP_PRIVATE, None, 0).unwrap();
+///     let mem = mmap_anonymous(None, one_k_non_zero, ProtFlags::PROT_NONE, MapFlags::MAP_PRIVATE)
+///         .unwrap();
 ///     mprotect(mem, ONE_K, ProtFlags::PROT_READ | ProtFlags::PROT_WRITE).unwrap();
 ///     std::slice::from_raw_parts_mut(mem as *mut u8, ONE_K)
 /// };

test/sys/test_mman.rs

@@ -1,16 +1,14 @@
-use nix::sys::mman::{mmap, MapFlags, ProtFlags};
-use std::{num::NonZeroUsize, os::unix::io::BorrowedFd};
+use nix::sys::mman::{mmap_anonymous, MapFlags, ProtFlags};
+use std::num::NonZeroUsize;
 
 #[test]
 fn test_mmap_anonymous() {
     unsafe {
-        let ptr = mmap::<BorrowedFd>(
+        let ptr = mmap_anonymous(
             None,
             NonZeroUsize::new(1).unwrap(),
             ProtFlags::PROT_READ | ProtFlags::PROT_WRITE,
-            MapFlags::MAP_PRIVATE | MapFlags::MAP_ANONYMOUS,
-            None,
-            0,
+            MapFlags::MAP_PRIVATE,
         )
         .unwrap() as *mut u8;
         assert_eq!(*ptr, 0x00u8);
@@ -29,13 +27,11 @@ fn test_mremap_grow() {
     let one_k_non_zero = NonZeroUsize::new(ONE_K).unwrap();
 
     let slice: &mut [u8] = unsafe {
-        let mem = mmap::<BorrowedFd>(
+        let mem = mmap_anonymous(
             None,
             one_k_non_zero,
             ProtFlags::PROT_READ | ProtFlags::PROT_WRITE,
-            MapFlags::MAP_ANONYMOUS | MapFlags::MAP_PRIVATE,
-            None,
-            0,
+            MapFlags::MAP_PRIVATE,
         )
         .unwrap();
         std::slice::from_raw_parts_mut(mem as *mut u8, ONE_K)
@@ -87,13 +83,11 @@ fn test_mremap_shrink() {
     const ONE_K: size_t = 1024;
     let ten_one_k = NonZeroUsize::new(10 * ONE_K).unwrap();
     let slice: &mut [u8] = unsafe {
-        let mem = mmap::<BorrowedFd>(
+        let mem = mmap_anonymous(
             None,
             ten_one_k,
             ProtFlags::PROT_READ | ProtFlags::PROT_WRITE,
-            MapFlags::MAP_ANONYMOUS | MapFlags::MAP_PRIVATE,
-            None,
-            0,
+            MapFlags::MAP_PRIVATE,
         )
         .unwrap();
         std::slice::from_raw_parts_mut(mem as *mut u8, ONE_K)