Use mmap for allocation (#190)

.travis.yml

@@ -11,6 +11,8 @@ matrix:
   - rust: stable
   - rust: stable
     env: TARGET=armv7-unknown-linux-gnueabihf
+  allow_failures:
+  - rust: nightly
 
 install:
 - if [ "$TRAVIS_RUST_VERSION" == "nightly" ]; then rustup target add wasm32-unknown-unknown; fi
@@ -26,6 +28,8 @@ script:
 - if [ "$TRAVIS_RUST_VERSION" == "nightly" ]; then cargo check --benches --manifest-path=benches/Cargo.toml; fi
 # Make sure `no_std` version checks.
 - if [ "$TRAVIS_RUST_VERSION" == "nightly" ]; then cargo +nightly check --no-default-features --features core; fi
+# Check that `vec_memory` feature works.
+- cargo check --features vec_memory
 - travis_wait 60 ./test.sh
 - ./doc.sh
 

Cargo.toml

@@ -17,6 +17,7 @@ memory_units = "0.3.0"
 libm = { version = "0.1.2", optional = true }
 num-rational = "0.2.2"
 num-traits = "0.2.8"
+libc = "0.2.58"
 
 [dev-dependencies]
 assert_matches = "1.1"
@@ -37,6 +38,12 @@ core = [
     "wasmi-validation/core",
     "libm"
 ]
+# Enforce using the linear memory implementation based on `Vec` instead of
+# mmap on unix systems.
+#
+# Useful for tests and if you need to minimize unsafe usage at the cost of performance on some
+# workloads.
+vec_memory = []
 
 [workspace]
 members = ["validation"]

src/memory/mmap_bytebuf.rs

@@ -0,0 +1,189 @@
+//! An implementation of a `ByteBuf` based on virtual memory.
+//!
+//! This implementation uses `mmap` on POSIX systems (and should use `VirtualAlloc` on windows).
+//! There are possibilities to improve the performance for the reallocating case by reserving
+//! memory up to maximum. This might be a problem for systems that don't have a lot of virtual
+//! memory (i.e. 32-bit platforms).
+
+use std::ptr::{self, NonNull};
+use std::slice;
+
+struct Mmap {
+    /// The pointer that points to the start of the mapping.
+    ///
+    /// This value doesn't change after creation.
+    ptr: NonNull<u8>,
+    /// The length of this mapping.
+    ///
+    /// Cannot be more than `isize::max_value()`. This value doesn't change after creation.
+    len: usize,
+}
+
+impl Mmap {
+    /// Create a new mmap mapping
+    ///
+    /// Returns `Err` if:
+    /// - `len` should not exceed `isize::max_value()`
+    /// - `len` should be greater than 0.
+    /// - `mmap` returns an error (almost certainly means out of memory).
+    fn new(len: usize) -> Result<Self, &'static str> {
+        if len > isize::max_value() as usize {
+            return Err("`len` should not exceed `isize::max_value()`");
+        }
+        if len == 0 {
+            return Err("`len` should be greater than 0");
+        }
+
+        let ptr_or_err = unsafe {
+            // Safety Proof:
+            // There are not specific safety proofs are required for this call, since the call
+            // by itself can't invoke any safety problems (however, misusing its result can).
+            libc::mmap(
+                // `addr` - let the system to choose the address at which to create the mapping.
+                ptr::null_mut(),
+                // the length of the mapping in bytes.
+                len,
+                // `prot` - protection flags: READ WRITE !EXECUTE
+                libc::PROT_READ | libc::PROT_WRITE,
+                // `flags`
+                // `MAP_ANON` - mapping is not backed by any file and initial contents are
+                // initialized to zero.
+                // `MAP_PRIVATE` - the mapping is private to this process.
+                libc::MAP_ANON | libc::MAP_PRIVATE,
+                // `fildes` - a file descriptor. Pass -1 as this is required for some platforms
+                // when the `MAP_ANON` is passed.
+                -1,
+                // `offset` - offset from the file.
+                0,
+            )
+        };
+
+        match ptr_or_err {
+            // With the current parameters, the error can only be returned in case of insufficient
+            // memory.
+            libc::MAP_FAILED => Err("mmap returned an error"),
+            _ => {
+                let ptr = NonNull::new(ptr_or_err as *mut u8).ok_or("mmap returned 0")?;
+                Ok(Self { ptr, len })
+            }
+        }
+    }
+
+    fn as_slice(&self) -> &[u8] {
+        unsafe {
+            // Safety Proof:
+            // - Aliasing guarantees of `self.ptr` are not violated since `self` is the only owner.
+            // - This pointer was allocated for `self.len` bytes and thus is a valid slice.
+            // - `self.len` doesn't change throughout the lifetime of `self`.
+            // - The value is returned valid for the duration of lifetime of `self`.
+            //   `self` cannot be destroyed while the returned slice is alive.
+            // - `self.ptr` is of `NonNull` type and thus `.as_ptr()` can never return NULL.
+            // - `self.len` cannot be larger than `isize::max_value()`.
+            slice::from_raw_parts(self.ptr.as_ptr(), self.len)
+        }
+    }
+
+    fn as_slice_mut(&mut self) -> &mut [u8] {
+        unsafe {
+            // Safety Proof:
+            // - See the proof for `Self::as_slice`
+            // - Additionally, it is not possible to obtain two mutable references for `self.ptr`
+            slice::from_raw_parts_mut(self.ptr.as_ptr(), self.len)
+        }
+    }
+}
+
+impl Drop for Mmap {
+    fn drop(&mut self) {
+        let ret_val = unsafe {
+            // Safety proof:
+            // - `self.ptr` was allocated by a call to `mmap`.
+            // - `self.len` was saved at the same time and it doesn't change throughout the lifetime
+            //   of `self`.
+            libc::munmap(self.ptr.as_ptr() as *mut libc::c_void, self.len)
+        };
+
+        // There is no reason for `munmap` to fail to deallocate a private annonymous mapping
+        // allocated by `mmap`.
+        // However, for the cases when it actually fails prefer to fail, in order to not leak
+        // and exhaust the virtual memory.
+        assert_eq!(ret_val, 0, "munmap failed");
+    }
+}
+
+pub struct ByteBuf {
+    mmap: Option<Mmap>,
+}
+
+impl ByteBuf {
+    pub fn new(len: usize) -> Result<Self, &'static str> {
+        let mmap = if len == 0 {
+            None
+        } else {
+            Some(Mmap::new(len)?)
+        };
+        Ok(Self { mmap })
+    }
+
+    pub fn realloc(&mut self, new_len: usize) -> Result<(), &'static str> {
+        let new_mmap = if new_len == 0 {
+            None
+        } else {
+            let mut new_mmap = Mmap::new(new_len)?;
+            if let Some(cur_mmap) = self.mmap.take() {
+                let src = cur_mmap.as_slice();
+                let dst = new_mmap.as_slice_mut();
+                let amount = src.len().min(dst.len());
+                dst[..amount].copy_from_slice(&src[..amount]);
+            }
+            Some(new_mmap)
+        };
+
+        self.mmap = new_mmap;
+        Ok(())
+    }
+
+    pub fn len(&self) -> usize {
+        self.mmap.as_ref().map(|m| m.len).unwrap_or(0)
+    }
+
+    pub fn as_slice(&self) -> &[u8] {
+        self.mmap.as_ref().map(|m| m.as_slice()).unwrap_or(&[])
+    }
+
+    pub fn as_slice_mut(&mut self) -> &mut [u8] {
+        self.mmap
+            .as_mut()
+            .map(|m| m.as_slice_mut())
+            .unwrap_or(&mut [])
+    }
+
+    pub fn erase(&mut self) -> Result<(), &'static str> {
+        let len = self.len();
+        if len > 0 {
+            // The order is important.
+            //
+            // 1. First we clear, and thus drop, the current mmap if any.
+            // 2. And then we create a new one.
+            //
+            // Otherwise we double the peak memory consumption.
+            self.mmap = None;
+            self.mmap = Some(Mmap::new(len)?);
+        }
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::ByteBuf;
+
+    const PAGE_SIZE: usize = 4096;
+
+    // This is not required since wasm memories can only grow but nice to have.
+    #[test]
+    fn byte_buf_shrink() {
+        let mut byte_buf = ByteBuf::new(PAGE_SIZE * 3).unwrap();
+        byte_buf.realloc(PAGE_SIZE * 2).unwrap();
+    }
+}