bytes: avoid torn writes with memcpy

Some memcpy implementations are copying bytes one at a time, which
is slow and also breaks the virtio specification by splitting writes
to the fields of the virtio descriptor. So, reimplement memcpy in Rust
and copy in larger pieces, according to the largest possible alignment
allowed by the pointer values.

Signed-off-by: Serban Iorga <[email protected]>
Signed-off-by: Andreea Florescu <[email protected]>
Signed-off-by: Alexandru Agache <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: bonzini

src/guest_memory.rs

@@ -847,9 +847,14 @@ impl<T: GuestMemory> Bytes<GuestAddress> for T {
 mod tests {
     use super::*;
     #[cfg(feature = "backend-mmap")]
+    use crate::bytes::ByteValued;
+    #[cfg(feature = "backend-mmap")]
     use crate::{GuestAddress, GuestMemoryMmap};
     #[cfg(feature = "backend-mmap")]
     use std::io::Cursor;
+    #[cfg(feature = "backend-mmap")]
+    use std::time::{Duration, Instant};
+
     use vmm_sys_util::tempfile::TempFile;
 
     #[cfg(feature = "backend-mmap")]
@@ -888,4 +893,124 @@ mod tests {
                 .unwrap()
         );
     }
+
+    // Runs the provided closure in a loop, until at least `duration` time units have elapsed.
+    #[cfg(feature = "backend-mmap")]
+    fn loop_timed<F>(duration: Duration, mut f: F)
+    where
+        F: FnMut() -> (),
+    {
+        // We check the time every `CHECK_PERIOD` iterations.
+        const CHECK_PERIOD: u64 = 1_000_000;
+        let start_time = Instant::now();
+
+        loop {
+            for _ in 0..CHECK_PERIOD {
+                f();
+            }
+            if start_time.elapsed() >= duration {
+                break;
+            }
+        }
+    }
+
+    // Helper method for the following test. It spawns a writer and a reader thread, which
+    // simultaneously try to access an object that is placed at the junction of two memory regions.
+    // The part of the object that's continuously accessed is a member of type T. The writer
+    // flips all the bits of the member with every write, while the reader checks that every byte
+    // has the same value (and thus it did not do a non-atomic access). The test succeeds if
+    // no mismatch is detected after performing accesses for a pre-determined amount of time.
+    #[cfg(feature = "backend-mmap")]
+    fn non_atomic_access_helper<T>()
+    where
+        T: ByteValued
+            + std::fmt::Debug
+            + From<u8>
+            + Into<u128>
+            + std::ops::Not<Output = T>
+            + PartialEq,
+    {
+        use std::mem;
+        use std::thread;
+
+        // A dummy type that's always going to have the same alignment as the first member,
+        // and then adds some bytes at the end.
+        #[derive(Clone, Copy, Debug, Default, PartialEq)]
+        struct Data<T> {
+            val: T,
+            some_bytes: [u8; 7],
+        }
+
+        // Some sanity checks.
+        assert_eq!(mem::align_of::<T>(), mem::align_of::<Data<T>>());
+        assert_eq!(mem::size_of::<T>(), mem::align_of::<T>());
+
+        unsafe impl<T: ByteValued> ByteValued for Data<T> {}
+
+        // Start of first guest memory region.
+        let start = GuestAddress(0);
+        let region_len = 1 << 12;
+
+        // The address where we start writing/reading a Data<T> value.
+        let data_start = GuestAddress((region_len - mem::size_of::<T>()) as u64);
+
+        let mem = GuestMemoryMmap::from_ranges(&[
+            (start, region_len),
+            (start.unchecked_add(region_len as u64), region_len),
+        ])
+        .unwrap();
+
+        // Need to clone this and move it into the new thread we create.
+        let mem2 = mem.clone();
+        // Just some bytes.
+        let some_bytes = [1u8, 2, 4, 16, 32, 64, 128];
+
+        let mut data = Data {
+            val: T::from(0u8),
+            some_bytes,
+        };
+
+        // Simple check that cross-region write/read is ok.
+        mem.write_obj(data, data_start).unwrap();
+        let read_data = mem.read_obj::<Data<T>>(data_start).unwrap();
+        assert_eq!(read_data, data);
+
+        let t = thread::spawn(move || {
+            let mut count: u64 = 0;
+
+            loop_timed(Duration::from_secs(3), || {
+                let data = mem2.read_obj::<Data<T>>(data_start).unwrap();
+
+                // Every time data is written to memory by the other thread, the value of
+                // data.val alternates between 0 and T::MAX, so the inner bytes should always
+                // have the same value. If they don't match, it means we read a partial value,
+                // so the access was not atomic.
+                let bytes = data.val.into().to_le_bytes();
+                for i in 1..mem::size_of::<T>() {
+                    if bytes[0] != bytes[i] {
+                        panic!(
+                            "val bytes don't match {:?} after {} iterations",
+                            &bytes[..mem::size_of::<T>()],
+                            count
+                        );
+                    }
+                }
+                count += 1;
+            });
+        });
+
+        // Write the object while flipping the bits of data.val over and over again.
+        loop_timed(Duration::from_secs(3), || {
+            mem.write_obj(data, data_start).unwrap();
+            data.val = !data.val;
+        });
+
+        t.join().unwrap()
+    }
+
+    #[cfg(feature = "backend-mmap")]
+    #[test]
+    fn test_non_atomic_access() {
+        non_atomic_access_helper::<u16>()
+    }
 }

src/volatile_memory.rs

@@ -463,6 +463,57 @@ impl<'a> VolatileSlice<'a> {
     }
 }
 
+// Return the largest value that `addr` is aligned to. Forcing this function to return 1 will
+// cause test_non_atomic_access to fail.
+fn alignment(addr: usize) -> usize {
+    // Rust is silly and does not let me write addr & -addr.
+    addr & (!addr + 1)
+}
+
+// Has the same safety requirements as `read_volatile` + `write_volatile`, namely:
+// - `src_addr` and `dst_addr` must be valid for reads/writes.
+// - `src_addr` and `dst_addr` must be properly aligned with respect to `align`.
+// - `src_addr` must point to a properly initialized value, which is true here because
+//   we're only using integer primitives.
+unsafe fn copy_single(align: usize, src_addr: usize, dst_addr: usize) {
+    match align {
+        8 => write_volatile(dst_addr as *mut u64, read_volatile(src_addr as *const u64)),
+        4 => write_volatile(dst_addr as *mut u32, read_volatile(src_addr as *const u32)),
+        2 => write_volatile(dst_addr as *mut u16, read_volatile(src_addr as *const u16)),
+        1 => write_volatile(dst_addr as *mut u8, read_volatile(src_addr as *const u8)),
+        _ => unreachable!(),
+    }
+}
+
+fn copy_slice(dst: &mut [u8], src: &[u8]) -> usize {
+    let total = min(src.len(), dst.len());
+    let mut left = total;
+
+    let mut src_addr = src.as_ptr() as usize;
+    let mut dst_addr = dst.as_ptr() as usize;
+    let align = min(alignment(src_addr), alignment(dst_addr));
+
+    let mut copy_aligned_slice = |min_align| {
+        while align >= min_align && left >= min_align {
+            // Safe because we check alignment beforehand, the memory areas are valid for
+            // reads/writes, and the source always contains a valid value.
+            unsafe { copy_single(min_align, src_addr, dst_addr) };
+            src_addr += min_align;
+            dst_addr += min_align;
+            left -= min_align;
+        }
+    };
+
+    if size_of::<usize>() > 4 {
+        copy_aligned_slice(8);
+    }
+    copy_aligned_slice(4);
+    copy_aligned_slice(2);
+    copy_aligned_slice(1);
+
+    total
+}
+
 impl Bytes<usize> for VolatileSlice<'_> {
     type E = Error;
 
@@ -482,13 +533,12 @@ impl Bytes<usize> for VolatileSlice<'_> {
         if addr >= self.size {
             return Err(Error::OutOfBounds { addr });
         }
-        unsafe {
-            // Guest memory can't strictly be modeled as a slice because it is
-            // volatile.  Writing to it with what compiles down to a memcpy
-            // won't hurt anything as long as we get the bounds checks right.
-            let mut slice: &mut [u8] = &mut self.as_mut_slice()[addr..];
-            slice.write(buf).map_err(Error::IOError)
-        }
+
+        // Guest memory can't strictly be modeled as a slice because it is
+        // volatile.  Writing to it with what is essentially a fancy memcpy
+        // won't hurt anything as long as we get the bounds checks right.
+        let slice = unsafe { self.as_mut_slice() }.split_at_mut(addr).1;
+        Ok(copy_slice(slice, buf))
     }
 
     /// # Examples
@@ -504,17 +554,16 @@ impl Bytes<usize> for VolatileSlice<'_> {
     ///     assert!(res.is_ok());
     ///     assert_eq!(res.unwrap(), 14);
     /// ```
-    fn read(&self, mut buf: &mut [u8], addr: usize) -> Result<usize> {
+    fn read(&self, buf: &mut [u8], addr: usize) -> Result<usize> {
         if addr >= self.size {
             return Err(Error::OutOfBounds { addr });
         }
-        unsafe {
-            // Guest memory can't strictly be modeled as a slice because it is
-            // volatile.  Writing to it with what compiles down to a memcpy
-            // won't hurt anything as long as we get the bounds checks right.
-            let slice: &[u8] = &self.as_slice()[addr..];
-            buf.write(slice).map_err(Error::IOError)
-        }
+
+        // Guest memory can't strictly be modeled as a slice because it is
+        // volatile.  Writing to it with what is essentially a fancy memcpy
+        // won't hurt anything as long as we get the bounds checks right.
+        let slice = unsafe { self.as_slice() }.split_at(addr).1;
+        Ok(copy_slice(buf, slice))
     }
 
     /// # Examples
@@ -1548,4 +1597,15 @@ mod tests {
             }
         );
     }
+
+    #[test]
+    fn alignment() {
+        let a = [0u8; 64];
+        let a = &a[a.as_ptr().align_offset(32)] as *const u8 as usize;
+        assert!(super::alignment(a) >= 32);
+        assert_eq!(super::alignment(a + 9), 1);
+        assert_eq!(super::alignment(a + 30), 2);
+        assert_eq!(super::alignment(a + 12), 4);
+        assert_eq!(super::alignment(a + 8), 8);
+    }
 }