Drop B: Bitmap type parameters from DescriptorChain fns

The `DescriptorChain::read` and `DescriptorChain::write` define a `B:
BitmapSlice` type parameter. However, the type of the bitmap that will
be associated with the `VolatileSlices` is already transitively
prescribed by the `M` type parameter on the `DescriptorChain` object
itself. To reconcile this, these functions need lengthy
`WithBitmapSlice` bounds (which tells the compiler "these two different
BitMap type variables are actually the same").

Drop the `B: Bitmap` slice parameter from these functions, and instead
simply return `Reader<'a, MS<M::Target>>`, where `MS<M::Target>`
resolves to the bitmap type induced by the guest memory implementation.
This avoids the `WithBitmapSlice` bounds completely.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

fuzz/common/src/vsock.rs

@@ -337,7 +337,7 @@ mod tests {
         let mut chain = vq.build_multiple_desc_chains(&q_descriptors).unwrap();
 
         let mut packet =
-            VsockPacket::from_tx_virtq_chain(&mem, &mut chain, MAX_PKT_BUF_SIZE).unwrap();
+            VsockPacket::<()>::from_tx_virtq_chain(&mem, &mut chain, MAX_PKT_BUF_SIZE).unwrap();
         create_basic_vsock_packet_ops(&mut packet, &mem, &mut functions);
         let vsock_fuzz_input = VsockInput {
             functions,
@@ -387,7 +387,7 @@ mod tests {
         let mut chain = vq.build_multiple_desc_chains(&q_descriptors).unwrap();
 
         let mut packet =
-            VsockPacket::from_rx_virtq_chain(&mem, &mut chain, MAX_PKT_BUF_SIZE).unwrap();
+            VsockPacket::<()>::from_rx_virtq_chain(&mem, &mut chain, MAX_PKT_BUF_SIZE).unwrap();
         let mut functions = Vec::new();
         create_basic_vsock_packet_ops(&mut packet, &mem, &mut functions);
         let vsock_fuzz_input = VsockInput {

fuzz/fuzz_targets/vsock.rs

@@ -28,10 +28,10 @@ fuzz_target!(|data: &[u8]| {
     if let Ok(mut chain) = vq.build_desc_chain(&descriptors) {
         let packet = match fuzz_input.init_function {
             InitFunction::FromRX => {
-                VsockPacket::from_rx_virtq_chain(&m, &mut chain, fuzz_input.pkt_max_data)
+                VsockPacket::<()>::from_rx_virtq_chain(&m, &mut chain, fuzz_input.pkt_max_data)
             }
             InitFunction::FromTX => {
-                VsockPacket::from_tx_virtq_chain(&m, &mut chain, fuzz_input.pkt_max_data)
+                VsockPacket::<()>::from_tx_virtq_chain(&m, &mut chain, fuzz_input.pkt_max_data)
             }
         };
         if let Ok(mut p) = packet {

virtio-queue/src/chain.rs

@@ -14,8 +14,8 @@ use std::fmt::{self, Debug};
 use std::mem::size_of;
 use std::ops::Deref;
 
-use vm_memory::bitmap::{BitmapSlice, WithBitmapSlice};
-use vm_memory::{Address, Bytes, GuestAddress, GuestMemory, GuestMemoryRegion};
+use vm_memory::bitmap::MS;
+use vm_memory::{Address, Bytes, GuestAddress, GuestMemory};
 
 use crate::{Descriptor, Error, Reader, Writer};
 use virtio_bindings::bindings::virtio_ring::VRING_DESC_ALIGN_SIZE;
@@ -90,21 +90,19 @@ where
     }
 
     /// Return a new instance of Writer
-    pub fn writer<'a, B: BitmapSlice>(self, mem: &'a M::Target) -> Result<Writer<'a, B>, Error>
+    pub fn writer(self, mem: &M::Target) -> Result<Writer<MS<M::Target>>, Error>
     where
         M::Target: Sized,
-        <<M::Target as GuestMemory>::R as GuestMemoryRegion>::B: WithBitmapSlice<'a, S = B>,
     {
-        Writer::new(mem, self).map_err(|_| Error::InvalidChain)
+        Writer::<MS<M::Target>>::new(mem, self).map_err(|_| Error::InvalidChain)
     }
 
     /// Return a new instance of Reader
-    pub fn reader<'a, B: BitmapSlice>(self, mem: &'a M::Target) -> Result<Reader<'a, B>, Error>
+    pub fn reader(self, mem: &M::Target) -> Result<Reader<MS<M::Target>>, Error>
     where
         M::Target: Sized,
-        <<M::Target as GuestMemory>::R as GuestMemoryRegion>::B: WithBitmapSlice<'a, S = B>,
     {
-        Reader::new(mem, self).map_err(|_| Error::InvalidChain)
+        Reader::<MS<M::Target>>::new(mem, self).map_err(|_| Error::InvalidChain)
     }
 
     /// Return an iterator that only yields the writable descriptors in the chain.

virtio-queue/src/descriptor_utils.rs

@@ -14,7 +14,7 @@ use std::ptr::copy_nonoverlapping;
 use std::{cmp, result};
 
 use crate::{DescriptorChain, Error};
-use vm_memory::bitmap::{BitmapSlice, WithBitmapSlice};
+use vm_memory::bitmap::{BitmapSlice, MS};
 use vm_memory::{
     Address, ByteValued, GuestMemory, GuestMemoryRegion, MemoryRegionAddress, VolatileSlice,
 };
@@ -157,10 +157,9 @@ pub struct Reader<'a, B = ()> {
 
 impl<'a, B: BitmapSlice> Reader<'a, B> {
     /// Construct a new Reader wrapper over `desc_chain`.
-    pub fn new<M, T>(mem: &'a M, desc_chain: DescriptorChain<T>) -> Result<Reader<'a, B>>
+    pub fn new<M, T>(mem: &'a M, desc_chain: DescriptorChain<T>) -> Result<Reader<'a, MS<'a, M>>>
     where
         M: GuestMemory,
-        <<M as GuestMemory>::R as GuestMemoryRegion>::B: WithBitmapSlice<'a, S = B>,
         T: Deref,
         T::Target: GuestMemory + Sized,
     {
@@ -186,7 +185,7 @@ impl<'a, B: BitmapSlice> Reader<'a, B> {
                     .get_slice(MemoryRegionAddress(offset.raw_value()), desc.len() as usize)
                     .map_err(Error::GuestMemoryError)
             })
-            .collect::<Result<VecDeque<VolatileSlice<'a, B>>>>()?;
+            .collect::<Result<VecDeque<_>>>()?;
         Ok(Reader {
             buffer: DescriptorChainConsumer {
                 buffers,
@@ -269,10 +268,9 @@ pub struct Writer<'a, B = ()> {
 
 impl<'a, B: BitmapSlice> Writer<'a, B> {
     /// Construct a new Writer wrapper over `desc_chain`.
-    pub fn new<M, T>(mem: &'a M, desc_chain: DescriptorChain<T>) -> Result<Writer<'a, B>>
+    pub fn new<M, T>(mem: &'a M, desc_chain: DescriptorChain<T>) -> Result<Writer<'a, MS<'a, M>>>
     where
         M: GuestMemory,
-        <<M as GuestMemory>::R as GuestMemoryRegion>::B: WithBitmapSlice<'a, S = B>,
         T: Deref,
         T::Target: GuestMemory + Sized,
     {
@@ -298,7 +296,7 @@ impl<'a, B: BitmapSlice> Writer<'a, B> {
                     .get_slice(MemoryRegionAddress(offset.raw_value()), desc.len() as usize)
                     .map_err(Error::GuestMemoryError)
             })
-            .collect::<Result<VecDeque<VolatileSlice<'a, B>>>>()?;
+            .collect::<Result<VecDeque<_>>>()?;
 
         Ok(Writer {
             buffer: DescriptorChainConsumer {
@@ -451,7 +449,7 @@ mod tests {
 
         let chain = queue.iter(&memory).unwrap().next().unwrap();
 
-        assert!(Reader::new(&memory, chain).is_err());
+        assert!(Reader::<()>::new(&memory, chain).is_err());
     }
 
     #[test]
@@ -473,7 +471,7 @@ mod tests {
             0,
         )
         .expect("create_descriptor_chain failed");
-        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");
+        let mut reader = Reader::<()>::new(&memory, chain).expect("failed to create Reader");
         assert_eq!(reader.available_bytes(), 106);
         assert_eq!(reader.bytes_read(), 0);
 
@@ -513,7 +511,7 @@ mod tests {
             0,
         )
         .expect("create_descriptor_chain failed");
-        let mut writer = Writer::new(&memory, chain).expect("failed to create Writer");
+        let mut writer = Writer::<()>::new(&memory, chain).expect("failed to create Writer");
         assert_eq!(writer.available_bytes(), 106);
         assert_eq!(writer.bytes_written(), 0);
 
@@ -543,7 +541,7 @@ mod tests {
 
         let chain = create_descriptor_chain(&memory, GuestAddress(0x0), vec![(Writable, 8)], 0)
             .expect("create_descriptor_chain failed");
-        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");
+        let mut reader = Reader::<()>::new(&memory, chain).expect("failed to create Reader");
         assert_eq!(reader.available_bytes(), 0);
         assert_eq!(reader.bytes_read(), 0);
 
@@ -562,7 +560,7 @@ mod tests {
 
         let chain = create_descriptor_chain(&memory, GuestAddress(0x0), vec![(Readable, 8)], 0)
             .expect("create_descriptor_chain failed");
-        let mut writer = Writer::new(&memory, chain).expect("failed to create Writer");
+        let mut writer = Writer::<()>::new(&memory, chain).expect("failed to create Writer");
         assert_eq!(writer.available_bytes(), 0);
         assert_eq!(writer.bytes_written(), 0);
 
@@ -593,8 +591,9 @@ mod tests {
             0,
         )
         .expect("create_descriptor_chain failed");
-        let mut reader = Reader::new(&memory, chain.clone()).expect("failed to create Reader");
-        let mut writer = Writer::new(&memory, chain).expect("failed to create Writer");
+        let mut reader =
+            Reader::<()>::new(&memory, chain.clone()).expect("failed to create Reader");
+        let mut writer = Writer::<()>::new(&memory, chain).expect("failed to create Writer");
 
         assert_eq!(reader.bytes_read(), 0);
         assert_eq!(writer.bytes_written(), 0);
@@ -636,7 +635,7 @@ mod tests {
             123,
         )
         .expect("create_descriptor_chain failed");
-        let mut writer = Writer::new(&memory, chain_writer).expect("failed to create Writer");
+        let mut writer = Writer::<()>::new(&memory, chain_writer).expect("failed to create Writer");
         if let Err(e) = writer.write_obj(secret) {
             panic!("write_obj should not fail here: {:?}", e);
         }
@@ -649,7 +648,7 @@ mod tests {
             123,
         )
         .expect("create_descriptor_chain failed");
-        let mut reader = Reader::new(&memory, chain_reader).expect("failed to create Reader");
+        let mut reader = Reader::<()>::new(&memory, chain_reader).expect("failed to create Reader");
         match reader.read_obj::<Le32>() {
             Err(e) => panic!("read_obj should not fail here: {:?}", e),
             Ok(read_secret) => assert_eq!(read_secret, secret),
@@ -671,7 +670,7 @@ mod tests {
         )
         .expect("create_descriptor_chain failed");
 
-        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");
+        let mut reader = Reader::<()>::new(&memory, chain).expect("failed to create Reader");
 
         let mut buf = vec![0; 1024];
 
@@ -705,13 +704,15 @@ mod tests {
             0,
         )
         .expect("create_descriptor_chain failed");
-        let mut reader = Reader::new(&memory, chain.clone()).expect("failed to create Reader");
+        let mut reader =
+            Reader::<()>::new(&memory, chain.clone()).expect("failed to create Reader");
 
         let other = reader.split_at(32).expect("failed to split Reader");
         assert_eq!(reader.available_bytes(), 32);
         assert_eq!(other.available_bytes(), 96);
 
-        let mut writer = Writer::new(&memory, chain.clone()).expect("failed to create Writer");
+        let mut writer =
+            Writer::<()>::new(&memory, chain.clone()).expect("failed to create Writer");
         let other = writer.split_at(64).expect("failed to split Writer");
         assert_eq!(writer.available_bytes(), 64);
         assert_eq!(other.available_bytes(), 4);
@@ -738,7 +739,7 @@ mod tests {
             0,
         )
         .expect("create_descriptor_chain failed");
-        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");
+        let mut reader = Reader::<()>::new(&memory, chain).expect("failed to create Reader");
 
         let other = reader.split_at(24).expect("failed to split Reader");
         assert_eq!(reader.available_bytes(), 24);
@@ -766,7 +767,7 @@ mod tests {
             0,
         )
         .expect("create_descriptor_chain failed");
-        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");
+        let mut reader = Reader::<()>::new(&memory, chain).expect("failed to create Reader");
 
         let other = reader.split_at(128).expect("failed to split Reader");
         assert_eq!(reader.available_bytes(), 128);
@@ -794,7 +795,7 @@ mod tests {
             0,
         )
         .expect("create_descriptor_chain failed");
-        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");
+        let mut reader = Reader::<()>::new(&memory, chain).expect("failed to create Reader");
 
         let other = reader.split_at(0).expect("failed to split Reader");
         assert_eq!(reader.available_bytes(), 0);
@@ -822,7 +823,7 @@ mod tests {
             0,
         )
         .expect("create_descriptor_chain failed");
-        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");
+        let mut reader = Reader::<()>::new(&memory, chain).expect("failed to create Reader");
 
         if reader.split_at(256).is_ok() {
             panic!("successfully split Reader with out of bounds offset");
@@ -843,7 +844,7 @@ mod tests {
             0,
         )
         .expect("create_descriptor_chain failed");
-        let mut reader = Reader::new(&memory, chain).expect("failed to create Reader");
+        let mut reader = Reader::<()>::new(&memory, chain).expect("failed to create Reader");
 
         let mut buf = [0u8; 64];
         assert_eq!(
@@ -866,7 +867,7 @@ mod tests {
             0,
         )
         .expect("create_descriptor_chain failed");
-        let mut writer = Writer::new(&memory, chain).expect("failed to create Writer");
+        let mut writer = Writer::<()>::new(&memory, chain).expect("failed to create Writer");
 
         let buf = [0xdeu8; 64];
         assert_eq!(