read_addrs() can return fewer bytes than requested (#115)

The `read_addrs()` method for targets now returns a number of bytes that
were read. This allows a target to gracefully signal the client that
memory at an address range is not accessible.

Signed-off-by: Thomas Scholtes <geigerzaehler@axiom.fm>

example_no_std/src/gdb.rs

@@ -77,10 +77,10 @@ impl MultiThreadBase for DummyTarget {
         _start_addr: u32,
         data: &mut [u8],
         _tid: Tid, // same address space for each core
-    ) -> TargetResult<(), Self> {
+    ) -> TargetResult<usize, Self> {
         print_str("> read_addrs");
         data.iter_mut().for_each(|b| *b = 0x55);
-        Ok(())
+        Ok(data.len())
     }
 
     #[inline(never)]

examples/armv4t/gdb/mod.rs

@@ -196,11 +196,22 @@ impl SingleThreadBase for Emu {
         Some(self)
     }
 
-    fn read_addrs(&mut self, start_addr: u32, data: &mut [u8]) -> TargetResult<(), Self> {
-        for (addr, val) in (start_addr..).zip(data.iter_mut()) {
+    fn read_addrs(&mut self, start_addr: u32, data: &mut [u8]) -> TargetResult<usize, Self> {
+        // These values are taken from the link script.
+        const MEMORY_ORIGIN: u32 = 0x5555_0000;
+        const MEMORY_LENGTH: u32 = 0x1000_0000;
+        const MEMORY_END: u32 = MEMORY_ORIGIN + MEMORY_LENGTH;
+
+        if !(MEMORY_ORIGIN..MEMORY_END).contains(&start_addr) {
+            return Err(TargetError::NonFatal);
+        }
+
+        let end_addr = std::cmp::min(start_addr + data.len() as u32, MEMORY_END);
+
+        for (addr, val) in (start_addr..end_addr).zip(data.iter_mut()) {
             *val = self.mem.r8(addr)
         }
-        Ok(())
+        Ok((end_addr - start_addr) as usize)
     }
 
     fn write_addrs(&mut self, start_addr: u32, data: &[u8]) -> TargetResult<(), Self> {

examples/armv4t_multicore/gdb.rs

@@ -92,11 +92,11 @@ impl MultiThreadBase for Emu {
         start_addr: u32,
         data: &mut [u8],
         _tid: Tid, // same address space for each core
-    ) -> TargetResult<(), Self> {
+    ) -> TargetResult<usize, Self> {
         for (addr, val) in (start_addr..).zip(data.iter_mut()) {
             *val = self.mem.r8(addr)
         }
-        Ok(())
+        Ok(data.len())
     }
 
     fn write_addrs(

src/stub/core_impl/base.rs

@@ -266,7 +266,7 @@ impl<T: Target, C: Connection> GdbStubImpl<T, C> {
 
                     let addr = addr + NumCast::from(i).ok_or(Error::TargetMismatch)?;
                     let data = &mut buf[..chunk_size];
-                    match target.base_ops() {
+                    let data_len = match target.base_ops() {
                         BaseOps::SingleThread(ops) => ops.read_addrs(addr, data),
                         BaseOps::MultiThread(ops) => {
                             ops.read_addrs(addr, data, self.current_mem_tid)
@@ -277,6 +277,8 @@ impl<T: Target, C: Connection> GdbStubImpl<T, C> {
                     n -= chunk_size;
                     i += chunk_size;
 
+                    // TODO: add more specific error variant?
+                    let data = data.get(..data_len).ok_or(Error::PacketBufferOverflow)?;
                     res.write_hex_buf(data)?;
                 }
                 HandlerStatus::Handled

src/target/ext/base/multithread.rs

@@ -43,17 +43,25 @@ pub trait MultiThreadBase: Target {
         None
     }
 
-    /// Read bytes from the specified address range.
+    /// Read bytes from the specified address range and return the number of
+    /// bytes that were read.
     ///
-    /// If the requested address range could not be accessed (e.g: due to
-    /// MMU protection, unhanded page fault, etc...), an appropriate non-fatal
-    /// error should be returned.
+    /// Implementations may return a number `n` that is less than `data.len()`
+    /// to indicate that memory starting at `start_addr + n` cannot be
+    /// accessed.
+    ///
+    /// Implemenations may also return an appropriate non-fatal error if the
+    /// requested address range could not be accessed (e.g: due to MMU
+    /// protection, unhanded page fault, etc...).
+    ///
+    /// Implementations must guarantee that the returned number is less than or
+    /// equal `data.len()`.
     fn read_addrs(
         &mut self,
         start_addr: <Self::Arch as Arch>::Usize,
         data: &mut [u8],
         tid: Tid,
-    ) -> TargetResult<(), Self>;
+    ) -> TargetResult<usize, Self>;
 
     /// Write bytes to the specified address range.
     ///

src/target/ext/base/singlethread.rs

@@ -32,16 +32,24 @@ pub trait SingleThreadBase: Target {
         None
     }
 
-    /// Read bytes from the specified address range.
+    /// Read bytes from the specified address range and return the number of
+    /// bytes that were read.
     ///
-    /// If the requested address range could not be accessed (e.g: due to
-    /// MMU protection, unhanded page fault, etc...), an appropriate
-    /// non-fatal error should be returned.
+    /// Implementations may return a number `n` that is less than `data.len()`
+    /// to indicate that memory starting at `start_addr + n` cannot be
+    /// accessed.
+    ///
+    /// Implemenations may also return an appropriate non-fatal error if the
+    /// requested address range could not be accessed (e.g: due to MMU
+    /// protection, unhanded page fault, etc...).
+    ///
+    /// Implementations must guarantee that the returned number is less than or
+    /// equal `data.len()`.
     fn read_addrs(
         &mut self,
         start_addr: <Self::Arch as Arch>::Usize,
         data: &mut [u8],
-    ) -> TargetResult<(), Self>;
+    ) -> TargetResult<usize, Self>;
 
     /// Write bytes to the specified address range.
     ///

src/target/mod.rs

@@ -151,7 +151,7 @@
 //!         &mut self,
 //!         start_addr: u32,
 //!         data: &mut [u8],
-//!     ) -> TargetResult<(), Self> { todo!() }
+//!     ) -> TargetResult<usize, Self> { todo!() }
 //!
 //!     fn write_addrs(
 //!         &mut self,
@@ -409,7 +409,7 @@ pub trait Target {
     /// #       &mut self,
     /// #       start_addr: u32,
     /// #       data: &mut [u8],
-    /// #   ) -> TargetResult<(), Self> { todo!() }
+    /// #   ) -> TargetResult<usize, Self> { todo!() }
     /// #
     /// #   fn write_addrs(
     /// #       &mut self,