tpm: save almost 8KB stack space

The largest possible PTM response is only 16 bytes. Size the output
buffer correctly.

In the socket read function, rely on the caller to provide a
sufficiently large buffer. That eliminates another large stack variable.

In total this saves almost 8KB stack space.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

tpm/src/emulator.rs

@@ -216,7 +216,16 @@ impl Emulator {
             ))
         })?;
 
-        let mut output = [0_u8; TPM_CRB_BUFFER_MAX];
+        // The largest response is 16 bytes so far.
+        if msg_len_out > 16 {
+            return Err(Error::RunControlCmd(anyhow!(
+                "Response size is too large for Cmd {:02X?}, max 16 wanted {}",
+                cmd,
+                msg_len_out
+            )));
+        }
+
+        let mut output = [0u8; 16];
 
         // Every Control Cmd gets atleast a result code in response. Read it
         let read_size = self.control_socket.read(&mut output).map_err(|e| {

tpm/src/socket.rs

@@ -3,7 +3,6 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 
-use crate::TPM_CRB_BUFFER_MAX;
 use anyhow::anyhow;
 use std::io::Read;
 use std::os::unix::io::{AsRawFd, RawFd};
@@ -122,23 +121,15 @@ impl SocketDev {
     }
 
     pub fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
-        let newbuf: &mut [u8] = &mut [0; TPM_CRB_BUFFER_MAX];
-
         if self.stream.is_none() {
             return Err(Error::ReadFromSocket(anyhow!(
                 "Stream for tpm socket was not initialized"
             )));
         }
         let mut socket = self.stream.as_ref().unwrap();
-        let size: usize = socket.read(newbuf).map_err(|e| {
+        let size: usize = socket.read(buf).map_err(|e| {
             Error::ReadFromSocket(anyhow!("Failed to read from socket. Error Code {:?}", e))
         })?;
-        if buf.len() < size {
-            return Err(Error::ReadFromSocket(anyhow!(
-                "Input buffer is of insufficient size"
-            )));
-        }
-        buf[0..size].clone_from_slice(&newbuf[0..size]);
         Ok(size)
     }
 }