Serial: add input path overriding stdin for --serial

Allowing the input to be specified for file-based serial ports allows
the user of pipes as input/output. That enables kgdb over serial.

TEST=
Build a kernel with support for gdb

```
make x86_64_defconfig
make kvmconfig
./scripts/config --enable GDB_SCRIPTS
./scripts/config --enable KGDB
./scripts/config --enable KGDB_SERIAL_CONSOLE
./scripts/config --enable KGDB_LOW_LEVEL_TRAP
./scripts/config --enable KGDB_KDB
./scripts/config --enable KDB_KEYBOARD
./scripts/config --enable GDB_SCRIPTS
./scripts/config --set-val KDB_CONTINUE_CATASTROPHIC 0
make -j33
```

Setup devices for PTYs

To make sure crosvm doesn't create an ordinary file if socat is started
after it, create these named pipes first:

```
mkfifo ~/console_{in,out} ~/kgdb_{in,out}
```

Set up two PTYs: ~/kgdb for the debugger, and ~/serial for the console.
PTY ~/kgdb connects to ~/kgdb{in,out}, and ~/serial connects to
~/console{in,out}

```
socat -d -d -d \
    'PIPE:$HOME/console_out,rdonly=1,nonblock=1,ignoreeof=1!!PIPE:$HOME/console_in,wronly=1' \
    PTY,link=$HOME/serial,ctty,raw,echo=0

socat -d -d -d \
    'PIPE:$HOME/kgdb_out,rdonly=1,nonblock=1,ignoreeof=1!!PIPE:$HOME/kgdb_in,wronly=1' \
    PTY,link=$HOME/kgdb,ctty,raw,echo=0
```

Start crosvm with serial ports pointed at ~/console{in,out} and ~/kgdb{in,out}.

```
cargo run run -p 'init=/bin/sh panic=0 kgdboc=ttyS1,115200 kgdbwait kgdbcon' \
    --serial type=file,path=$HOME/console_out,num=1,console=true,stdin=false,input=$HOME/console_in \
    --serial type=file,path=$HOME/kgdb_out,input=$HOME/kgdb_in,num=2,console=false,stdin=false \
    -r ~/rootfs.img \
    ~/src/linux/arch/x86/boot/bzImage
```

Start GDB

```
gdb vmlinux -ex "target remote /home/dgreid/kgdb"
```

To break into gdb, open up the serial console, mount /proc and send a SysRq

```
minicom -D ~/serial
mount -t proc none /proc
echo g > /proc/sysrq-trigger
```

Change-Id: I18a9c1087d38301df49de08eeae2f8559b03463a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2151856
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Tested-by: Dylan Reid <dgreid@chromium.org>
Commit-Queue: Dylan Reid <dgreid@chromium.org>

1 files changed, 11 insertions, 2 deletions

diff --git a/devices/src/serial.rs b/devices/src/serial.rs
index 6629d7e..caf1ee0 100644
--- a/devices/src/serial.rs
+++ b/devices/src/serial.rs
@@ -130,6 +130,7 @@ impl FromStr for SerialType {
 pub struct SerialParameters {
     pub type_: SerialType,
     pub path: Option<PathBuf>,
+    pub input: Option<PathBuf>,
     pub num: u8,
     pub console: bool,
     pub stdin: bool,
@@ -149,7 +150,11 @@ impl SerialParameters {
     ) -> std::result::Result<Serial, Error> {
         let evt_fd = evt_fd.try_clone().map_err(Error::CloneEventFd)?;
         keep_fds.push(evt_fd.as_raw_fd());
-        let input: Option<Box<dyn io::Read + Send>> = if self.stdin {
+        let input: Option<Box<dyn io::Read + Send>> = if let Some(input_path) = &self.input {
+            let input_file = File::open(input_path.as_path()).map_err(Error::FileError)?;
+            keep_fds.push(input_file.as_raw_fd());
+            Some(Box::new(input_file))
+        } else if self.stdin {
             keep_fds.push(stdin().as_raw_fd());
             // This wrapper is used in place of the libstd native version because we don't want
             // buffering for stdin.
@@ -181,12 +186,12 @@ impl SerialParameters {
                 ))
             }
             SerialType::File => match &self.path {
-                None => Err(Error::PathRequired),
                 Some(path) => {
                     let file = File::create(path.as_path()).map_err(Error::FileError)?;
                     keep_fds.push(file.as_raw_fd());
                     Ok(Serial::new(evt_fd, input, Some(Box::new(file))))
                 }
+                _ => Err(Error::PathRequired),
             },
             SerialType::UnixSocket => Err(Error::Unimplemented(SerialType::UnixSocket)),
         }
@@ -198,6 +203,7 @@ pub const DEFAULT_SERIAL_PARAMS: [SerialParameters; 4] = [
     SerialParameters {
         type_: SerialType::Stdout,
         path: None,
+        input: None,
         num: 1,
         console: true,
         stdin: true,
@@ -205,6 +211,7 @@ pub const DEFAULT_SERIAL_PARAMS: [SerialParameters; 4] = [
     SerialParameters {
         type_: SerialType::Sink,
         path: None,
+        input: None,
         num: 2,
         console: false,
         stdin: false,
@@ -212,6 +219,7 @@ pub const DEFAULT_SERIAL_PARAMS: [SerialParameters; 4] = [
     SerialParameters {
         type_: SerialType::Sink,
         path: None,
+        input: None,
         num: 3,
         console: false,
         stdin: false,
@@ -219,6 +227,7 @@ pub const DEFAULT_SERIAL_PARAMS: [SerialParameters; 4] = [
     SerialParameters {
         type_: SerialType::Sink,
         path: None,
+        input: None,
         num: 4,
         console: false,
         stdin: false,