Pass args and envs using a config file

When using "krun_set_exec", the arguments and environment variables are
passed to the guest using the kernel command line. In addition to being
limited in length, this is also only compatible with ASCII strings.

libkrun is also capable or reading both arguments and environment
variables from a JSON config file, which doesn't suffer from any of
those limitations, so switch to using this approach instead.

Depends on containers/libkrun#220

Signed-off-by: Sergio Lopez <[email protected]>

Author: slp

crates/krun/src/bin/krun.rs

@@ -1,4 +1,5 @@
 use std::ffi::{c_char, CString};
+use std::io::Write;
 use std::os::fd::{IntoRawFd, OwnedFd};
 use std::path::Path;
 use std::{cmp, env};
@@ -11,7 +12,7 @@ use krun::launch::{launch_or_lock, LaunchResult};
 use krun::net::{connect_to_passt, start_passt};
 use krun::types::MiB;
 use krun_sys::{
-    krun_add_disk, krun_add_vsock_port, krun_create_ctx, krun_set_exec, krun_set_gpu_options,
+    krun_add_disk, krun_add_vsock_port, krun_create_ctx, krun_set_env, krun_set_gpu_options,
     krun_set_log_level, krun_set_passt_fd, krun_set_root, krun_set_vm_config, krun_set_workdir,
     krun_start_enter, VIRGLRENDERER_DRM, VIRGLRENDERER_THREAD_SYNC,
     VIRGLRENDERER_USE_ASYNC_FENCE_CB, VIRGLRENDERER_USE_EGL,
@@ -23,6 +24,21 @@ use rustix::io::Errno;
 use rustix::process::{
     geteuid, getgid, getrlimit, getuid, sched_setaffinity, setrlimit, CpuSet, Resource,
 };
+use serde::{Deserialize, Serialize};
+use tempfile::NamedTempFile;
+
+#[derive(Serialize, Deserialize)]
+pub struct KrunConfig {
+    #[serde(rename = "Cmd")]
+    args: Vec<String>,
+    #[serde(rename = "Env")]
+    envs: Vec<String>,
+}
+#[derive(Serialize, Deserialize)]
+pub struct KrunBaseConfig {
+    #[serde(rename = "Config")]
+    config: KrunConfig,
+}
 
 fn add_ro_disk(ctx_id: u32, label: &str, path: &str) -> Result<()> {
     let path_cstr = CString::new(path).unwrap();
@@ -294,90 +310,66 @@ fn main() -> Result<()> {
     let krun_guest_path = find_krun_exec("krun-guest")?;
     let krun_server_path = find_krun_exec("krun-server")?;
 
-    let mut krun_guest_args: Vec<CString> = vec![
-        CString::new(username).expect("username should not contain NUL character"),
-        CString::new(format!("{}", getuid().as_raw()))
-            .expect("uid should not contain NUL character"),
-        CString::new(format!("{}", getgid().as_raw()))
-            .expect("gid should not contain NUL character"),
+    let mut krun_guest_args: Vec<String> = vec![
+        krun_guest_path,
+        username,
+        format!("{uid}", uid = getuid().as_raw()),
+        format!("{gid}", gid = getgid().as_raw()),
+        krun_server_path,
+        command
+            .to_str()
+            .context("Failed to process command as it contains invalid UTF-8")?
+            .to_string(),
     ];
-
-    krun_guest_args.push(krun_server_path);
-    krun_guest_args.push(
-        CString::new(
-            command
-                .to_str()
-                .context("Failed to process command as it contains invalid UTF-8")?,
-        )
-        .context("Failed to process command as it contains NUL character")?,
-    );
-    let command_argc = command_args.len();
     for arg in command_args {
-        let s = CString::new(arg)
-            .context("Failed to process command arg as it contains NUL character")?;
-        krun_guest_args.push(s);
+        krun_guest_args.push(arg);
     }
 
-    let krun_guest_args: Vec<*const c_char> = {
-        const KRUN_GUEST_ARGS_FIXED: usize = 4;
-        // SAFETY: All pointers must be stored in the same allocation.
-        // See https://doc.rust-lang.org/std/slice/fn.from_raw_parts.html#safety
-        let mut vec = Vec::with_capacity(KRUN_GUEST_ARGS_FIXED + command_argc + 1);
-        for s in &krun_guest_args {
-            vec.push(s.as_ptr());
-        }
-        vec.push(std::ptr::null());
-        vec
-    };
-
     let mut env = prepare_env_vars(env).context("Failed to prepare environment variables")?;
     env.insert(
         "KRUN_SERVER_PORT".to_owned(),
         options.server_port.to_string(),
     );
-    let env: Vec<CString> = {
-        let mut vec = Vec::with_capacity(env.len());
-        for (key, value) in env {
-            let s = CString::new(format!("{key}={value}")).with_context(|| {
-                format!("Failed to process `{key}` env var as it contains NUL character")
-            })?;
-            vec.push(s);
-        }
-        vec
+
+    let mut krun_config = KrunConfig {
+        args: Vec::new(),
+        envs: Vec::new(),
     };
-    let env: Vec<*const c_char> = {
-        // SAFETY: All pointers must be stored in the same allocation.
-        // See https://doc.rust-lang.org/std/slice/fn.from_raw_parts.html#safety
-        let mut vec = Vec::with_capacity(env.len() + 1);
-        for s in &env {
-            vec.push(s.as_ptr());
-        }
-        vec.push(std::ptr::null());
-        vec
+    for arg in krun_guest_args {
+        krun_config.args.push(arg);
+    }
+    for (key, value) in env {
+        krun_config.envs.push(format!("{}={}", key, value));
+    }
+    let krun_config = KrunBaseConfig {
+        config: krun_config,
     };
 
+    // SAFETY: `config_file` lifetime needs to exceed krun_start_enter's
+    let mut config_file = NamedTempFile::new()
+        .context("Failed to create a temporary file to store the guest config")?;
+    write!(
+        config_file,
+        "{}",
+        serde_json::to_string(&krun_config)
+            .context("Failed to transform KrunConfig into a JSON string")?
+    )
+    .context("Failed to write to temporary config file")?;
+
+    let krun_config_env = CString::new(format!("KRUN_CONFIG={}", config_file.path().display()))
+        .context("Failed to process config_file var as it contains NUL character")?;
+    let env: Vec<*const c_char> = vec![krun_config_env.as_ptr(), std::ptr::null()];
+
     {
-        // Specify the path of the binary to be executed in the isolated context,
-        // relative to the root path.
+        // Sets environment variables to be configured in the context of the executable.
         //
         // SAFETY:
-        // * `krun_guest_path` is a pointer to a `CString` with long enough lifetime.
-        // * `krun_guest_args` is a pointer to a `Vec` of pointers to `CString`s all
-        //   with long enough lifetime.
         // * `env` is a pointer to a `Vec` of pointers to `CString`s all with long
         //   enough lifetime.
-        let err = unsafe {
-            krun_set_exec(
-                ctx_id,
-                krun_guest_path.as_ptr(),
-                krun_guest_args.as_ptr(),
-                env.as_ptr(),
-            )
-        };
+        let err = unsafe { krun_set_env(ctx_id, env.as_ptr()) };
         if err < 0 {
             let err = Errno::from_raw_os_error(-err);
-            return Err(err)
-                .context("Failed to configure the parameters for the executable to be run");
+            return Err(err).context("Failed to set the environment variables in the guest");
         }
     }
 

crates/krun/src/env.rs

@@ -1,6 +1,5 @@
 use std::collections::HashMap;
 use std::env::{self, VarError};
-use std::ffi::CString;
 use std::fs;
 use std::io::ErrorKind;
 use std::path::Path;
@@ -83,7 +82,7 @@ pub fn prepare_env_vars(env: Vec<(String, Option<String>)>) -> Result<HashMap<St
     Ok(env_map)
 }
 
-pub fn find_krun_exec<P>(program: P) -> Result<CString>
+pub fn find_krun_exec<P>(program: P) -> Result<String>
 where
     P: AsRef<Path>,
 {
@@ -97,10 +96,9 @@ where
         let path = path.context("Failed to get path of current running executable")?;
         path.with_file_name(program)
     };
-    let path = CString::new(path.to_str().with_context(|| {
+    let path = path.to_str().with_context(|| {
         format!("Failed to process {program:?} path as it contains invalid UTF-8")
-    })?)
-    .with_context(|| format!("Failed to process {program:?} path as it contains NUL character"))?;
+    })?;
 
-    Ok(path)
+    Ok(path.to_string())
 }