fix(chroot): resolve bare command names through PATH before exec

execve(2) does not search PATH, so `ecr alpine -- echo hi` failed with
ENOENT: the old code only used PATH as an existence check and then
still exec'd the bare name, which the kernel resolved relative to the
working directory.  Pre-dates the crate split, but bare names are the
natural CLI usage so it needs to work.

Programs are now resolved execvp-style: a bare name is looked up in
the caller-composed PATH (empty components skipped rather than treated
as the cwd), paths containing '/' are used as-is with a friendlier
error than a raw ENOENT.

crates/ecr/src/chroot.rs

@@ -1,7 +1,8 @@
 use crate::veprintln;
 use anyhow::{anyhow, Context, Result};
 use nix::unistd::{chroot, execve};
-use std::path::Path;
+use std::os::unix::ffi::OsStrExt;
+use std::path::{Path, PathBuf};
 
 /// Compose the default environment for a rootfs: HOME, USER, SHELL (detected
 /// inside `rootfs`), TERM (inherited from the host process) and PATH.
@@ -92,32 +93,51 @@ pub fn run_chroot(
     veprintln!("Entering chroot at {}", rootfs.display());
     veprintln!("Working directory: {}", working_dir.display());
 
-    // Check if the program exists
-    if !Path::new(&program).exists() {
-        // Try to find it in the caller-composed PATH
-        let found = env.iter().find(|(k, _)| k == "PATH").and_then(|(_, path)| {
-            path.split(':')
-                .map(|p| std::path::PathBuf::from(p).join(&program))
-                .find(|p| p.exists())
-        });
-
-        if found.is_none() {
-            return Err(anyhow!("Program not found: {}", program));
-        }
-    }
+    // execve(2) does not search PATH: resolve the program through the
+    // caller-composed environment like execvp(3) would.
+    let program_path = resolve_program(&program, env)?;
 
     // Exec the program directly with the caller-composed, isolated environment.
     // execve never returns on success.
-    let program_cstr = std::ffi::CString::new(program.as_str()).context("Invalid program name")?;
+    let program_cstr = std::ffi::CString::new(program_path.as_os_str().as_bytes())
+        .with_context(|| format!("Invalid program path: {}", program_path.display()))?;
 
     let result = execve(&program_cstr, &args, &env_cstrings);
 
     match result {
         Ok(_) => Ok(()), // Never reached
-        Err(e) => Err(anyhow!("Failed to exec {}: {}", program, e)),
+        Err(e) => Err(anyhow!("Failed to exec {}: {}", program_path.display(), e)),
     }
 }
 
+/// Resolve the program to exec.  Commands containing a '/' are used as-is
+/// (existing check gives a clearer error than a raw ENOENT); a bare command
+/// name is looked up in the caller-composed PATH, like execvp(3) would.
+fn resolve_program(program: &str, env: &[(String, String)]) -> Result<PathBuf> {
+    if program.contains('/') {
+        let path = PathBuf::from(program);
+        if !path.exists() {
+            return Err(anyhow!("Program not found: {}", program));
+        }
+        return Ok(path);
+    }
+
+    let path_var = env
+        .iter()
+        .find(|(k, _)| k == "PATH")
+        .map(|(_, v)| v.as_str())
+        .unwrap_or("/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin");
+
+    // Empty PATH components are skipped rather than treated as the current
+    // directory (execvp semantics) — cwd-relative lookup is a footgun here.
+    path_var
+        .split(':')
+        .filter(|dir| !dir.is_empty())
+        .map(|dir| PathBuf::from(dir).join(program))
+        .find(|candidate| candidate.exists())
+        .ok_or_else(|| anyhow!("Program not found: {} (PATH={})", program, path_var))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -126,6 +146,55 @@ mod tests {
         env.iter().find(|(k, _)| k == key).map(|(_, v)| v.as_str())
     }
 
+    #[test]
+    fn resolve_program_bare_name_via_path() {
+        let dir = tempfile::tempdir().unwrap();
+        std::fs::write(dir.path().join("true"), b"binary").unwrap();
+        let env = vec![(
+            "PATH".to_string(),
+            format!("/nonexistent:{}", dir.path().display()),
+        )];
+        assert_eq!(
+            resolve_program("true", &env).unwrap(),
+            dir.path().join("true")
+        );
+    }
+
+    #[test]
+    fn resolve_program_bare_name_defaults_to_standard_path() {
+        // No PATH in the composed env: the standard fallback must still
+        // resolve a program present on any Linux host.
+        let resolved = resolve_program("sh", &[]).unwrap();
+        assert!(resolved.exists());
+    }
+
+    #[test]
+    fn resolve_program_bare_name_missing() {
+        let env = vec![("PATH".to_string(), "/nonexistent".to_string())];
+        assert!(resolve_program("definitely-missing", &env).is_err());
+    }
+
+    #[test]
+    fn resolve_program_absolute_path_passthrough() {
+        let dir = tempfile::tempdir().unwrap();
+        std::fs::write(dir.path().join("tool"), b"binary").unwrap();
+        let path = dir.path().join("tool");
+        assert_eq!(resolve_program(path.to_str().unwrap(), &[]).unwrap(), path);
+    }
+
+    #[test]
+    fn resolve_program_absolute_path_missing() {
+        assert!(resolve_program("/nonexistent/tool", &[]).is_err());
+    }
+
+    #[test]
+    fn resolve_program_skips_empty_path_components() {
+        // An empty PATH component must not mean "current directory";
+        // Cargo.toml exists in the test process cwd and must not resolve.
+        let env = vec![("PATH".to_string(), ":".to_string())];
+        assert!(resolve_program("Cargo.toml", &env).is_err());
+    }
+
     #[test]
     fn test_default_env_entries() {
         let dir = tempfile::tempdir().unwrap();