Produce sane CARGO when invoked through ld.so

Fixes rust-lang#10113

src/cargo/util/config/mod.rs

@@ -409,6 +409,32 @@ impl Config {
                 }
 
                 let exe = from_current_exe()
+                    .ok()
+                    .filter(|exe| {
+                        if let Some(name) = exe.file_name().and_then(|name| name.to_str()) {
+                            if name.starts_with("ld-")
+                                && (name.ends_with(".so") || name.contains(".so."))
+                            {
+                                // Cargo seems to be executed directly through the dynamic linker
+                                // (e.g., /usr/lib64/ld-linux-x86-64.so.2), which is then what
+                                // current_exe points at. That's not the path to Cargo itself
+                                // though, so we need to fall back to argv[0], which the step that
+                                // set up the invocation of Cargo this way has hopefully set
+                                // correctly (likely with exec -a).
+                                //
+                                // See
+                                //
+                                //   https://github.com/rust-lang/cargo/issues/10113
+                                //
+                                // and
+                                //
+                                //   https://linux.die.net/man/8/ld-linux.so
+                                return false;
+                            }
+                        }
+                        true
+                    })
+                    .ok_or(())
                     .or_else(|_| from_argv())
                     .with_context(|| "couldn't get the path to cargo executable")?;
                 Ok(exe)

tests/testsuite/build_script.rs

@@ -3,8 +3,8 @@
 use cargo_test_support::compare::assert_match_exact;
 use cargo_test_support::paths::CargoPathExt;
 use cargo_test_support::registry::Package;
-use cargo_test_support::tools;
-use cargo_test_support::{basic_manifest, cross_compile, is_coarse_mtime, project};
+use cargo_test_support::{basic_manifest, cargo_exe, cross_compile, is_coarse_mtime, project};
+use cargo_test_support::{execs, process, tools};
 use cargo_test_support::{rustc_host, sleep_ms, slow_cpu_multiplier, symlink_supported};
 use cargo_util::paths::remove_dir_all;
 use std::env;
@@ -137,6 +137,44 @@ fn custom_build_env_vars() {
     p.cargo("build --features bar_feat").run();
 }
 
+#[cargo_test]
+fn issue_10113() {
+    use std::path::Path;
+
+    let p = project()
+        .file(
+            "build.rs",
+            r#"
+            use std::env;
+            use std::path::Path;
+
+            fn main() {
+                let cargo = dbg!(env::var("CARGO").unwrap());
+                let cargo = Path::new(&cargo);
+                assert!(cargo.ends_with("cargo"));
+                assert!(cargo.exists());
+            }
+        "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("build").run();
+
+    let ld = Path::new("/usr/lib64/ld-linux-x86-64.so.2");
+    if cfg!(target_os = "linux") {
+        assert!(ld.exists());
+        let mut proc = process(ld);
+        proc.cwd(p.root());
+        proc.arg(&cargo_exe());
+        proc.arg("build");
+        // Touch the file so build is re-run.
+        p.change_file("src/lib.rs", "");
+        let mut execs = execs().with_process_builder(proc);
+        execs.run();
+    }
+}
+
 #[cargo_test]
 fn custom_build_env_var_rustflags() {
     let rustflags = "--cfg=special";