diff --git a/src/bootstrap/src/core/sanity.rs b/src/bootstrap/src/core/sanity.rs
index bd21fafc999be..e0790c5bf7091 100644
--- a/src/bootstrap/src/core/sanity.rs
+++ b/src/bootstrap/src/core/sanity.rs
@@ -379,13 +379,5 @@ $ pacman -R cmake && pacman -S mingw-w64-x86_64-cmake
         cmd_finder.must_have(s);
     }
 
-    // this warning is useless in CI,
-    // and CI probably won't have the right branches anyway.
-    if !build_helper::ci::CiEnv::is_ci() {
-        if let Err(e) = warn_old_master_branch(&build.config.git_config(), &build.config.src)
-            .map_err(|e| e.to_string())
-        {
-            eprintln!("unable to check if upstream branch is old: {e}");
-        }
-    }
+    warn_old_master_branch(&build.config.git_config(), &build.config.src);
 }
diff --git a/src/tools/build_helper/src/git.rs b/src/tools/build_helper/src/git.rs
index 15d863caf0c5f..10c5476cd8f3b 100644
--- a/src/tools/build_helper/src/git.rs
+++ b/src/tools/build_helper/src/git.rs
@@ -204,21 +204,20 @@ pub fn get_git_untracked_files(
 ///
 /// This can result in formatting thousands of files instead of a dozen,
 /// so we should warn the user something is wrong.
-pub fn warn_old_master_branch(
-    config: &GitConfig<'_>,
-    git_dir: &Path,
-) -> Result<(), Box<dyn std::error::Error>> {
-    use std::time::Duration;
-    const WARN_AFTER: Duration = Duration::from_secs(60 * 60 * 24 * 10);
-    let updated_master = updated_master_branch(config, Some(git_dir))?;
-    let branch_path = git_dir.join(".git/refs/remotes").join(&updated_master);
-    match std::fs::metadata(branch_path) {
-        Ok(meta) => {
-            if meta.modified()?.elapsed()? > WARN_AFTER {
-                eprintln!("warning: {updated_master} has not been updated in 10 days");
-            } else {
-                return Ok(());
+pub fn warn_old_master_branch(config: &GitConfig<'_>, git_dir: &Path) {
+    if crate::ci::CiEnv::is_ci() {
+        // this warning is useless in CI,
+        // and CI probably won't have the right branches anyway.
+        return;
+    }
+    // this will be overwritten by the actual name, if possible
+    let mut updated_master = "the upstream master branch".to_string();
+    match warn_old_master_branch_(config, git_dir, &mut updated_master) {
+        Ok(branch_is_old) => {
+            if !branch_is_old {
+                return;
             }
+            // otherwise fall through and print the rest of the warning
         }
         Err(err) => {
             eprintln!("warning: unable to check if {updated_master} is old due to error: {err}")
@@ -226,7 +225,38 @@ pub fn warn_old_master_branch(
     }
     eprintln!(
         "warning: {updated_master} is used to determine if files have been modified\n\
-               warning: if it is not updated, this may cause files to be needlessly reformatted"
+         warning: if it is not updated, this may cause files to be needlessly reformatted"
     );
-    Ok(())
+}
+
+pub fn warn_old_master_branch_(
+    config: &GitConfig<'_>,
+    git_dir: &Path,
+    updated_master: &mut String,
+) -> Result<bool, Box<dyn std::error::Error>> {
+    use std::time::Duration;
+    *updated_master = updated_master_branch(config, Some(git_dir))?;
+    let branch_path = git_dir.join(".git/refs/remotes").join(&updated_master);
+    const WARN_AFTER: Duration = Duration::from_secs(60 * 60 * 24 * 10);
+    let meta = match std::fs::metadata(&branch_path) {
+        Ok(meta) => meta,
+        Err(err) => {
+            let gcd = git_common_dir(&git_dir)?;
+            if branch_path.starts_with(&gcd) {
+                return Err(Box::new(err));
+            }
+            std::fs::metadata(Path::new(&gcd).join("refs/remotes").join(&updated_master))?
+        }
+    };
+    if meta.modified()?.elapsed()? > WARN_AFTER {
+        eprintln!("warning: {updated_master} has not been updated in 10 days");
+        Ok(true)
+    } else {
+        Ok(false)
+    }
+}
+
+fn git_common_dir(dir: &Path) -> Result<String, String> {
+    output_result(Command::new("git").arg("-C").arg(dir).arg("rev-parse").arg("--git-common-dir"))
+        .map(|x| x.trim().to_string())
 }