From 3e801e61930de2116d02cb8d91f1a98a4728d560 Mon Sep 17 00:00:00 2001
From: Kevin Valerio <kvalerio@protonmail.com>
Date: Wed, 17 Jun 2026 15:25:30 +0200
Subject: [PATCH] fix(bpf): avoid extending narrow extern C returns

---
 compiler/rustc_target/src/callconv/bpf.rs     |  3 +-
 .../codegen-llvm/bpf-abi-extern-narrow-ret.rs | 50 +++++++++++++++++++
 2 files changed, 51 insertions(+), 2 deletions(-)
 create mode 100644 tests/codegen-llvm/bpf-abi-extern-narrow-ret.rs

diff --git a/compiler/rustc_target/src/callconv/bpf.rs b/compiler/rustc_target/src/callconv/bpf.rs
index 3624f406704e9..7123ba2946ffb 100644
--- a/compiler/rustc_target/src/callconv/bpf.rs
+++ b/compiler/rustc_target/src/callconv/bpf.rs
@@ -6,8 +6,6 @@ use crate::callconv::{ArgAbi, FnAbi};
 fn classify_ret<Ty>(ret: &mut ArgAbi<'_, Ty>) {
     if ret.layout.is_aggregate() || ret.layout.size.bits() > 64 {
         ret.make_indirect();
-    } else {
-        ret.extend_integer_width_to(32);
     }
 }
 
@@ -45,5 +43,6 @@ where
 pub(crate) fn compute_rust_abi_info<Ty>(fn_abi: &mut FnAbi<'_, Ty>) {
     if !fn_abi.ret.is_ignore() {
         classify_ret(&mut fn_abi.ret);
+        fn_abi.ret.extend_integer_width_to(32);
     }
 }
diff --git a/tests/codegen-llvm/bpf-abi-extern-narrow-ret.rs b/tests/codegen-llvm/bpf-abi-extern-narrow-ret.rs
new file mode 100644
index 0000000000000..fc1323e5621e3
--- /dev/null
+++ b/tests/codegen-llvm/bpf-abi-extern-narrow-ret.rs
@@ -0,0 +1,50 @@
+//@ add-minicore
+//@ needs-llvm-components: bpf
+//@ compile-flags: --target bpfel-unknown-none
+
+#![feature(no_core)]
+#![crate_type = "lib"]
+#![no_core]
+
+extern crate minicore;
+use minicore::*;
+
+unsafe extern "C" {
+    fn c_ret_u8() -> u8;
+    fn c_ret_u16() -> u16;
+    fn c_ret_i8() -> i8;
+    fn c_ret_i16() -> i16;
+}
+
+// CHECK-LABEL: define {{.*}} @observe_u8(
+// CHECK: call noundef i8 @c_ret_u8()
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn observe_u8() -> u64 {
+    c_ret_u8() as u64
+}
+
+// CHECK-LABEL: define {{.*}} @observe_u16(
+// CHECK: call noundef i16 @c_ret_u16()
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn observe_u16() -> u64 {
+    c_ret_u16() as u64
+}
+
+// CHECK-LABEL: define {{.*}} @observe_i8(
+// CHECK: call noundef i8 @c_ret_i8()
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn observe_i8() -> i64 {
+    c_ret_i8() as i64
+}
+
+// CHECK-LABEL: define {{.*}} @observe_i16(
+// CHECK: call noundef i16 @c_ret_i16()
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn observe_i16() -> i64 {
+    c_ret_i16() as i64
+}
+
+// CHECK: declare noundef i8 @c_ret_u8()
+// CHECK: declare noundef i16 @c_ret_u16()
+// CHECK: declare noundef i8 @c_ret_i8()
+// CHECK: declare noundef i16 @c_ret_i16()