symcheck: Only ELFv1 ppc64 doesn't set .note.GNU-stack

`powerpc64-unknown-linux-gnu` currently is an ELFv1 target, while
`powerpc64-unknown-linux-musl` is an ELFv2 target.

Big-endian and little-endian ELFv2 targets both behave normally: they emit
`.note.GNU-stack`. Therefore, currently the tests would fail on big-endian
powerpc64 with ELFv2 ABI.

To determine whether we need to special-case powerpc64, we should check
the ABI instead of the endianness. The problem here is that the `e_flags`
part of the ELF header is actually `0` in the output of `cc -O0
-ffunction-sections -fdata-sections -fPIC -m64 -mabi=elfv2 -Wall -Wextra
-o missing_gnu_stack_section.o -c missing_gnu_stack_section.S`, the output
of that command is bit-for-bit identical on ELFv1 and ELFv2 hosts. In
order to know when to allow an unset `.note.GNU-stack` we therefore must
set `.abiversion 2` to be able to tell them apart from the ELF header.

This makes all tests pass on `powerpc64-unknown-linux-musl`.

library/compiler-builtins/crates/symbol-check/src/main.rs

@@ -13,8 +13,8 @@
 
 use object::read::archive::ArchiveFile;
 use object::{
-    Architecture, BinaryFormat, Endianness, File as ObjFile, Object, ObjectSection, ObjectSymbol,
-    Result as ObjResult, SectionFlags, Symbol, SymbolKind, SymbolScope, U32, elf,
+    Architecture, BinaryFormat, Endianness, File as ObjFile, FileFlags, Object, ObjectSection,
+    ObjectSymbol, Result as ObjResult, SectionFlags, Symbol, SymbolKind, SymbolScope, U32, elf,
 };
 use regex::Regex;
 use serde_json::Value;
@@ -526,12 +526,15 @@ fn check_elf_exe_stack(obj: &ObjFile) -> Result<(), ExeStack> {
         return Ok(());
     }
 
+    let FileFlags::Elf { e_flags, .. } = obj.flags() else {
+        unreachable!("only elf files are being checked");
+    };
+
     // If there is no `.note.GNU-stack` and no executable sections, behavior differs by platform.
     match obj.architecture() {
         // PPC64 doesn't set `.note.GNU-stack` since GNU nested functions don't need a trampoline,
-        // <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=21098>. From experimentation, it seems
-        // like this only applies to big endian.
-        Architecture::PowerPc64 if obj.endianness() == Endianness::Big => Ok(()),
+        // <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=21098>. This only applies to ELFv1.
+        Architecture::PowerPc64 if e_flags & elf::EF_PPC64_ABI != 2 => Ok(()),
 
         _ => Err(ExeStack::MissingGnuStackSec),
     }

library/compiler-builtins/crates/symbol-check/tests/all.rs

@@ -111,8 +111,11 @@ fn test_missing_gnu_stack_section() {
 
         let assert = t.symcheck_exe().arg(obj).arg("--no-visibility").assert();
 
-        if t.is_ppc64be() || t.no_os() || t.binary_obj_format() != BinaryFormat::Elf {
-            // Ppc64be doesn't emit `.note.GNU-stack`, not relevant without an OS, and non-elf
+        if (t.is_ppc64be() && t.is_glibc())
+            || t.no_os()
+            || t.binary_obj_format() != BinaryFormat::Elf
+        {
+            // Ppc64be ELFv1 doesn't emit `.note.GNU-stack`, not relevant without an OS, and non-elf
             // targets don't use `.note.GNU-stack`.
             assert.success();
             return;
@@ -143,8 +146,8 @@ fn test_exe_gnu_stack_section() {
 
         let assert = t.symcheck_exe().arg(obj).arg("--no-visibility").assert();
 
-        if t.is_ppc64be() || t.no_os() {
-            // Ppc64be doesn't emit `.note.GNU-stack`, not relevant without an OS.
+        if (t.is_ppc64be() && t.is_glibc()) || t.no_os() {
+            // Ppc64be ELFv1 doesn't emit `.note.GNU-stack`, not relevant without an OS.
             assert.success();
             return;
         }
@@ -316,6 +319,10 @@ fn is_ppc64be(&self) -> bool {
         self.triple.starts_with("powerpc64-")
     }
 
+    fn is_glibc(&self) -> bool {
+        self.triple.contains("-linux-gnu")
+    }
+
     /// True if the target needs `--no-os` passed to symcheck.
     fn no_os(&self) -> bool {
         self.triple.contains("-none")

library/compiler-builtins/crates/symbol-check/tests/input/missing_gnu_stack_section.S

@@ -4,6 +4,12 @@
  * executability is implied (usually yes on Linux).
  */
 
+/* Unless we set the ABI version, we have no way to differentiate between
+   big-endian ppc64 ELFv1 or ELFv2 since e_flags is otherwise unspecified */
+#if defined(__powerpc64__) && _CALL_ELF == 2
+.abiversion 2
+#endif
+
 .global func
 
 #ifdef __wasm__