[lld][elf] warn on missing .note.GNU-stack section

[nickdesaulniers]

Since https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=ba951afb99912da01a6e8434126b8fac7aa75107, GNU ld (BFD) will warn when an ELF object is missing the .note.GNU-stack section.

Generally, we'd like code to not have an executable stack. Because some code could rely on that, we need to start migrating codebases to be built without executable stack.

We see this for the Linux kernel for asm sources. We plan to add -Wa,--noexecstack to KBUILD. See also:
https://lore.kernel.org/linux-block/CAKwvOdmD10yK0r1H-M2PcnZgy3M0aA9gdkY0BErDOQ+KpBRxVQ@mail.gmail.com/

[llvmbot]

@llvm/issue-subscribers-lld-elf

[MaskRay]

This is a wontfix. lld, since its beginning, has taken the stance to ignore input .note.GNU-stack and default to -z noexecstack.

The GNU ld transition is more complex because it has the traditional tri-state for a long time and some archaic ports (LLVM does not support) may not finish the transition so smoothly. The diagnostic is only meaningful to GNU ld.

In addition, there are quite few groups using ld.lld exclusively. There are quite few assembly files which do not have .note.GNU-stack. If we have such a diagnostic, it'd just break these builds when --fatal-warnings is specificed.

Note: clang uses do not need -Wa,--noexecstack. See https://reviews.llvm.org/D113840

[nickdesaulniers]

Thanks for the info, will include it in a kernel commit.

Note: clang uses do not need -Wa,--noexecstack.

Well, if the default -fuse-ld is GNU BFD, they do need it to avoid the warning with GNU BFD. I guess they don't need it when linking with LLD, but it also doesn't hurt.

[MaskRay]

I recommend that kernel uses -z noexecstack for LDFLAGS and doesn't add -Wa,--noexecstack.

-z noexecstack supports all linkers. --no-warn-execstack is only supported by GNU ld>=2.39 and I am not sure it's valuable enough to be added to ld.lld (new GNU ld option purely for its transition purpose transiting away from a dangerous traditional behavior that ld.lld never supports in the first place).

-Wa,--noexecstack is not necessary: if the built relocatable files are destined to be used with ld with -z noexecstack, the input .note.GNU-stack sections are really redundant. It would be a difficult story if the relocatable files can "leak" to the outside world where a GNU ld without -z noexecstack may be used. But IMO this is not the case for the kernel.