[LLD] Extend special OpenBSD support, but scope under ELFOSABI

[Ericson2314]

• Add support for .openbsd.mutable

  (rebaser's note) adapted from:
  openbsd/src@bd249b5
  New auto-coalescing sections removed

  In the linkers, collect objects in section "openbsd.mutable" and place
  them into a page-aligned region in the bss, with the right markers for
  kernel/ld.so to identify the region and skip making it immutable. While
  here, fix readelf/objdump versions to show all of this. ok miod kettenis

• Add support for .openbsd.syscalls

  (rebaser's note) adapted from:
  openbsd/src@42a61ac

  Collect .openbsd.syscalls sections into a new PT_OPENBSD_SYSCALLS
  segment. This will be used soon to pin system calls to designated call
  sites.

  ok deraadt@

• Scope OpenBSD special section handling under that ELFOSABI

  As a preexisting comment in ELF/Writer.cpp says:

  section names shouldn't be significant in ELF in spirit.

  so scoping OSABI-specific magic name hacks to just the OSABI in
  question limits the degree to which we deviate from that "spirit" for
  all other OSABIs.

  OpenBSD in particular is very fast moving, having added a number of
  special sections, etc. in recent years. It is unclear how possible /
  reasonable it is for upstream to implement all these features in any
  event, but scoping like this at least mitigates the fallout for other
  OSABIs systems which wish to be more slow-moving.

Depends on #98158

[llvmbot]

@llvm/pr-subscribers-mc
@llvm/pr-subscribers-lld

@llvm/pr-subscribers-lld-elf

Author: John Ericson (Ericson2314)

Changes

> (rebaser's note) adapted from:
> openbsd/src@bd249b5
>
> In the linkers, collect objects in section "openbsd.mutable" and place them into a page-aligned region in the bss, with the right markers for kernel/ld.so to identify the region and skip making it immutable. While here, fix readelf/objdump versions to show all of this. ok miod kettenis

> (rebaser's note) adapted from:
> openbsd/src@42a61ac
>
> Collect .openbsd.syscalls sections into a new PT_OPENBSD_SYSCALLS segment. This will be used soon to pin system calls to designated call sites.
>
> ok deraadt@

Note that I think it would seem better if all this OpenBSD stuff was conditioned on config->osabi == ELFOSABI_FREEBSD, but the -m flag for ld.llld is quite underpowered compared to --target.

---

Full diff: https://github.com/llvm/llvm-project/pull/97122.diff

3 Files Affected:

• (modified) lld/ELF/LinkerScript.cpp (+2-1)
• (modified) lld/ELF/ScriptParser.cpp (+2)
• (modified) lld/ELF/Writer.cpp (+11)

diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 1ec796a3bdd99..e1f38eccb1518 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -107,7 +107,8 @@ static StringRef getOutputSectionName(const InputSectionBase *s) {
   for (StringRef v :
        {".data.rel.ro", ".data", ".rodata", ".bss.rel.ro", ".bss", ".ldata",
         ".lrodata", ".lbss", ".gcc_except_table", ".init_array", ".fini_array",
-        ".tbss", ".tdata", ".ARM.exidx", ".ARM.extab", ".ctors", ".dtors"})
+        ".tbss", ".tdata", ".ARM.exidx", ".ARM.extab", ".ctors", ".dtors",
+        ".openbsd.randomdata", ".openbsd.mutable"})
     if (isSectionPrefix(v, s->name))
       return v;
 
diff --git a/lld/ELF/ScriptParser.cpp b/lld/ELF/ScriptParser.cpp
index db46263115242..41bd9a95053f7 100644
--- a/lld/ELF/ScriptParser.cpp
+++ b/lld/ELF/ScriptParser.cpp
@@ -1628,7 +1628,9 @@ unsigned ScriptParser::readPhdrType() {
                      .Case("PT_GNU_EH_FRAME", PT_GNU_EH_FRAME)
                      .Case("PT_GNU_STACK", PT_GNU_STACK)
                      .Case("PT_GNU_RELRO", PT_GNU_RELRO)
+                     .Case("PT_OPENBSD_MUTABLE", PT_OPENBSD_MUTABLE)
                      .Case("PT_OPENBSD_RANDOMIZE", PT_OPENBSD_RANDOMIZE)
+                     .Case("PT_OPENBSD_SYSCALLS", PT_OPENBSD_SYSCALLS)
                      .Case("PT_OPENBSD_WXNEEDED", PT_OPENBSD_WXNEEDED)
                      .Case("PT_OPENBSD_BOOTDATA", PT_OPENBSD_BOOTDATA)
                      .Default(-1);
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 640cb2a445f7d..917ed48bf0ec4 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -2245,11 +2245,22 @@ SmallVector<PhdrEntry *, 0> Writer<ELFT>::createPhdrs(Partition &part) {
     addHdr(PT_GNU_EH_FRAME, part.ehFrameHdr->getParent()->getPhdrFlags())
         ->add(part.ehFrameHdr->getParent());
 
+  // PT_OPENBSD_MUTABLE is an OpenBSD-specific feature. That makes
+  // the dynamic linker fill the segment with zero data, like bss, but
+  // it can be treated differently.
+  if (OutputSection *cmd = findSection(".openbsd.mutable", partNo))
+    addHdr(PT_OPENBSD_MUTABLE, cmd->getPhdrFlags())->add(cmd);
+
   // PT_OPENBSD_RANDOMIZE is an OpenBSD-specific feature. That makes
   // the dynamic linker fill the segment with random data.
   if (OutputSection *cmd = findSection(".openbsd.randomdata", partNo))
     addHdr(PT_OPENBSD_RANDOMIZE, cmd->getPhdrFlags())->add(cmd);
 
+  // PT_OPENBSD_SYSCALLS is an OpenBSD-specific feature. That makes
+  // the kernel and dynamic linker register system call sites.
+  if (OutputSection *cmd = findSection(".openbsd.syscalls", partNo))
+    addHdr(PT_OPENBSD_SYSCALLS, cmd->getPhdrFlags())->add(cmd);
+
   if (config->zGnustack != GnuStackKind::None) {
     // PT_GNU_STACK is a special section to tell the loader to make the
     // pages for the stack non-executable. If you really want an executable

[Ericson2314]

I'm happy to report that with change, my cross compiled statically linked hello world ran successfully :)

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[Ericson2314]

@MaskRay After testing that my cross compiled binary still runs, I:

• Removed the section-coalescing parts of the first two commits.

• Added a new commit which:

  • Gated the PT_-assignments under a config->osabi == ELFOSABI_OPENBSD check

  • Modified LLVM to set ELFOSABI_OPENBSD for OpenBSD targets just like it does ELFOSABI_FREEBSD for FreeBSD ones

I am reasonably happy with this. Per the 3rd commits message, without weighing in on the degree to which it is possible/reasonable for upstream to track something as fast-moving as OpenBSD, the config->osabi == ELFOSABI_OPENBSD adds a stronger "firewall" between OpenBSD and everything else.

[Ericson2314]

I rebased atop #98158 so as to separate the LLVM an LLD parts, in case that is easier for review.

[Ericson2314]

OK @MaskRay I think this is ready now?

• (Previously) addressed your 3 prior review comments
• Rebased atop merged [MC] set OpenBSD's ELFOSABI by default #98158 so it's just the one commit
• changed -triple to be x86_64-unknown-freebsd (with unknown) to match what you did in 06cecdc

[Ericson2314]

(Rebased because one thing in #98158 needed fixing.)

[Ericson2314]

I found another such possibly-affected test. I need to make sure it is working --- even if the section rename is not necessary, I figure either keeping it, or making the target openbsd not linux is a good idea.

[MaskRay]

It should be fine to keep this unchanged. We can add the osabi check if the .openbsd.* list ever gets longer.

[Ericson2314]

@MaskRay I would sort of like to change it for consistency.

I ended up duplicating the test I had to change so we have a Linux and OpenBSD version, and this makes me feel better about coverage, etc.

[Ericson2314]

(And checked locally that both the new tests worked, and the old test failed.)

[Ericson2314]

Linux CI is stuck in the queue, Windows has

PASS: lld :: ELF/openbsd-phdr.s (1928 of 2949)
...
PASS: lld :: ELF/relro-openbsd.s (2129 of 2949)
PASS: lld :: ELF/relro.s (2130 of 2949)

and there is no thing host-specific about this, so I think we are good!