[LLD][ELF] Add support for OpenBSD PT_OPENBSD_MUTABLE segment type

[brad]

Summary

Add support to the linker for 'PT_OPENBSD_MUTABLE' segment type.

https://github.com/openbsd/src/commit/bd249b5664da50f0178adea78250a7a0d8ea6566

[MaskRay]

This needs a test. You may merge it with openbsd-randomize.s and possibly rename the test file.
The OpenBSD section/program header types are settled? I think the kernel has no usage yet.
Hope that David's comments in https://lobste.rs/s/wck0g4/openbsd_immutable_userland_mappings are useful ;-)

[MaskRay]

lld/test/ELF/linkerscript/openbsd-mutable.s

1 ↗

(On Diff #476658)

This can be shared with openbsd-randomdata

lld/test/ELF/openbsd-mutable.s

1 ↗

(On Diff #476658)

ditto. llvm-readelf -l is preferred for new tests.

[MaskRay]

This is what I was thinking of: https://github.com/llvm/llvm-project/blob/main/lld/test/ELF/openbsd-phdr.s

[brad]

In D137921#3935680, @MaskRay wrote:

Hope that David's comments in https://lobste.rs/s/wck0g4/openbsd_immutable_userland_mappings are useful ;-)

https://github.com/openbsd/src/commit/9ba2c65f00ed07a3f3367ffa7061b2f43f010a11.patch

[MaskRay]

It seems that OpenBSD is adding more and more OS specific program headers. The pace is kinda fast.
It's true that GNU is adding a lot program headers as well but many have gain support from many other OSes....
I think we can wait a bit before committing to add this permanent thing..

If one wants to play with PT_OPENBSD_MUTABLE, in a PHDRS command, a program header can have a numeric type (https://sourceware.org/binutils/docs/ld/PHDRS.html#:~:text=expression)...

lld/ELF/ScriptParser.cpp

unsigned ret = StringSwitch<unsigned>(tok)
                      .Case("PT_INTERP", PT_INTERP)
                      .Case("PT_NOTE", PT_NOTE)
                      .Case("PT_SHLIB", PT_SHLIB)
                      .Case("PT_PHDR", PT_PHDR)
                      .Case("PT_TLS", PT_TLS)
                      .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_WXNEEDED", PT_OPENBSD_WXNEEDED)
                      .Case("PT_OPENBSD_BOOTDATA", PT_OPENBSD_BOOTDATA)
                      .Default(-1);
 
   if (ret == (unsigned)-1) {
     setError("invalid program header type: " + tok);
     return PT_NULL;

lld/ELF/Writer.cpp

if (relRo->firstSec)
     ret.push_back(relRo);
 
   // PT_GNU_EH_FRAME is a special section pointing on .eh_frame_hdr.
   if (part.ehFrame->isNeeded() && part.ehFrameHdr &&
       part.ehFrame->getParent() && part.ehFrameHdr->getParent())
     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);
 
   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