[ELF] Support linking ET_EXEC

[MaskRay]

Summary

GNU ld supports linking an ET_EXEC as if it were an ET_REL. This is an
extremely rare feature, but we can implement it with just one more line.

As a real world use case, the final link phase of seabios does:

ld -N -T out/romlayout32flat.lds out/rom16.strip.o out/rom32seg.strip.o out/code32flat.o -o out/rom.o

out/rom16.strip.o and out/rom32seg.strip.o are ET_EXEC created by previous steps.

romlayout32flat.lds collects the pieces and produces rom.o which
contains code of all x86 memory models.

With this lld patch and two seabios patches, I manage to link seabios with lld.

• https://mail.coreboot.org/hyperkitty/list/[e-mail]/thread/CJR7WU6TTHVQEG5XHCRI75EA6SBV72SW/
• https://mail.coreboot.org/hyperkitty/list/[e-mail]/thread/GUYWC4VOYH76JOYGBNT7J4FSHLKG53EO/

[grimar]

I think a one-line patch even for such a rare feature is OK, so LGTM. But lets wait for a second opinion too.

lld/ELF/Driver.cpp

262

I wonder if we want a comment here.

lld/test/ELF/executable.test

19 ↗

(On Diff #250349)

Nit: this could be a single line:

# RUN: echo "SECTIONS { .foo : { *(.text) } }" > %t.script

This test file could be *.s.

[MaskRay]

Add a comment

[MaskRay]

Rename executable.test to executable.s and switch the role of assembly/linker script

[psmith]

I can see that this would work if the user were very careful and as it is a legitimate use case we should support it. In a previous linker we were asked to make this case an error as a customer had done this by mistake and spent several hours wondering why their program wasn't working. On balance I think it isn't worth a warning along the lines of "warning: file X is an executable" as I don't think that this happens much in practice, but something to think about if other users have problems.

Looks good to me.

[MaskRay]

In D76174#1923326, @psmith wrote:

I can see that this would work if the user were very careful and as it is a legitimate use case we should support it. In a previous linker we were asked to make this case an error as a customer had done this by mistake and spent several hours wondering why their program wasn't working. On balance I think it isn't worth a warning along the lines of "warning: file X is an executable" as I don't think that this happens much in practice, but something to think about if other users have problems.

Looks good to me.

Thanks for the feedback. On second thought, I think I shall abandon the patch. This is easy to workaround on seabios' side

https://mail.coreboot.org/hyperkitty/list/[e-mail]/thread/HHIUPUXRIZ3KLTK4TPLG2V4PFP32HRBE/

If seabios accepts my 3 patches, they can link seabios with lld 7. Projects like FreeBSD 12 (lld 8.0.1) can immediately benefit from it.

lld/ELF/Driver.cpp

case file_magic::elf_shared_object:
     //
     // If a file was specified by -lfoo, the directory part is not
     // significant, as a user did not specify it. This behavior is
     // compatible with GNU.
     files.push_back(
         make<SharedFile>(mbref, withLOption ? path::filename(path) : path));
     return;
   case file_magic::bitcode:
+  case file_magic::elf_executable: // handled the same way as an ET_REL
   case file_magic::elf_relocatable:
     if (inLib)
       files.push_back(make<LazyObjFile>(mbref, "", 0));
     else
       files.push_back(createObjectFile(mbref));
     break;
   default:
     error(path + ": unknown file type");

lld/test/ELF/executable.s

+# REQUIRES: x86
+## Test an ET_EXEC can be linked like a relocatable object.
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+# RUN: ld.lld %t.o -o %t.exe
+
+# RUN: ld.lld %t.exe -o %t
+# RUN: cmp %t.exe %t
+
+## This demonstrates a use case that a linker script can
+## uses the section contents to create another output.
+## seabios has such a use case.
+
+# RUN: echo 'SECTIONS { .foo : {*(.text)}}' > %t.script
+# RUN: ld.lld -T %t.script %t.exe %t.exe -o %t2
+# RUN: llvm-readelf -x .foo %t2 | FileCheck %s
+
+# CHECK: 0x00000000 90cccccc 90
+
+nop

lld/test/ELF/invalid/core.test

+## Reject core files.
+# RUN: yaml2obj %s -o %t
+# RUN: not ld.lld %t -o /dev/null 2>&1 | FileCheck %s
+
+# CHECK: unknown file type
+
+!ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_CORE
+  Machine: EM_X86_64

lld/test/ELF/invalid/executable.s

-# REQUIRES: x86
-# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
-# RUN: ld.lld -o %t1.exe %t.o
-# RUN: not ld.lld -o /dev/null %t1.exe 2>&1 | FileCheck %s
-# CHECK: unknown file type
-
-.global _start
-_start:
-  ret