[ELF] Add PT_RISCV_ATTRIBUTES program header

[MaskRay]

Summary

Close https://github.com/llvm/llvm-project/issues/63084

Unlike AArch32, RISC-V defines PT_RISCV_ATTRIBUTES to include the
SHT_RISCV_ATTRIBUTES section. There is no real-world use case yet.

We place PT_RISCV_ATTRIBUTES after PT_GNU_STACK, similar to PT_ARM_EXIDX. GNU ld
places PT_RISCV_ATTRIBUTES earlier, but the placement should not matter.

Link: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/71

[MaskRay]

The experimental partitions feature is not tested. I don't think anyone cares about its interaction with PT_RISCV_ATTRIBUTES and don't think a test for the , partNo part is necessary.

[MaskRay]

add Dxxxx link to release note
fix test

[psykose]

tested applied on 16.0.5, works for the original strip failure case for me as well.

[asb]

LGTM.

[MaskRay]

Relatedly, llvm-objcopy --remove-section=.riscv.attributes probably should change the PT_RISCV_ATTRIBUTES program header to a PT_NONE. I'll handle this later.

[MaskRay]

I was misled. I forgot to ask myself again why this program header type is needed:)

I created https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/386 to propose that we remove PT_RISCV_ATTRIBUTES instead.

lld/test/ELF/riscv-attributes.s

72

GNU ld sets p_memsz to 0. I forgot to notice it.

lld/ELF/Writer.cpp

for (Partition &part : partitions) {
         addPhdrForSection(part, SHT_ARM_EXIDX, PT_ARM_EXIDX, PF_R);
       }
       if (config->emachine == EM_MIPS) {
         // Add separate segments for MIPS-specific sections.
         addPhdrForSection(part, SHT_MIPS_REGINFO, PT_MIPS_REGINFO, PF_R);
         addPhdrForSection(part, SHT_MIPS_OPTIONS, PT_MIPS_OPTIONS, PF_R);
         addPhdrForSection(part, SHT_MIPS_ABIFLAGS, PT_MIPS_ABIFLAGS, PF_R);
       }
+      if (config->emachine == EM_RISCV)
+        addPhdrForSection(part, SHT_RISCV_ATTRIBUTES, PT_RISCV_ATTRIBUTES,
+                          PF_R);
     }
     Out::programHeaders->size = sizeof(Elf_Phdr) * mainPart->phdrs.size();
 
     // Find the TLS segment. This happens before the section layout loop so that
     // Android relocation packing can look up TLS symbol addresses. We only need
     // to care about the main partition here because all TLS symbols were moved
     // to the main partition (see MarkLive.cpp).
     for (PhdrEntry *p : mainPart->phdrs)

lld/docs/ReleaseNotes.rst

 Non-comprehensive list of changes in this release
 =================================================
 
 ELF Improvements
 ----------------
 
 * ``--remap-inputs=`` and ``--remap-inputs-file=`` are added to remap input files.
   (`D148859 <https://reviews.llvm.org/D148859>`_)
+* ``PT_RISCV_ATTRIBUTES`` is added to include the SHT_RISCV_ATTRIBUTES section.
+  (`D152065 <https://reviews.llvm.org/D152065>`_)
 
 Breaking changes
 ----------------
 
 COFF Improvements
 -----------------
 
 MinGW Improvements

lld/test/ELF/riscv-attributes.s

 # REQUIRES: riscv
 
 # RUN: rm -rf %t && split-file %s %t && cd %t
 # RUN: llvm-mc -filetype=obj -triple=riscv64 a.s -o a.o
 # RUN: ld.lld -e 0 a.o -o out 2>&1 | count 0
-# RUN: llvm-readobj --arch-specific out | FileCheck %s
+# RUN: llvm-readelf -S -l --arch-specific out | FileCheck %s --check-prefixes=HDR,CHECK
 # RUN: ld.lld -e 0 a.o a.o -o out1 2>&1 | count 0
 # RUN: llvm-readobj --arch-specific out1 | FileCheck %s
 # RUN: ld.lld -r a.o a.o -o out1 2>&1 | count 0
 # RUN: llvm-readobj --arch-specific out1 | FileCheck %s
 
 # RUN: llvm-mc -filetype=obj -triple=riscv64 b.s -o b.o
 # RUN: llvm-mc -filetype=obj -triple=riscv64 c.s -o c.o
 # RUN: ld.lld a.o b.o c.o -o out2

 # UNKNOWN13:         warning: unknown13a.o:(.riscv.attributes): invalid tag 0xd at offset 0x10
 
 # RUN: llvm-mc -filetype=obj -triple=riscv64 unknown22.s -o unknown22.o
 # RUN: llvm-mc -filetype=obj -triple=riscv64 unknown22a.s -o unknown22a.o
 # RUN: ld.lld -e 0 unknown22.o unknown22.o unknown22a.o -o unknown22 2>&1 | FileCheck %s --check-prefix=UNKNOWN22 --implicit-check-not=warning:
 # UNKNOWN22-COUNT-2: warning: unknown22.o:(.riscv.attributes): invalid tag 0x16 at offset 0x10
 # UNKNOWN22:         warning: unknown22a.o:(.riscv.attributes): invalid tag 0x16 at offset 0x10
 
+# HDR:      Name              Type             Address          Off    Size   ES Flg Lk Inf Al
+# HDR:      .riscv.attributes RISCV_ATTRIBUTES 0000000000000000 000158 00003e 00      0   0  1{{$}}
+
+# HDR:      Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
+# HDR:      LOAD           0x000000 0x0000000000010000 0x0000000000010000 0x000158 0x000158 R   0x1000
+# HDR-NEXT: GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0
+# HDR-NEXT: ATTRIBUTES     0x000158 0x0000000000000000 0x0000000000000000 0x00003e 0x00003e R   0x1{{$}}
+
 # CHECK:      BuildAttributes {
 # CHECK-NEXT:   FormatVersion: 0x41
 # CHECK-NEXT:   Section 1 {
 # CHECK-NEXT:     SectionLength: 61
 # CHECK-NEXT:     Vendor: riscv
 # CHECK-NEXT:     Tag: Tag_File (0x1)
 # CHECK-NEXT:     Size: 51
 # CHECK-NEXT:     FileAttributes {