Use lld as the linker and add a linker script

This patch aims only to add linker script semantics and should retain
linking equivalence. It does not aim to produce address for address
equality with `ld`. Functional changes in linker output should follow in
additional patches.

Before this patch, `ld` produced the following:

```
Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000002898 0x0000000000002898  R      0x1000
  LOAD           0x0000000000003000 0x0000000000003000 0x0000000000003000
                 0x00000000000340ae 0x00000000000340ae  RWE    0x1000
  LOAD           0x0000000000038000 0x0000000000038000 0x0000000000038000
                 0x0000000000007520 0x0000000000007520  R      0x1000
  LOAD           0x000000000003f728 0x0000000000040728 0x0000000000040728
                 0x00000000000018d8 0x00000000000020d8  RW     0x1000
  DYNAMIC        0x00000000000408c0 0x00000000000418c0 0x00000000000418c0
                 0x0000000000000110 0x0000000000000110  RW     0x8
  NOTE           0x0000000000000238 0x0000000000000238 0x0000000000000238
                 0x0000000000000024 0x0000000000000024  R      0x4
  GNU_EH_FRAME   0x000000000003c638 0x000000000003c638 0x000000000003c638
                 0x0000000000000a04 0x0000000000000a04  R      0x4
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     0x10
  GNU_RELRO      0x000000000003f728 0x0000000000040728 0x0000000000040728
                 0x00000000000018d8 0x00000000000018d8  R      0x1

 Section to Segment mapping:
  Segment Sections...
   00     .note.gnu.build-id .gnu.hash .dynsym .dynstr .rela.dyn
   01     .init .text
   02     .rodata .eh_frame_hdr .eh_frame
   03     .data.rel.ro .dynamic .got .bss
   04     .dynamic
   05     .note.gnu.build-id
   06     .eh_frame_hdr
   07
   08     .data.rel.ro .dynamic .got
```

After this patch, `lld` + linker script produce the following:

```
Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000003830 0x0000000000003830  R      0x1000
  LOAD           0x0000000000004000 0x0000000000004000 0x0000000000004000
                 0x00000000000340ae 0x00000000000340ae  RWE    0x1000
  LOAD           0x0000000000039000 0x0000000000039000 0x0000000000039000
                 0x0000000000007464 0x0000000000007464  R      0x1000
  LOAD           0x0000000000041000 0x0000000000041000 0x0000000000041000
                 0x0000000000001920 0x0000000000002800  RW     0x1000
  DYNAMIC        0x0000000000042198 0x0000000000042198 0x0000000000042198
                 0x00000000000000d0 0x00000000000000d0  RW     0x8
  NOTE           0x0000000000001000 0x0000000000001000 0x0000000000001000
                 0x0000000000000018 0x0000000000000018  R      0x4
  GNU_EH_FRAME   0x000000000003d42c 0x000000000003d42c 0x000000000003d42c
                 0x0000000000000a04 0x0000000000000a04  R      0x4
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     0x0
  GNU_RELRO      0x0000000000041000 0x0000000000041000 0x0000000000041000
                 0x0000000000001920 0x0000000000002000  R      0x1

 Section to Segment mapping:
  Segment Sections...
   00     .note.gnu.build-id .gnu.hash .dynsym .dynstr .rela.dyn
   01     .init .text
   02     .rodata .eh_frame_hdr .eh_frame
   03     .data.rel.ro .dynamic .got .bss
   04     .dynamic
   05     .note.gnu.build-id
   06     .eh_frame_hdr
   07
   08     .data.rel.ro .dynamic .got
```

Signed-off-by: Nathaniel McCallum <[email protected]>

.cargo/config

@@ -4,4 +4,10 @@ rustflags = [
     "-C", "relocation-model=pic",
     "-C", "link-args=-Wl,--sort-section=alignment -nostartfiles",
     "-C", "link-self-contained=no",
+    "-C", "link-arg=-Tlink.script",
+
+    # Using GNU ld on Fedora 34 produces this assertion:
+    #  = note: /usr/bin/ld: BFD version 2.35.2-3.fc34 assertion fail elf.c:6278
+    #          collect2: error: ld returned 1 exit status
+    #"-C", "link-arg=-fuse-ld=lld",
 ]

.github/workflows/test.yml

@@ -10,6 +10,8 @@ jobs:
         with:
           toolchain: ${{ matrix.toolchain }}
           override: true
+      - name: Install lld
+        run: sudo apt install lld
       - uses: actions-rs/cargo@v1
         with:
           command: build

link.script

@@ -0,0 +1,58 @@
+ENTRY(_start)
+
+PHDRS {
+  preamble PT_LOAD FILEHDR PHDRS;
+  text PT_LOAD;
+  rodata PT_LOAD;
+  data PT_LOAD;
+
+  dynamic PT_DYNAMIC;
+  note PT_NOTE;
+
+  frame 0x6474e550;          /* GNU_EH_FRAME */
+  stack 0x6474e551 FLAGS(6); /* GNU_STACK */
+  relro 0x6474e552 FLAGS(4); /* GNU_RELRO */
+}
+
+SECTIONS {
+  . = SIZEOF_HEADERS;
+
+  . = ALIGN(CONSTANT(MAXPAGESIZE));
+  .interp             : { *(.interp) } :preamble
+  .note.gnu.build-id  : { *(.note.gnu.build-id) } :preamble :note
+  .hash               : { *(.hash) } :preamble
+  .gnu.hash           : { *(.gnu.hash) } :preamble
+  .dynsym             : { *(.dynsym) } :preamble
+  .dynstr             : { *(.dynstr) } :preamble
+  .gnu.version        : { *(.gnu.version) } :preamble
+  .gnu.version_d      : { *(.gnu.version_d) } :preamble
+  .gnu.version_r      : { *(.gnu.version_r) } :preamble
+  .rela.dyn           : { *(.rela.*) } :preamble
+  .rela.plt           : { *(.rela.plt) *(.rela.iplt) } :preamble
+
+  . = ALIGN(CONSTANT(MAXPAGESIZE));
+  .init               : { KEEP (*(SORT_NONE(.init))) } :text
+  .fini               : { KEEP (*(SORT_NONE(.fini))) } :text
+  .text               : { *(.text .text.*) } :text
+  .plt                : { *(.plt) *(.iplt) } :text
+  .plt.got            : { *(.plt.got) } :text
+  .plt.sec            : { *(.plt.sec) } :text
+
+  . = ALIGN(CONSTANT(MAXPAGESIZE));
+  .rodata             : { *(.rodata .rodata.*) } :rodata
+  .rodata1            : { *(.rodata1) } :rodata
+  .eh_frame_hdr       : { *(.eh_frame_hdr) *(.eh_frame_entry .eh_frame_entry.*) } :rodata :frame
+  .eh_frame           : ONLY_IF_RO { KEEP (*(.eh_frame)) *(.eh_frame.*) } :rodata
+
+  . = ALIGN(CONSTANT(MAXPAGESIZE));
+  .data.rel.ro        : { *(.data.rel.ro .data.rel.ro.*) } :data :relro
+  .dynamic            : { *(.dynamic) } :data :relro :dynamic
+  .got                : { *(.got) } :data :relro
+  .got.plt            : { *(.got.plt) } :data :relro
+
+  . = ALIGN(CONSTANT(MAXPAGESIZE)); /* Align here? New section? */
+  .eh_frame           : ONLY_IF_RW { KEEP (*(.eh_frame)) *(.eh_frame.*) } :data
+  .data               : { *(.data .data.*) } :data
+  .data1              : { *(.data1) } :data
+  .bss                : { *(.bss .bss.*) } :data
+}