arm64: Force position-independent veneers

Certain portions of code always need to be position-independent
regardless of CONFIG_RELOCATABLE, including code which is executed in an
idmap or which is executed before relocations are applied. In some
kernel configurations the LLD linker generates position-dependent
veneers for such code, and when executed these result in early boot-time
failures.

Marc Zyngier encountered a boot failure resulting from this when
building a (particularly cursed) configuration with LLVM, as he reported
to the list:

  https://lore.kernel.org/linux-arm-kernel/86wmjwvatn.wl-maz@kernel.org/

In Marc's kernel configuration, the .head.text and .rodata.text sections
end up more than 128MiB apart, requiring a veneer to branch between the
two:

| [mark@lakrids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w _text
| ffff800080000000 g       .head.text     0000000000000000 _text
| [mark@lakrids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w primary_entry
| ffff8000889df0e0 g       .rodata.text   000000000000006c primary_entry,

... consequently, LLD inserts a position-dependent veneer for the branch
from _stext (in .head.text) to primary_entry (in .rodata.text):

| ffff800080000000 <_text>:
| ffff800080000000:       fa405a4d        ccmp    x18, #0x0, #0xd, pl     // pl = nfrst
| ffff800080000004:       14003fff        b       ffff800080010000 <__AArch64AbsLongThunk_primary_entry>
...
| ffff800080010000 <__AArch64AbsLongThunk_primary_entry>:
| ffff800080010000:       58000050        ldr     x16, ffff800080010008 <__AArch64AbsLongThunk_primary_entry+0x8>
| ffff800080010004:       d61f0200        br      x16
| ffff800080010008:       889df0e0        .word   0x889df0e0
| ffff80008001000c:       ffff8000        .word   0xffff8000

... and as this is executed early in boot before the kernel is mapped in
TTBR1 this results in a silent boot failure.

Fix this by passing '--pic-veneer' to the linker, which will cause the
linker to use position-independent veneers, e.g.

| ffff800080000000 <_text>:
| ffff800080000000:       fa405a4d        ccmp    x18, #0x0, #0xd, pl     // pl = nfrst
| ffff800080000004:       14003fff        b       ffff800080010000 <__AArch64ADRPThunk_primary_entry>
...
| ffff800080010000 <__AArch64ADRPThunk_primary_entry>:
| ffff800080010000:       f004e3f0        adrp    x16, ffff800089c8f000 <__idmap_text_start>
| ffff800080010004:       91038210        add     x16, x16, #0xe0
| ffff800080010008:       d61f0200        br      x16

I've opted to pass '--pic-veneer' unconditionally, as:

* In addition to solving the boot failure, these sequences are generally
  nicer as they require fewer instructions and don't need to perform
  data accesses.

* While the position-independent veneer sequences have a limited +/-2GiB
  range, this is not a new restriction. Even kernels built with
  CONFIG_RELOCATABLE=n are limited to 2GiB in size as we have several
  structues using 32-bit relative offsets and PPREL32 relocations, which
  are similarly limited to +/-2GiB in range. These include extable
  entries, jump table entries, and alt_instr entries.

* GNU LD defaults to using position-independent veneers, and supports
  the same '--pic-veneer' option, so this change is not expected to
  adversely affect GNU LD.

I've tested with GNU LD 2.30 to 2.42 inclusive and LLVM 13.0.1 to 19.1.0
inclusive, using the kernel.org binaries from:

* https://mirrors.edge.kernel.org/pub/tools/crosstool/
* https://mirrors.edge.kernel.org/pub/tools/llvm/Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reported-by: Marc Zyngier <maz@kernel.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Will Deacon <will@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20240927101838.3061054-1-mark.rutland@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Makefile

@@ -10,7 +10,7 @@
 #
 # Copyright (C) 1995-2001 by Russell King
 
-LDFLAGS_vmlinux	:=--no-undefined -X
+LDFLAGS_vmlinux	:=--no-undefined -X --pic-veneer
 
 ifeq ($(CONFIG_RELOCATABLE), y)
 # Pass --no-apply-dynamic-relocs to restore pre-binutils-2.27 behaviour