arm64: fix .idmap.text assertion for large kernels

When building a kernel with many debug options enabled (which happens in
test configurations use by myself and syzbot), the kernel can become
large enough that portions of .text can be more than 128M away from
.idmap.text (which is placed inside the .rodata section). Where idmap
code branches into .text, the linker will place veneers in the
.idmap.text section to make those branches possible.

Unfortunately, as Ard reports, GNU LD has bseen observed to add 4K of
padding when adding such veneers, e.g.

| .idmap.text    0xffffffc01e48e5c0      0x32c arch/arm64/mm/proc.o
|                0xffffffc01e48e5c0                idmap_cpu_replace_ttbr1
|                0xffffffc01e48e600                idmap_kpti_install_ng_mappings
|                0xffffffc01e48e800                __cpu_setup
| *fill*         0xffffffc01e48e8ec        0x4
| .idmap.text.stub
|                0xffffffc01e48e8f0       0x18 linker stubs
|                0xffffffc01e48f8f0                __idmap_text_end = .
|                0xffffffc01e48f000                . = ALIGN (0x1000)
| *fill*         0xffffffc01e48f8f0      0x710
|                0xffffffc01e490000                idmap_pg_dir = .

This makes the __idmap_text_start .. __idmap_text_end region bigger than
the 4K we require it to fit within, and triggers an assertion in arm64's
vmlinux.lds.S, which breaks the build:

| LD      .tmp_vmlinux.kallsyms1
| aarch64-linux-gnu-ld: ID map text too big or misaligned
| make[1]: *** [scripts/Makefile.vmlinux:35: vmlinux] Error 1
| make: *** [Makefile:1264: vmlinux] Error 2

Avoid this by using an `ADRP+ADD+BLR` sequence for branches out of
.idmap.text, which avoids the need for veneers. These branches are only
executed once per boot, and only when the MMU is on, so there should be
no noticeable performance penalty in replacing `BL` with `ADRP+ADD+BLR`.

At the same time, remove the "x" and "w" attributes when placing code in
.idmap.text, as these are not necessary, and this will prevent the
linker from assuming that it is safe to place PLTs into .idmap.text,
causing it to warn if and when there are out-of-range branches within
.idmap.text, e.g.

|   LD      .tmp_vmlinux.kallsyms1
| arch/arm64/kernel/head.o: in function `primary_entry':
| (.idmap.text+0x1c): relocation truncated to fit: R_AARCH64_CALL26 against symbol `dcache_clean_poc' defined in .text section in arch/arm64/mm/cache.o
| arch/arm64/kernel/head.o: in function `init_el2':
| (.idmap.text+0x88): relocation truncated to fit: R_AARCH64_CALL26 against symbol `dcache_clean_poc' defined in .text section in arch/arm64/mm/cache.o
| make[1]: *** [scripts/Makefile.vmlinux:34: vmlinux] Error 1
| make: *** [Makefile:1252: vmlinux] Error 2

Thus, if future changes add out-of-range branches in .idmap.text, it
should be easy enough to identify those from the resulting linker
errors.

Reported-by:  <syzbot+f8ac312e31226e23302b@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/linux-arm-kernel/00000000000028ea4105f4e2ef54@google.com/


Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Will Deacon <will@kernel.org>
Tested-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20230220162317.1581208-1-mark.rutland@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>