[PATCH 5/8] arm64: alternative: Add support for patching adrp instructions

On 23 August 2016 at 11:16, Suzuki K Poulose [off-list ref] wrote:

On 22/08/16 12:45, Ard Biesheuvel wrote:

quoted

On 18 August 2016 at 15:10, Suzuki K Poulose [off-list ref]
wrote:

quoted

adrp uses PC-relative address offset to a page (of 4K size) of
a symbol. If it appears in an alternative code patched in, we
should adjust the offset to reflect the address where it will
be run from. This patch adds support for fixing the offset
for adrp instructions.

Cc: Will Deacon <redacted>
Cc: Marc Zyngier <redacted>
Cc: Andre Przywara <andre.przywara@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
---
 arch/arm64/kernel/alternative.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/arm64/kernel/alternative.c

b/arch/arm64/kernel/alternative.c
index d2ee1b2..71c6962 100644

--- a/arch/arm64/kernel/alternative.c
+++ b/arch/arm64/kernel/alternative.c

@@ -80,6 +80,19 @@ static u32 get_alt_insn(struct alt_instr *alt, u32

*insnptr, u32 *altinsnptr)
                        offset = target - (unsigned long)insnptr;
                        insn = aarch64_set_branch_offset(insn, offset);
                }
+       } else if (aarch64_insn_is_adrp(insn)) {
+               s32 orig_offset, new_offset;
+               unsigned long target;
+
+               /*
+                * If we're replacing an adrp instruction, which uses
PC-relative
+                * immediate addressing, adjust the offset to reflect the
new
+                * PC. adrp operates on 4K aligned addresses.
+                */
+               orig_offset  = aarch64_insn_adrp_get_offset(insn);
+               target = ((unsigned long)altinsnptr & ~0xfffUL) +
orig_offset;
+               new_offset = target - ((unsigned long)insnptr &
~0xfffUL);
+               insn = aarch64_insn_adrp_set_offset(insn, new_offset);

Are orig_offset and new_offset guaranteed to be equal modulo 4 KB?
Otherwise, you will have to track down and patch the associated :lo12:
add/ldr instruction as well.

We are modifying the alternative instruction to accommodate for the new PC,
where this instruction will be executed from, while the referenced symbol
remains the same. Hence the associated :lo12: doesn't change. Does that
address your concern ? Or did I miss something ?

Ah, of course. Yes, that should work fine, given that the symbol stays
in place, and so its offset into the containing 4 KB page does not
change either.

Thanks,
Ard.