On 23 August 2016 at 11:16, Suzuki K Poulose <[email protected]> wrote: > On 22/08/16 12:45, Ard Biesheuvel wrote: >> >> On 18 August 2016 at 15:10, Suzuki K Poulose <[email protected]> >> wrote: >>> >>> 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 <[email protected]> >>> Cc: Marc Zyngier <[email protected]> >>> Cc: Andre Przywara <[email protected]> >>> Cc: Mark Rutland <[email protected]> >>> Signed-off-by: Suzuki K Poulose <[email protected]> >>> --- >>> 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.
