Commit 7616fc8b authored by Marc Zyngier's avatar Marc Zyngier Committed by Catalin Marinas

arm64: alternative: Allow immediate branch as alternative instruction

Since all branches are PC-relative on AArch64, these instructions
cannot be used as an alternative with the simplistic approach
we currently have (the immediate has been computed from
the .altinstr_replacement section, and end-up being completely off
if the target is outside of the replacement sequence).

This patch handles the branch instructions in a different way,
using the insn framework to recompute the immediate, and generate
the right displacement in the above case.
Acked-by: default avatarWill Deacon <>
Signed-off-by: default avatarMarc Zyngier <>
Signed-off-by: default avatarCatalin Marinas <>
parent b0dd9c02
......@@ -24,8 +24,13 @@
#include <asm/cacheflush.h>
#include <asm/alternative.h>
#include <asm/cpufeature.h>
#include <asm/insn.h>
#include <linux/stop_machine.h>
#define __ALT_PTR(a,f) (u32 *)((void *)&(a)->f + (a)->f)
#define ALT_ORIG_PTR(a) __ALT_PTR(a, orig_offset)
#define ALT_REPL_PTR(a) __ALT_PTR(a, alt_offset)
extern struct alt_instr __alt_instructions[], __alt_instructions_end[];
struct alt_region {
......@@ -33,13 +38,63 @@ struct alt_region {
struct alt_instr *end;
* Check if the target PC is within an alternative block.
static bool branch_insn_requires_update(struct alt_instr *alt, unsigned long pc)
unsigned long replptr;
if (kernel_text_address(pc))
return 1;
replptr = (unsigned long)ALT_REPL_PTR(alt);
if (pc >= replptr && pc <= (replptr + alt->alt_len))
return 0;
* Branching into *another* alternate sequence is doomed, and
* we're not even trying to fix it up.
static u32 get_alt_insn(struct alt_instr *alt, u32 *insnptr, u32 *altinsnptr)
u32 insn;
insn = le32_to_cpu(*altinsnptr);
if (aarch64_insn_is_branch_imm(insn)) {
s32 offset = aarch64_get_branch_offset(insn);
unsigned long target;
target = (unsigned long)altinsnptr + offset;
* If we're branching inside the alternate sequence,
* do not rewrite the instruction, as it is already
* correct. Otherwise, generate the new instruction.
if (branch_insn_requires_update(alt, target)) {
offset = target - (unsigned long)insnptr;
insn = aarch64_set_branch_offset(insn, offset);
return insn;
static int __apply_alternatives(void *alt_region)
struct alt_instr *alt;
struct alt_region *region = alt_region;
u8 *origptr, *replptr;
u32 *origptr, *replptr;
for (alt = region->begin; alt < region->end; alt++) {
u32 insn;
int i, nr_inst;
if (!cpus_have_cap(alt->cpufeature))
......@@ -47,11 +102,17 @@ static int __apply_alternatives(void *alt_region)
pr_info_once("patching kernel code\n");
origptr = (u8 *)&alt->orig_offset + alt->orig_offset;
replptr = (u8 *)&alt->alt_offset + alt->alt_offset;
memcpy(origptr, replptr, alt->alt_len);
origptr = ALT_ORIG_PTR(alt);
replptr = ALT_REPL_PTR(alt);
nr_inst = alt->alt_len / sizeof(insn);
for (i = 0; i < nr_inst; i++) {
insn = get_alt_insn(alt, origptr + i, replptr + i);
*(origptr + i) = cpu_to_le32(insn);
(uintptr_t)(origptr + alt->alt_len));
(uintptr_t)(origptr + nr_inst));
return 0;
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