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: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 66 insertions, 5 deletions

diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
index 28f8365edc4c..221b98312f0c 100644
--- a/arch/arm64/kernel/alternative.c
+++ b/arch/arm64/kernel/alternative.c
@@ -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.
+         */
+        BUG();
+}
+
+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))
                         continue;
 
@@ -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);
+                }
+
                 flush_icache_range((uintptr_t)origptr,
-                                   (uintptr_t)(origptr + alt->alt_len));
+                                   (uintptr_t)(origptr + nr_inst));
         }
 
         return 0;