We've so far relied on a patching infrastructure that only gave us
a single alternative, without any way to finely control what gets
patched. For a single feature, this is an all or nothing thing.
It would be interesting to have a more fine grained way of patching
the kernel though, where we could dynamically tune the code that gets
injected.
In order to achive this, let's introduce a new form of alternative
that is associated with a callback. This callback gets the instruction
sequence number and the old instruction as a parameter, and returns
the new instruction. This callback is always called, as the patching
decision is now done at runtime (not patching is equivalent to returning
the same instruction).
Patching with a callback is declared with the new ALTERNATIVE_CB
and alternative_cb directives:
asm volatile(ALTERNATIVE_CB("mov %0, #0\n", callback)
: "r" (v));
or
alternative_cb callback
mov x0, #0
alternative_cb_end
where callback is the C function computing the alternative.
Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
---
arch/arm64/include/asm/alternative.h | 36 ++++++++++++++++++++++++++----
arch/arm64/include/asm/alternative_types.h | 3 +++
arch/arm64/kernel/alternative.c | 21 +++++++++++++----
3 files changed, 52 insertions(+), 8 deletions(-)
diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h
index 395befde7595..04f66f6173fc 100644
--- a/arch/arm64/include/asm/alternative.h
+++ b/arch/arm64/include/asm/alternative.h
@@ -18,10 +18,14 @@
void __init apply_alternatives_all(void);
void apply_alternatives(void *start, size_t length);
-#define ALTINSTR_ENTRY(feature) \
+#define ALTINSTR_ENTRY(feature,cb) \
" .align " __stringify(ALTINSTR_ALIGN) "\n" \
" .word 661b - .\n" /* label */ \
+ " .if " __stringify(cb) " == 0\n" \
" .word 663f - .\n" /* new instruction */ \
+ " .else\n" \
+ " .word " __stringify(cb) "- .\n" /* callback */ \
+ " .endif\n" \
" .hword " __stringify(feature) "\n" /* feature bit */ \
" .byte 662b-661b\n" /* source len */ \
" .byte 664f-663f\n" /* replacement len */
@@ -39,15 +43,18 @@ void apply_alternatives(void *start, size_t length);
* but most assemblers die if insn1 or insn2 have a .inst. This should
* be fixed in a binutils release posterior to 2.25.51.0.2 (anything
* containing commit 4e4d08cf7399b606 or c1baaddf8861).
+ *
+ * Alternatives with callbacks do not generate replacement instructions.
*/
-#define __ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg_enabled) \
+#define __ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg_enabled, cb) \
".if "__stringify(cfg_enabled)" == 1\n" \
"661:\n\t" \
oldinstr "\n" \
"662:\n" \
".pushsection .altinstructions,\"a\"\n" \
- ALTINSTR_ENTRY(feature) \
+ ALTINSTR_ENTRY(feature,cb) \
".popsection\n" \
+ " .if " __stringify(cb) " == 0\n" \
".pushsection .altinstr_replacement, \"a\"\n" \
"663:\n\t" \
newinstr "\n" \
@@ -55,11 +62,17 @@ void apply_alternatives(void *start, size_t length);
".popsection\n\t" \
".org . - (664b-663b) + (662b-661b)\n\t" \
".org . - (662b-661b) + (664b-663b)\n" \
+ ".else\n\t" \
+ "663:\n\t" \
+ "664:\n\t" \
+ ".endif\n" \
".endif\n"
#define _ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg, ...) \
- __ALTERNATIVE_CFG(oldinstr, newinstr, feature, IS_ENABLED(cfg))
+ __ALTERNATIVE_CFG(oldinstr, newinstr, feature, IS_ENABLED(cfg), 0)
+#define ALTERNATIVE_CB(oldinstr, cb) \
+ __ALTERNATIVE_CFG(oldinstr, "NOT_AN_INSTRUCTION", ARM64_NCAPS, 1, cb)
#else
#include <asm/assembler.h>
@@ -127,6 +140,14 @@ void apply_alternatives(void *start, size_t length);
661:
.endm
+.macro alternative_cb cb
+ .set .Lasm_alt_mode, 0
+ .pushsection .altinstructions, "a"
+ altinstruction_entry 661f, \cb, ARM64_NCAPS, 662f-661f, 0
+ .popsection
+661:
+.endm
+
/*
* Provide the other half of the alternative code sequence.
*/
@@ -152,6 +173,13 @@ void apply_alternatives(void *start, size_t length);
.org . - (662b-661b) + (664b-663b)
.endm
+/*
+ * Callback-based alternative epilogue
+ */
+.macro alternative_cb_end
+662:
+.endm
+
/*
* Provides a trivial alternative or default sequence consisting solely
* of NOPs. The number of NOPs is chosen automatically to match the
diff --git a/arch/arm64/include/asm/alternative_types.h b/arch/arm64/include/asm/alternative_types.h
index 26cf76167f2d..513f3985d455 100644
--- a/arch/arm64/include/asm/alternative_types.h
+++ b/arch/arm64/include/asm/alternative_types.h
@@ -2,6 +2,9 @@
#ifndef __ASM_ALTERNATIVE_TYPES_H
#define __ASM_ALTERNATIVE_TYPES_H
+struct alt_instr;
+typedef u32 (*alternative_cb_t)(struct alt_instr *alt, int index, u32 new_insn);
+
struct alt_instr {
s32 orig_offset; /* offset to original instruction */
s32 alt_offset; /* offset to replacement instruction */
diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
index 6dd0a3a3e5c9..cd299af96c95 100644
--- a/arch/arm64/kernel/alternative.c
+++ b/arch/arm64/kernel/alternative.c
@@ -110,25 +110,38 @@ static void __apply_alternatives(void *alt_region, bool use_linear_alias)
struct alt_instr *alt;
struct alt_region *region = alt_region;
__le32 *origptr, *replptr, *updptr;
+ alternative_cb_t alt_cb;
for (alt = region->begin; alt < region->end; alt++) {
u32 insn;
int i, nr_inst;
- if (!cpus_have_cap(alt->cpufeature))
+ /* Use ARM64_NCAPS as an unconditional patch */
+ if (alt->cpufeature < ARM64_NCAPS &&
+ !cpus_have_cap(alt->cpufeature))
continue;
- BUG_ON(alt->alt_len != alt->orig_len);
+ if (alt->cpufeature == ARM64_NCAPS)
+ BUG_ON(alt->alt_len != 0);
+ else
+ BUG_ON(alt->alt_len != alt->orig_len);
pr_info_once("patching kernel code\n");
origptr = ALT_ORIG_PTR(alt);
replptr = ALT_REPL_PTR(alt);
+ alt_cb = ALT_REPL_PTR(alt);
updptr = use_linear_alias ? lm_alias(origptr) : origptr;
- nr_inst = alt->alt_len / sizeof(insn);
+ nr_inst = alt->orig_len / sizeof(insn);
for (i = 0; i < nr_inst; i++) {
- insn = get_alt_insn(alt, origptr + i, replptr + i);
+ if (alt->cpufeature == ARM64_NCAPS) {
+ insn = le32_to_cpu(updptr[i]);
+ insn = alt_cb(alt, i, insn);
+ } else {
+ insn = get_alt_insn(alt, origptr + i,
+ replptr + i);
+ }
updptr[i] = cpu_to_le32(insn);
}
--
2.14.2
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