ARM: kprobes: Infrastructure for table driven decoding of CPU instructions
The existing ARM instruction decoding functions are a mass of if/else
code. Rather than follow this pattern for Thumb instruction decoding
this patch implements an infrastructure for a new table driven scheme.
This has several advantages:
- Reduces the kernel size by approx 2kB. (The ARM instruction decoding
will eventually have -3.1kB code, +1.3kB data; with similar or better
estimated savings for Thumb decoding.)
- Allows programmatic checking of decoding consistency and test case
coverage.
- Provides more uniform source code and is therefore, arguably, clearer.
For a detailed explanation of how decoding tables work see the in-source
documentation in kprobes.h, and also for kprobe_decode_insn().
Signed-off-by: Jon Medhurst <tixy@yxit.co.uk>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
diff --git a/arch/arm/kernel/kprobes-common.c b/arch/arm/kernel/kprobes-common.c
index 87e91ce..1cb6a82 100644
--- a/arch/arm/kernel/kprobes-common.c
+++ b/arch/arm/kernel/kprobes-common.c
@@ -140,3 +140,261 @@
&__check_hi, &__check_ls, &__check_ge, &__check_lt,
&__check_gt, &__check_le, &__check_al, &__check_al
};
+
+
+/*
+ * Prepare an instruction slot to receive an instruction for emulating.
+ * This is done by placing a subroutine return after the location where the
+ * instruction will be placed. We also modify ARM instructions to be
+ * unconditional as the condition code will already be checked before any
+ * emulation handler is called.
+ */
+static kprobe_opcode_t __kprobes
+prepare_emulated_insn(kprobe_opcode_t insn, struct arch_specific_insn *asi,
+ bool thumb)
+{
+#ifdef CONFIG_THUMB2_KERNEL
+ if (thumb) {
+ u16 *thumb_insn = (u16 *)asi->insn;
+ thumb_insn[1] = 0x4770; /* Thumb bx lr */
+ thumb_insn[2] = 0x4770; /* Thumb bx lr */
+ return insn;
+ }
+ asi->insn[1] = 0xe12fff1e; /* ARM bx lr */
+#else
+ asi->insn[1] = 0xe1a0f00e; /* mov pc, lr */
+#endif
+ /* Make an ARM instruction unconditional */
+ if (insn < 0xe0000000)
+ insn = (insn | 0xe0000000) & ~0x10000000;
+ return insn;
+}
+
+/*
+ * Write a (probably modified) instruction into the slot previously prepared by
+ * prepare_emulated_insn
+ */
+static void __kprobes
+set_emulated_insn(kprobe_opcode_t insn, struct arch_specific_insn *asi,
+ bool thumb)
+{
+#ifdef CONFIG_THUMB2_KERNEL
+ if (thumb) {
+ u16 *ip = (u16 *)asi->insn;
+ if (is_wide_instruction(insn))
+ *ip++ = insn >> 16;
+ *ip++ = insn;
+ return;
+ }
+#endif
+ asi->insn[0] = insn;
+}
+
+/*
+ * When we modify the register numbers encoded in an instruction to be emulated,
+ * the new values come from this define. For ARM and 32-bit Thumb instructions
+ * this gives...
+ *
+ * bit position 16 12 8 4 0
+ * ---------------+---+---+---+---+---+
+ * register r2 r0 r1 -- r3
+ */
+#define INSN_NEW_BITS 0x00020103
+
+/* Each nibble has same value as that at INSN_NEW_BITS bit 16 */
+#define INSN_SAMEAS16_BITS 0x22222222
+
+/*
+ * Validate and modify each of the registers encoded in an instruction.
+ *
+ * Each nibble in regs contains a value from enum decode_reg_type. For each
+ * non-zero value, the corresponding nibble in pinsn is validated and modified
+ * according to the type.
+ */
+static bool __kprobes decode_regs(kprobe_opcode_t* pinsn, u32 regs)
+{
+ kprobe_opcode_t insn = *pinsn;
+ kprobe_opcode_t mask = 0xf; /* Start at least significant nibble */
+
+ for (; regs != 0; regs >>= 4, mask <<= 4) {
+
+ kprobe_opcode_t new_bits = INSN_NEW_BITS;
+
+ switch (regs & 0xf) {
+
+ case REG_TYPE_NONE:
+ /* Nibble not a register, skip to next */
+ continue;
+
+ case REG_TYPE_ANY:
+ /* Any register is allowed */
+ break;
+
+ case REG_TYPE_SAMEAS16:
+ /* Replace register with same as at bit position 16 */
+ new_bits = INSN_SAMEAS16_BITS;
+ break;
+
+ case REG_TYPE_SP:
+ /* Only allow SP (R13) */
+ if ((insn ^ 0xdddddddd) & mask)
+ goto reject;
+ break;
+
+ case REG_TYPE_PC:
+ /* Only allow PC (R15) */
+ if ((insn ^ 0xffffffff) & mask)
+ goto reject;
+ break;
+
+ case REG_TYPE_NOSP:
+ /* Reject SP (R13) */
+ if (((insn ^ 0xdddddddd) & mask) == 0)
+ goto reject;
+ break;
+
+ case REG_TYPE_NOSPPC:
+ case REG_TYPE_NOSPPCX:
+ /* Reject SP and PC (R13 and R15) */
+ if (((insn ^ 0xdddddddd) & 0xdddddddd & mask) == 0)
+ goto reject;
+ break;
+
+ case REG_TYPE_NOPCWB:
+ if (!is_writeback(insn))
+ break; /* No writeback, so any register is OK */
+ /* fall through... */
+ case REG_TYPE_NOPC:
+ case REG_TYPE_NOPCX:
+ /* Reject PC (R15) */
+ if (((insn ^ 0xffffffff) & mask) == 0)
+ goto reject;
+ break;
+ }
+
+ /* Replace value of nibble with new register number... */
+ insn &= ~mask;
+ insn |= new_bits & mask;
+ }
+
+ *pinsn = insn;
+ return true;
+
+reject:
+ return false;
+}
+
+static const int decode_struct_sizes[NUM_DECODE_TYPES] = {
+ [DECODE_TYPE_TABLE] = sizeof(struct decode_table),
+ [DECODE_TYPE_CUSTOM] = sizeof(struct decode_custom),
+ [DECODE_TYPE_SIMULATE] = sizeof(struct decode_simulate),
+ [DECODE_TYPE_EMULATE] = sizeof(struct decode_emulate),
+ [DECODE_TYPE_OR] = sizeof(struct decode_or),
+ [DECODE_TYPE_REJECT] = sizeof(struct decode_reject)
+};
+
+/*
+ * kprobe_decode_insn operates on data tables in order to decode an ARM
+ * architecture instruction onto which a kprobe has been placed.
+ *
+ * These instruction decoding tables are a concatenation of entries each
+ * of which consist of one of the following structs:
+ *
+ * decode_table
+ * decode_custom
+ * decode_simulate
+ * decode_emulate
+ * decode_or
+ * decode_reject
+ *
+ * Each of these starts with a struct decode_header which has the following
+ * fields:
+ *
+ * type_regs
+ * mask
+ * value
+ *
+ * The least significant DECODE_TYPE_BITS of type_regs contains a value
+ * from enum decode_type, this indicates which of the decode_* structs
+ * the entry contains. The value DECODE_TYPE_END indicates the end of the
+ * table.
+ *
+ * When the table is parsed, each entry is checked in turn to see if it
+ * matches the instruction to be decoded using the test:
+ *
+ * (insn & mask) == value
+ *
+ * If no match is found before the end of the table is reached then decoding
+ * fails with INSN_REJECTED.
+ *
+ * When a match is found, decode_regs() is called to validate and modify each
+ * of the registers encoded in the instruction; the data it uses to do this
+ * is (type_regs >> DECODE_TYPE_BITS). A validation failure will cause decoding
+ * to fail with INSN_REJECTED.
+ *
+ * Once the instruction has passed the above tests, further processing
+ * depends on the type of the table entry's decode struct.
+ *
+ */
+int __kprobes
+kprobe_decode_insn(kprobe_opcode_t insn, struct arch_specific_insn *asi,
+ const union decode_item *table, bool thumb)
+{
+ const struct decode_header *h = (struct decode_header *)table;
+ const struct decode_header *next;
+ bool matched = false;
+
+ insn = prepare_emulated_insn(insn, asi, thumb);
+
+ for (;; h = next) {
+ enum decode_type type = h->type_regs.bits & DECODE_TYPE_MASK;
+ u32 regs = h->type_regs.bits >> DECODE_TYPE_BITS;
+
+ if (type == DECODE_TYPE_END)
+ return INSN_REJECTED;
+
+ next = (struct decode_header *)
+ ((uintptr_t)h + decode_struct_sizes[type]);
+
+ if (!matched && (insn & h->mask.bits) != h->value.bits)
+ continue;
+
+ if (!decode_regs(&insn, regs))
+ return INSN_REJECTED;
+
+ switch (type) {
+
+ case DECODE_TYPE_TABLE: {
+ struct decode_table *d = (struct decode_table *)h;
+ next = (struct decode_header *)d->table.table;
+ break;
+ }
+
+ case DECODE_TYPE_CUSTOM: {
+ struct decode_custom *d = (struct decode_custom *)h;
+ return (*d->decoder.decoder)(insn, asi);
+ }
+
+ case DECODE_TYPE_SIMULATE: {
+ struct decode_simulate *d = (struct decode_simulate *)h;
+ asi->insn_handler = d->handler.handler;
+ return INSN_GOOD_NO_SLOT;
+ }
+
+ case DECODE_TYPE_EMULATE: {
+ struct decode_emulate *d = (struct decode_emulate *)h;
+ asi->insn_handler = d->handler.handler;
+ set_emulated_insn(insn, asi, thumb);
+ return INSN_GOOD;
+ }
+
+ case DECODE_TYPE_OR:
+ matched = true;
+ break;
+
+ case DECODE_TYPE_REJECT:
+ default:
+ return INSN_REJECTED;
+ }
+ }
+ }