[PATCH v5 6/9] riscv/kprobe: Add code to check if kprobe can be optimized

Chen Guokai
2022-12-24T15:23:54Z

From: Liao Chang <liaochang1@huawei.com>

This patch add code to check if kprobe can be optimized, regular kprobe
replaces single instruction with EBREAK or C.EBREAK, it just requires
the instrumented instruction support execute out-of-line or simulation,
while optimized kprobe patch AUIPC/JALR pair to do a long jump, it makes
everything more compilated, espeically for kernel that is hybrid RVI and
RVC binary, although AUIPC/JALR just need 8 bytes space, the bytes to
patch are 10 bytes long at worst case to ensure no RVI would be
truncated, so there are four methods to patch optimized kprobe.

- Replace 2 RVI with AUIPC/JALR.
- Replace 4 RVC with AUIPC/JALR.
- Replace 2 RVC and 1 RVI with AUIPC/JALR.
- Replace 3 RVC and 1 RVI with AUIPC/JALR, and patch C.NOP into last
two bytes for alignment.

So it has to find out a instruction window large enough to patch
AUIPC/JALR from the address instrumented breakpoint, meanwhile, ensure
no instruction has chance to jump into the range of patched window.

Signed-off-by: Liao Chang <liaochang1@huawei.com>
Co-developed-by: Chen Guokai <chenguokai17@mails.ucas.ac.cn>
Signed-off-by: Chen Guokai <chenguokai17@mails.ucas.ac.cn>
---
arch/riscv/kernel/probes/opt.c | 98 ++++++++++++++++++++++++++++++++--
1 file changed, 93 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/kernel/probes/opt.c b/arch/riscv/kernel/probes/opt.c
index a0d2ab39e3fa..258a283c906d 100644
--- a/arch/riscv/kernel/probes/opt.c
+++ b/arch/riscv/kernel/probes/opt.c
@@ -271,15 +271,103 @@ static void find_free_registers(struct kprobe *kp, struct optimized_kprobe *op,
*ra = (kw == 1UL) ? 0 : __builtin_ctzl(kw & ~1UL);
}

+static bool insn_jump_into_range(unsigned long addr, unsigned long start,
+ unsigned long end)
+{
+ kprobe_opcode_t insn = *(kprobe_opcode_t *)addr;
+ unsigned long target, offset = GET_INSN_LENGTH(insn);
+
+#ifdef CONFIG_RISCV_ISA_C
+ if (offset == RVC_INSN_LEN) {
+ if (riscv_insn_is_c_beqz(insn) || riscv_insn_is_c_bnez(insn))
+ target = addr + rvc_branch_imme(insn);
+ else if (riscv_insn_is_c_jal(insn) || riscv_insn_is_c_j(insn))
+ target = addr + rvc_jal_imme(insn);
+ else
+ target = 0;
+ return (target >= start) && (target < end);
+ }
+#endif
+
+ if (riscv_insn_is_branch(insn))
+ target = addr + rvi_branch_imme(insn);
+ else if (riscv_insn_is_jal(insn))
+ target = addr + rvi_jal_imme(insn);
+ else
+ target = 0;
+ return (target >= start) && (target < end);
+}
+
+static int search_copied_insn(unsigned long paddr, struct optimized_kprobe *op)
+{
+ int i = 1;
+ unsigned long offset = GET_INSN_LENGTH(*(kprobe_opcode_t *)paddr);
+
+ while ((i++ < MAX_COPIED_INSN) && (offset < 2 * RVI_INSN_LEN)) {
+ if (riscv_probe_decode_insn((probe_opcode_t *)paddr + offset,
+ NULL) != INSN_GOOD)
+ return -1;
+ offset += GET_INSN_LENGTH(*(kprobe_opcode_t *)(paddr + offset));
+ }
+
+ op->optinsn.length = offset;
+ return 0;
+}
+
/*
- * If two free registers can be found at the beginning of both
- * the start and the end of replaced code, it can be optimized
- * Also, in-function jumps need to be checked to make sure that
- * there is no jump to the second instruction to be replaced
+ * The kprobe can be optimized when no in-function jump reaches to the
+ * instructions replaced by optimized jump instructions(AUIPC/JALR).
*/
static bool can_optimize(unsigned long paddr, struct optimized_kprobe *op)
{
- return false;
+ int ret;
+ unsigned long addr, size = 0, offset = 0;
+ struct kprobe *kp = get_kprobe((kprobe_opcode_t *)paddr);
+
+ /*
+ * Skip optimization if kprobe has been disarmed or instrumented
+ * instruction support XOI.
+ */
+ if (!kp || (riscv_probe_decode_insn(&kp->opcode, NULL) != INSN_GOOD))
+ return false;
+
+ /*
+ * Find a instruction window large enough to contain a pair
+ * of AUIPC/JALR, and ensure each instruction in this window
+ * supports XOI.
+ */
+ ret = search_copied_insn(paddr, op);
+ if (ret)
+ return false;
+
+ if (!kallsyms_lookup_size_offset(paddr, &size, &offset))
+ return false;
+
+ /* Check there is enough space for relative jump(AUIPC/JALR) */
+ if (size - offset <= op->optinsn.length)
+ return false;
+
+ /*
+ * Decode instructions until function end, check any instruction
+ * don't jump into the window used to emit optprobe(AUIPC/JALR).
+ */
+ addr = paddr - offset;
+ while (addr < paddr) {
+ if (insn_jump_into_range(addr, paddr + RVC_INSN_LEN,
+ paddr + op->optinsn.length))
+ return false;
+ addr += GET_INSN_LENGTH(*(kprobe_opcode_t *)addr);
+ }
+
+ addr = paddr + op->optinsn.length;
+ while (addr < paddr - offset + size) {
+ if (insn_jump_into_range(addr, paddr + RVC_INSN_LEN,
+ paddr + op->optinsn.length))
+ return false;
+ addr += GET_INSN_LENGTH(*(kprobe_opcode_t *)addr);
+ }
+
+ return true;
}

int arch_prepared_optinsn(struct arch_optimized_insn *optinsn)
--
2.34.1