[PATCH 1/8] riscv/kprobe: Prepare the skeleton to implement RISCV OPTPROBES feature

[Chen Guokai]

From: Liao Chang <liaochang1@xxxxxxxxxx>

Prepare skeleton to implement optimized kprobe on RISCV, it consists
of Makfile, Kconfig and some architecture specific files: kprobe.h and
opt.c opt.c includes some macro, type definition and functions required
by kprobe framework, opt_trampoline.S provides a piece of assembly code
template used to construct the detour buffer as the target of long jump
instruction(s) for each optimzed kprobe.

Since the jump range of PC-relative instruction JAL is +/-1M, that is
too small to reach the detour buffer, hence the foudamental idea to
address OPTPROBES on RISCV is to replace 'EBREAK' with 'AUIPC+JALR'. which
means it needs to clobber one more instruction beside the kprobe
instruction, furthermore, RISCV supports hybird RVI and RVC in single
kernel binary, so in theory a pair of 'AUIPC/JALR' is about to clobber
10 bytes(3 RVC and 1 RVI, 2 bytes is padding for alignment) at worst
case. The second hardsome problem is looking for one integer register as
the destination of 'AUIPC/JALR' without any side-effect.

More solution details will be introduced in the coming commits.

Co-developed-by: Chen Guokai <chenguokai17@xxxxxxxxxxxxxxxx>
Signed-off-by: Chen Guokai <chenguokai17@xxxxxxxxxxxxxxxx>
Signed-off-by: Liao Chang <liaochang1@xxxxxxxxxx>
---
 arch/riscv/Kconfig                        |  1 +
 arch/riscv/include/asm/kprobes.h          | 32 ++++++++++++++
 arch/riscv/kernel/probes/Makefile         |  1 +
 arch/riscv/kernel/probes/opt.c            | 51 +++++++++++++++++++++++
 arch/riscv/kernel/probes/opt_trampoline.S | 12 ++++++
 5 files changed, 97 insertions(+)
 create mode 100644 arch/riscv/kernel/probes/opt.c
 create mode 100644 arch/riscv/kernel/probes/opt_trampoline.S

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 6b48a3ae9843..ca29306c93e2 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -99,6 +99,7 @@ config RISCV
 	select HAVE_KPROBES if !XIP_KERNEL
 	select HAVE_KPROBES_ON_FTRACE if !XIP_KERNEL
 	select HAVE_KRETPROBES if !XIP_KERNEL
+	select HAVE_OPTPROBES if !XIP_KERNEL
 	select HAVE_MOVE_PMD
 	select HAVE_MOVE_PUD
 	select HAVE_PCI
diff --git a/arch/riscv/include/asm/kprobes.h b/arch/riscv/include/asm/kprobes.h
index 217ef89f22b9..22b73a2fd1fd 100644
--- a/arch/riscv/include/asm/kprobes.h
+++ b/arch/riscv/include/asm/kprobes.h
@@ -43,5 +43,37 @@ bool kprobe_single_step_handler(struct pt_regs *regs);
 void __kretprobe_trampoline(void);
 void __kprobes *trampoline_probe_handler(struct pt_regs *regs);
 
+#ifdef CONFIG_OPTPROBES
+
+/* optinsn template addresses */
+extern __visible kprobe_opcode_t optprobe_template_entry[];
+extern __visible kprobe_opcode_t optprobe_template_end[];
+
+#define MAX_OPTINSN_SIZE				\
+	((unsigned long)optprobe_template_end -		\
+	 (unsigned long)optprobe_template_entry)
+
+/*
+ * For RVI and RVC hybird encoding kernel, althought long jump just needs
+ * 2 RVI instructions(AUIPC+JALR), optimized instructions is 10 bytes long
+ * at most to ensure no RVI would be truncated actually, so it means four
+ * combinations:
+ * - 2 RVI
+ * - 4 RVC
+ * - 2 RVC + 1 RVI
+ * - 3 RVC + 1 RVI (truncated, need padding)
+ */
+#define MAX_COPIED_INSN		4
+#define MAX_OPTIMIZED_LENGTH	10
+
+struct arch_optimized_insn {
+	kprobe_opcode_t copied_insn[MAX_COPIED_INSN];
+	/* detour code buffer */
+	kprobe_opcode_t *insn;
+	unsigned long length;
+	int rd;
+};
+
+#endif /* CONFIG_OPTPROBES */
 #endif /* CONFIG_KPROBES */
 #endif /* _ASM_RISCV_KPROBES_H */
diff --git a/arch/riscv/kernel/probes/Makefile b/arch/riscv/kernel/probes/Makefile
index 7f0840dcc31b..6255b4600875 100644
--- a/arch/riscv/kernel/probes/Makefile
+++ b/arch/riscv/kernel/probes/Makefile
@@ -3,4 +3,5 @@ obj-$(CONFIG_KPROBES)		+= kprobes.o decode-insn.o simulate-insn.o
 obj-$(CONFIG_KPROBES)		+= kprobes_trampoline.o
 obj-$(CONFIG_KPROBES_ON_FTRACE)	+= ftrace.o
 obj-$(CONFIG_UPROBES)		+= uprobes.o decode-insn.o simulate-insn.o
+obj-$(CONFIG_OPTPROBES)		+= opt.o opt_trampoline.o
 CFLAGS_REMOVE_simulate-insn.o = $(CC_FLAGS_FTRACE)
diff --git a/arch/riscv/kernel/probes/opt.c b/arch/riscv/kernel/probes/opt.c
new file mode 100644
index 000000000000..56c8a227c857
--- /dev/null
+++ b/arch/riscv/kernel/probes/opt.c
@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ *  Kernel Probes Jump Optimization (Optprobes)
+ *
+ * Copyright (C) Guokai Chen, 2022
+ * Author: Guokai Chen chenguokai17@xxxxxxxxxxxxxxxx
+ */
+
+#define pr_fmt(fmt)	"optprobe: " fmt
+
+#include <linux/kprobes.h>
+#include <asm/kprobes.h>
+
+int arch_prepared_optinsn(struct arch_optimized_insn *optinsn)
+{
+	return 0;
+}
+
+int arch_check_optimized_kprobe(struct optimized_kprobe *op)
+{
+	return 0;
+}
+
+int arch_prepare_optimized_kprobe(struct optimized_kprobe *op,
+				  struct kprobe *orig)
+{
+	return 0;
+}
+
+void arch_remove_optimized_kprobe(struct optimized_kprobe *op)
+{
+}
+
+void arch_optimize_kprobes(struct list_head *oplist)
+{
+}
+
+void arch_unoptimize_kprobes(struct list_head *oplist,
+			     struct list_head *done_list)
+{
+}
+
+void arch_unoptimize_kprobe(struct optimized_kprobe *op)
+{
+}
+
+int arch_within_optimized_kprobe(struct optimized_kprobe *op,
+				 kprobe_opcode_t *addr)
+{
+	return 0;
+}
diff --git a/arch/riscv/kernel/probes/opt_trampoline.S b/arch/riscv/kernel/probes/opt_trampoline.S
new file mode 100644
index 000000000000..16160c4367ff
--- /dev/null
+++ b/arch/riscv/kernel/probes/opt_trampoline.S
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2022 Guokai Chen
+ */
+
+#include <linux/linkage.h>
+
+#incldue <asm/csr.h>
+#include <asm/asm-offsets.h>
+
+SYM_ENTRY(optprobe_template_entry, SYM_L_GLOBAL, SYM_A_NONE)
+SYM_ENTRY(optprobe_template_end, SYM_L_GLOBAL, SYM_A_NONE)
-- 
2.25.1