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Subject[PATCH -next 2/2] riscv: implemented branch simulate instructions
Date
To test the kprobe-based event tracing, we prepare
a kernel module 'kprobe_test.ko' to add the probes.
The assembly codes (partially) of the module are as follows:
...
0000000000000000 <kprobe_test_branch>:
...

0000000000000080 <.L7>:
80: 414986bb subw a3,s3,s4
84: 4785 li a5,1
86: 00d7c363 blt a5,a3,8c <.L8>
8a: 2485 addiw s1,s1,1

000000000000008c <.L8>:
8c: 012987bb addw a5,s3,s2
90: 4711 li a4,4
92: 00f75363 bge a4,a5,98 <.L9>
96: 2485 addiw s1,s1,1
...

00000000000000aa <.L11>:
aa: 01299363 bne s3,s2,b0 <.L12>
ae: 2485 addiw s1,s1,1

00000000000000b0 <.L12>:
b0: 012a0363 beq s4,s2,b6 <.L13>
b4: 2485 addiw s1,s1,1
...

00000000000000c2 <.L14>:
c2: 0009861b sext.w a2,s3
c6: 013a7363 bgeu s4,s3,cc <.L15>
ca: 2485 addiw s1,s1,1
...

00000000000000d2 <.L16>:
d2: 00e7e363 bltu a5,a4,d8 <.L17>
d6: 2485 addiw s1,s1,1
...

Test the kprobe-based event tracing in qemu-system-riscv64:
First, install the kprobe test module:
insmod /root/kprobe_test.ko

Then, add probes as new events at the branch instructions,
i.e. blt, bge, bne, beq, bgeu and bltu.
The following errors occur due to the instructions not allowed to probe yet:
echo "p:blt kprobe_test:kprobe_test_branch+0x86 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
sh: write error: Invalid argument
echo "p:bge kprobe_test:kprobe_test_branch+0x92 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
sh: write error: Invalid argument
echo "p:bne kprobe_test:kprobe_test_branch+0xaa epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
sh: write error: Invalid argument
echo "p:beq kprobe_test:kprobe_test_branch+0xb0 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
sh: write error: Invalid argument
echo "p:bgeu kprobe_test:kprobe_test_branch+0xc6 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
sh: write error: Invalid argument
echo "p:bltu kprobe_test:kprobe_test_branch+0xd2 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
sh: write error: Invalid argument

This patch implemented the branch simulate instructions and allowed to probe them.
Merge this patch and perform the test again, the test results are as follows:
First, add probes at the branch instructions:
echo "p:blt kprobe_test:kprobe_test_branch+0x86 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
echo "p:bge kprobe_test:kprobe_test_branch+0x92 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
echo "p:bne kprobe_test:kprobe_test_branch+0xaa epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
echo "p:beq kprobe_test:kprobe_test_branch+0xb0 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
echo "p:bgeu kprobe_test:kprobe_test_branch+0xc6 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
echo "p:bltu kprobe_test:kprobe_test_branch+0xd2 epc=%epc opcode=+0(%epc):x32" >> /sys/kernel/debug/tracing/kprobe_events
echo 1 > /sys/kernel/debug/tracing/events/kprobes/blt/enable
echo 1 > /sys/kernel/debug/tracing/events/kprobes/bge/enable
echo 1 > /sys/kernel/debug/tracing/events/kprobes/bne/enable
echo 1 > /sys/kernel/debug/tracing/events/kprobes/beq/enable
echo 1 > /sys/kernel/debug/tracing/events/kprobes/bgeu/enable
echo 1 > /sys/kernel/debug/tracing/events/kprobes/bltu/enable

Then, do something to run to the probes.
After that, see the traced information:
cat /sys/kernel/debug/tracing/trace
sysctl-63 [001] d... 2505.263969: blt: (kprobe_test_branch+0x86/0x10e [kprobe_test]) epc=0xffffffff016122f8 opcode=0x100073
sysctl-63 [001] d... 2505.263991: bge: (kprobe_test_branch+0x92/0x10e [kprobe_test]) epc=0xffffffff01612304 opcode=0x100073
sysctl-63 [001] d... 2505.264001: bne: (kprobe_test_branch+0xaa/0x10e [kprobe_test]) epc=0xffffffff0161231c opcode=0x100073
sysctl-63 [001] d... 2505.264011: beq: (kprobe_test_branch+0xb0/0x10e [kprobe_test]) epc=0xffffffff01612322 opcode=0x100073
sysctl-63 [001] d... 2505.264019: bgeu: (kprobe_test_branch+0xc6/0x10e [kprobe_test]) epc=0xffffffff01612338 opcode=0x100073
sysctl-63 [001] d... 2505.264027: bltu: (kprobe_test_branch+0xd2/0x10e [kprobe_test]) epc=0xffffffff01612344 opcode=0x100073

Now we can see the traced information.
The actual address of the symbol 'kprobe_test_branch' is as follows:
cat /proc/kallsyms | grep kprobe_test_branch
ffffffff01612272 t kprobe_test_branch [kprobe_test]

Based on the traced information and the actual address of the symbol
'kprobe_test_branch', we can also see that the branch instructions
have been replaced by 'ebreak(0x100073)' instructions.

The pesudoinstructions of the branch instructions,
i.e. bnez, beqz, blez, bgez, bltz, bgtz, bleu, bgtu and ble
are allowed to probe as well.

--------

Signed-off-by: Chen Lifu <chenlifu@huawei.com>
---
arch/riscv/kernel/probes/decode-insn.c | 3 +-
arch/riscv/kernel/probes/simulate-insn.c | 78 ++++++++++++++++++++++++
2 files changed, 79 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/kernel/probes/decode-insn.c b/arch/riscv/kernel/probes/decode-insn.c
index 5eb03fb61450..64f6183b4717 100644
--- a/arch/riscv/kernel/probes/decode-insn.c
+++ b/arch/riscv/kernel/probes/decode-insn.c
@@ -38,11 +38,10 @@ riscv_probe_decode_insn(probe_opcode_t *addr, struct arch_probe_insn *api)
RISCV_INSN_REJECTED(c_ebreak, insn);
#endif

- RISCV_INSN_REJECTED(branch, insn);
-
RISCV_INSN_SET_SIMULATE(jal, insn);
RISCV_INSN_SET_SIMULATE(jalr, insn);
RISCV_INSN_SET_SIMULATE(auipc, insn);
+ RISCV_INSN_SET_SIMULATE(branch, insn);

return INSN_GOOD;
}
diff --git a/arch/riscv/kernel/probes/simulate-insn.c b/arch/riscv/kernel/probes/simulate-insn.c
index b81719522d5c..efc989b28859 100644
--- a/arch/riscv/kernel/probes/simulate-insn.c
+++ b/arch/riscv/kernel/probes/simulate-insn.c
@@ -117,3 +117,81 @@ bool __kprobes simulate_auipc(u32 opcode, unsigned long addr, struct pt_regs *re

return true;
}
+
+#define branch_rs1_idx(opcode) \
+ (((opcode) >> 15) & 0x1f)
+
+#define branch_rs2_idx(opcode) \
+ (((opcode) >> 20) & 0x1f)
+
+#define branch_funct3(opcode) \
+ (((opcode) >> 12) & 0x7)
+
+#define branch_imm(opcode) \
+ (((((opcode) >> 8 ) & 0xf ) << 1 ) | \
+ ((((opcode) >> 25) & 0x3f) << 5 ) | \
+ ((((opcode) >> 7 ) & 0x1 ) << 11) | \
+ ((((opcode) >> 31) & 0x1 ) << 12))
+
+#define branch_offset(opcode) \
+ sign_extend32((branch_imm(opcode)), 12)
+
+#define BRANCH_BEQ 0x0
+#define BRANCH_BNE 0x1
+#define BRANCH_BLT 0x4
+#define BRANCH_BGE 0x5
+#define BRANCH_BLTU 0x6
+#define BRANCH_BGEU 0x7
+
+bool __kprobes simulate_branch(u32 opcode, unsigned long addr, struct pt_regs *regs)
+{
+ /*
+ * branch instructions:
+ * 31 30 25 24 20 19 15 14 12 11 8 7 6 0
+ * | imm[12] | imm[10:5] | rs2 | rs1 | funct3 | imm[4:1] | imm[11] | opcode |
+ * 1 6 5 5 3 4 1 7
+ * imm[12|10:5] rs2 rs1 000 imm[4:1|11] 1100011 BEQ
+ * imm[12|10:5] rs2 rs1 001 imm[4:1|11] 1100011 BNE
+ * imm[12|10:5] rs2 rs1 100 imm[4:1|11] 1100011 BLT
+ * imm[12|10:5] rs2 rs1 101 imm[4:1|11] 1100011 BGE
+ * imm[12|10:5] rs2 rs1 110 imm[4:1|11] 1100011 BLTU
+ * imm[12|10:5] rs2 rs1 111 imm[4:1|11] 1100011 BGEU
+ */
+
+ s32 offset;
+ s32 offset_tmp;
+ unsigned long rs1_val;
+ unsigned long rs2_val;
+
+ if (!rv_insn_reg_get_val(regs, branch_rs1_idx(opcode), &rs1_val) ||
+ !rv_insn_reg_get_val(regs, branch_rs2_idx(opcode), &rs2_val))
+ return false;
+
+ offset_tmp = branch_offset(opcode);
+ switch (branch_funct3(opcode)) {
+ case BRANCH_BEQ:
+ offset = (rs1_val == rs2_val) ? offset_tmp : 4;
+ break;
+ case BRANCH_BNE:
+ offset = (rs1_val != rs2_val) ? offset_tmp : 4;
+ break;
+ case BRANCH_BLT:
+ offset = ((long)rs1_val < (long)rs2_val) ? offset_tmp : 4;
+ break;
+ case BRANCH_BGE:
+ offset = ((long)rs1_val >= (long)rs2_val) ? offset_tmp : 4;
+ break;
+ case BRANCH_BLTU:
+ offset = (rs1_val < rs2_val) ? offset_tmp : 4;
+ break;
+ case BRANCH_BGEU:
+ offset = (rs1_val >= rs2_val) ? offset_tmp : 4;
+ break;
+ default:
+ return false;
+ }
+
+ instruction_pointer_set(regs, addr + offset);
+
+ return true;
+}
--
2.32.0
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