[PATCH] riscv/fault: Dump user opcode bytes on fatal faults

[Yunhui Cui]

We encountered such a problem(logs are below). We are more curious about
which instruction execution caused the exception. After dumping it
through show_opcodes(), we found that it was caused by a floating-point
instruction.

In this way, we found the problem: in the system bringup , it is
precisely that we have not enabled the floating point function.

So when an exception occurs, it is necessary to dump the instruction
that caused the exception, like x86/fault (ba54d856a9d8).

Logs:
[    0.822481] Run /init as init process
[    0.837569] init[1]: unhandled signal 4 code 0x1 at 0x000000000005e028 in bb[10000+5fe000]
[    0.932292] CPU: 0 PID: 1 Comm: init Not tainted 5.14.0-rc4-00048-g4a843c9043e8-dirty #138
[    0.932949] Hardware name:  , BIOS
[    0.933399] epc : 000000000005e028 ra : 000000000007c7c4 sp : 0000003fffd45da0
[    0.933855]  gp : ffffffff816ea2d8 tp : 0000000000000000 t0 : 0000000000000000
[    0.934303]  t1 : 0000003fffd35df0 t2 : 0000000000000000 s0 : 0000000000000000
[    0.934734]  s1 : 0000000000000000 a0 : 0000003fffd36190 a1 : 0000003fffd45e18
[    0.935200]  a2 : 0000000000000000 a3 : 0000000000000000 a4 : 0000000000000000
[    0.935622]  a5 : 0000000000000000 a6 : 0000000000000000 a7 : 0000000000000000
[    0.936073]  s2 : 0000000000000000 s3 : 0000000000000000 s4 : 0000000000000000
[    0.936495]  s5 : 0000000000000000 s6 : 0000000000000000 s7 : 0000000000000000
[    0.936947]  s8 : 0000000000000000 s9 : 0000000000000000 s10: 0000000000000000
[    0.937487]  s11: 0000000000d14980 t3 : 0000000000000000 t4 : 0000000000000000
[    0.937954]  t5 : 0000000000000000 t6 : 0000000000000000
[    0.938510] status: 0000000200000020 badaddr: 00000000f0028053 cause: 0000000000000002

Signed-off-by: Yunhui Cui <cuiyunhui at bytedance.com>
---
 arch/riscv/include/asm/bug.h |  1 +
 arch/riscv/kernel/process.c  | 30 ++++++++++++++++++++++++++++++
 arch/riscv/kernel/traps.c    |  1 +
 3 files changed, 32 insertions(+)

diff --git a/arch/riscv/include/asm/bug.h b/arch/riscv/include/asm/bug.h
index d3804a2f9aad..77655dd10a2c 100644
--- a/arch/riscv/include/asm/bug.h
+++ b/arch/riscv/include/asm/bug.h
@@ -86,6 +86,7 @@ struct pt_regs;
 struct task_struct;
 
 void __show_regs(struct pt_regs *regs);
+void show_opcodes(struct pt_regs *regs);
 void die(struct pt_regs *regs, const char *str);
 void do_trap(struct pt_regs *regs, int signo, int code, unsigned long addr);
 
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 03ac3aa611f5..759dc74fe160 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -83,6 +83,36 @@ void show_regs(struct pt_regs *regs)
 		dump_backtrace(regs, NULL, KERN_DEFAULT);
 }
 
+static int copy_code(struct pt_regs *regs, u8 *buf, unsigned long src,
+		     unsigned int nbytes)
+{
+	if (!user_mode(regs))
+		return copy_from_kernel_nofault(buf, (u8 *)src, nbytes);
+
+	/* The user space code from other tasks cannot be accessed. */
+	if (regs != task_pt_regs(current))
+		return -EPERM;
+
+	return copy_from_user_nofault(buf, (void __user *)src, nbytes);
+}
+
+void show_opcodes(struct pt_regs *regs)
+{
+	u8 opcodes[4];
+
+	switch (copy_code(regs, opcodes, regs->epc, sizeof(opcodes))) {
+	case 0:
+		pr_info("Opcode: %4ph", opcodes);
+		break;
+	case -EPERM:
+		pr_err("Unable to access userspace of other tasks");
+		break;
+	default:
+		pr_err("Failed to access opcode");
+		break;
+	}
+}
+
 void start_thread(struct pt_regs *regs, unsigned long pc,
 	unsigned long sp)
 {
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index 0a98fd0ddfe9..a6f6851e4e87 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -68,6 +68,7 @@ void do_trap(struct pt_regs *regs, int signo, int code, unsigned long addr)
 		print_vma_addr(KERN_CONT " in ", instruction_pointer(regs));
 		pr_cont("\n");
 		__show_regs(regs);
+		show_opcodes(regs);
 	}
 
 	force_sig_fault(signo, code, (void __user *)addr);
-- 
2.20.1