2023-03-29 08:40:55

by yunhui cui

Subject: [PATCH v2] riscv: Dump user opcode bytes on fatal faults

We encountered such a problem that when the system starts to execute
init, init exits unexpectedly with error message: "unhandled signal 4
code 0x1 ...".

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(CONFIG_FPU
is set, but not enalbe COMPAT_HWCAP_ISA_F/D in the dts or acpi).

Like commit ba54d856a9d8 ("x86/fault: Dump user opcode bytes on fatal
faults"), when an exception occurs, it is necessary to dump the
instruction that caused the exception.

Signed-off-by: Yunhui Cui <[email protected]>
---
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 1aaea81fb141..56dab998d05d 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 774ffde386ab..9ba9f8719605 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;
+ }
+}
+
#ifdef CONFIG_COMPAT
static bool compat_mode_supported __read_mostly;

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f6fda94e8e59..892826234ee9 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -100,6 +100,7 @@ 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

2023-04-11 12:36:42

by yunhui cui

Subject: Re: [PATCH v2] riscv: Dump user opcode bytes on fatal faults

Dear All,

Any comments on this patch?

Thanks,
Yunhui

2023-08-16 21:35:09

by Björn Töpel

Subject: Re: [PATCH v2] riscv: Dump user opcode bytes on fatal faults

Hi Yunhui,

Waking up the dead! ;-)

Yunhui Cui <[email protected]> writes:

> We encountered such a problem that when the system starts to execute
> init, init exits unexpectedly with error message: "unhandled signal 4
> code 0x1 ...".
>
> 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(CONFIG_FPU
> is set, but not enalbe COMPAT_HWCAP_ISA_F/D in the dts or acpi).
>
> Like commit ba54d856a9d8 ("x86/fault: Dump user opcode bytes on fatal
> faults"), when an exception occurs, it is necessary to dump the
> instruction that caused the exception.

X86's show_opcodes() is used both for kernel oops:es, and userland
unhandled signals. On RISC-V there's dump_kernel_instr() added in commit
eb165bfa8eaf ("riscv: Add instruction dump to RISC-V splats").

Wdyt about reworking that function, so that it works for userland epc as
well? I think it's useful to have the surrounding instruction context,
and not just on instruction.


Björn