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Subject: Re: [PATCH v3] RISC-V: Add fixup to support fast call of
 crash_kexec()
To:     Kefeng Wang <wangkefeng.wang@huawei.com>, paul.walmsley@sifive.com,
        palmer@dabbelt.com, aou@eecs.berkeley.edu,
        philipp.tomsich@vrull.eu, ebiederm@xmission.com, heiko@sntech.de,
        vitaly.wool@konsulko.com, tongtiangen@huawei.com, guoren@kernel.org
Cc:     linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
References: <20220606123750.2884245-1-xianting.tian@linux.alibaba.com>
 <b2a45f97-0f97-6621-bb7e-11f51282c468@huawei.com>
From:   Xianting Tian <xianting.tian@linux.alibaba.com>
Message-ID: <b1098514-d185-e649-6dea-3c2f01e97ea8@linux.alibaba.com>
Date:   Tue, 7 Jun 2022 09:46:01 +0800
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在 2022/6/7 上午9:21, Kefeng Wang 写道:
>
> On 2022/6/6 20:37, Xianting Tian wrote:
>> Currently, almost all archs (x86, arm64, mips...) support fast call
>> of crash_kexec() when "regs && kexec_should_crash()" is true. But
>> RISC-V not, it can only enter crash system via panic(). However panic()
>> doesn't pass the regs of the real accident scene to crash_kexec(),
>> it caused we can't get accurate backtrace via gdb,
>>     $ riscv64-linux-gnu-gdb vmlinux vmcore
>>     Reading symbols from vmlinux...
>>     [New LWP 95]
>>     #0  console_unlock () at kernel/printk/printk.c:2557
>>     2557                    if (do_cond_resched)
>>     (gdb) bt
>>     #0  console_unlock () at kernel/printk/printk.c:2557
>>     #1  0x0000000000000000 in ?? ()
>>
>> With the patch we can get the accurate backtrace,
>>     $ riscv64-linux-gnu-gdb vmlinux vmcore
>>     Reading symbols from vmlinux...
>>     [New LWP 95]
>>     #0  0xffffffe00063a4e0 in test_thread (data=<optimized out>) at 
>> drivers/test_crash.c:81
>>     81             *(int *)p = 0xdead;
>>     (gdb)
>>     (gdb) bt
>>     #0  0xffffffe00064d5c0 in test_thread (data=<optimized out>) at 
>> drivers/test_crash.c:81
>>     #1  0x0000000000000000 in ?? ()
>>
>> Test code to produce NULL address dereference in test_crash.c,
>>     void *p = NULL;
>>     *(int *)p = 0xdead;
>>
>> Fixes: 76d2a0493a17 ("RISC-V: Init and Halt Code")
>> Reviewed-by: Guo Ren <guoren@kernel.org>
>> Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
>> ---
>> Changes from v1:
>> - simplify the commit message
>> Changes from v2:
>> - add fixup in title
>> ---
>>   arch/riscv/kernel/traps.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
>> index b40426509244..39d0f8bba4b4 100644
>> --- a/arch/riscv/kernel/traps.c
>> +++ b/arch/riscv/kernel/traps.c
>> @@ -16,6 +16,7 @@
>>   #include <linux/mm.h>
>>   #include <linux/module.h>
>>   #include <linux/irq.h>
>> +#include <linux/kexec.h>
>>     #include <asm/asm-prototypes.h>
>>   #include <asm/bug.h>
>> @@ -44,6 +45,9 @@ void die(struct pt_regs *regs, const char *str)
>>         ret = notify_die(DIE_OOPS, str, regs, 0, regs->cause, SIGSEGV);
>>   +    if (regs && kexec_should_crash(current))
>> +        crash_kexec(regs);
>> +
>
> It seems that the regs won't be null, right? except that,

Autually both regs won't be null, But if it is triggered by panic() , 
the regs are got via riscv_crash_save_regs(), which are the regs of that 
moment, but not the real accident scene.

>
> Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>
>>       bust_spinlocks(0);
>>       add_taint(TAINT_DIE, LOCKDEP_NOW_UNRELIABLE);
>>       spin_unlock_irq(&die_lock);