linux-kernel - Re: [PATCH v3] scripts/gdb: make lx-dmesg command work (reliably)

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Date:   Wed, 31 May 2017 14:10:08 +0900
From:   Kieran Bingham <kieran@...uared.org.uk>
To:     Jan Kiszka <jan.kiszka@...mens.com>,
        André Draszik <git@...red.net>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3] scripts/gdb: make lx-dmesg command work (reliably)

Hi André, Jan, Andrew,

On 26/05/17 20:28, Jan Kiszka wrote:
> On 2017-05-26 13:22, André Draszik wrote:
>> lx-dmesg needs access to the log_buf symbol from printk.c.
>> Unfortunately, the symbol log_buf also exists in BPF's
>> verifier.c and hence gdb can pick one or the other. If it
>> happens to pick BPF's log_buf, lx-dmesg doesn't work:
>>
>>   (gdb) lx-dmesg
>>   Python Exception <class 'gdb.MemoryError'> Cannot access memory at address 0x0:
>>   Error occurred in Python command: Cannot access memory at address 0x0
>>   (gdb) p log_buf
>>   $15 = 0x0
>>
>> Luckily, GDB has a way to deal with this, see
>>   https://sourceware.org/gdb/onlinedocs/gdb/Symbols.html
>>
>>   (gdb) info variables ^log_buf$
>>   All variables matching regular expression "^log_buf$":
>>
>>   File <linux.git>/kernel/bpf/verifier.c:
>>   static char *log_buf;
>>
>>   File <linux.git>/kernel/printk/printk.c:
>>   static char *log_buf;
>>   (gdb) p 'verifier.c'::log_buf
>>   $1 = 0x0
>>   (gdb) p 'printk.c'::log_buf
>>   $2 = 0x811a6aa0 <__log_buf> ""
>>   (gdb) p &log_buf
>>   $3 = (char **) 0x8120fe40 <log_buf>
>>   (gdb) p &'verifier.c'::log_buf
>>   $4 = (char **) 0x8120fe40 <log_buf>
>>   (gdb) p &'printk.c'::log_buf
>>   $5 = (char **) 0x8048b7d0 <log_buf>
>>
>> By being explicit about the location of the symbol, we
>> can make lx-dmesg work again. While at it, do the same
>> for the other symbols we need from printk.c
>>
>> Signed-off-by: André Draszik <git@...red.net>

Nice to see you again André :D

>> ---
>> v3: pep8 compliant
>> v2: Commit message slightly updated
>> ---
>>  scripts/gdb/linux/dmesg.py | 9 +++++----
>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/scripts/gdb/linux/dmesg.py b/scripts/gdb/linux/dmesg.py
>> index f9b92ece7834..5afd1098e33a 100644
>> --- a/scripts/gdb/linux/dmesg.py
>> +++ b/scripts/gdb/linux/dmesg.py
>> @@ -23,10 +23,11 @@ class LxDmesg(gdb.Command):
>>          super(LxDmesg, self).__init__("lx-dmesg", gdb.COMMAND_DATA)
>>  
>>      def invoke(self, arg, from_tty):
>> -        log_buf_addr = int(str(gdb.parse_and_eval("log_buf")).split()[0], 16)
>> -        log_first_idx = int(gdb.parse_and_eval("log_first_idx"))
>> -        log_next_idx = int(gdb.parse_and_eval("log_next_idx"))
>> -        log_buf_len = int(gdb.parse_and_eval("log_buf_len"))
>> +        log_buf_addr = int(str(gdb.parse_and_eval(
>> +            "'printk.c'::log_buf")).split()[0], 16)
>> +        log_first_idx = int(gdb.parse_and_eval("'printk.c'::log_first_idx"))
>> +        log_next_idx = int(gdb.parse_and_eval("'printk.c'::log_next_idx"))
>> +        log_buf_len = int(gdb.parse_and_eval("'printk.c'::log_buf_len"))
>>  
>>          inf = gdb.inferiors()[0]
>>          start = log_buf_addr + log_first_idx
>>
> 
> Acked-by: Jan Kiszka <jan.kiszka@...mens.com>

Sorry for the delay - I'm in Tokyo for Linux OSS Japan.

This looks good to me and I've verified it locally.

Tested-by: Kieran Bingham <kieran@...gham.xyz>

> 
> Andrew, please include in your queue.
>
> Jan
> 

-- 
Regards

Kieran Bingham