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Subject: Re: [PATCH] scripts/gdb: make lx-dmesg command work (reliably)
To: =?UTF-8?Q?Andr=c3=a9_Draszik?= <git@andred.net>,
        linux-kernel@vger.kernel.org
Cc: Kieran Bingham <kieran@ksquared.org.uk>
References: <20170526110454.1119-1-git@andred.net>
From: Jan Kiszka <jan.kiszka@siemens.com>
Message-ID: <1c56ad8f-2128-3fe2-7a22-fb2074084a5e@siemens.com>
Date: Fri, 26 May 2017 13:12:29 +0200
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On 2017-05-26 13:04, 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> ""
> 
> 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@andred.net>
> ---
>  scripts/gdb/linux/dmesg.py | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/scripts/gdb/linux/dmesg.py b/scripts/gdb/linux/dmesg.py
> index f9b92ece7834..11f2397d40ed 100644
> --- a/scripts/gdb/linux/dmesg.py
> +++ b/scripts/gdb/linux/dmesg.py
> @@ -23,10 +23,10 @@ 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)

Overlong line. Please stay pep8 compliant.

> +        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
> 

Looks good otherwise, thanks for addressing this!

Jan

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