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

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Date:   Fri, 26 May 2017 13:28:41 +0200
From:   Jan Kiszka <jan.kiszka@...mens.com>
To:     André Draszik <git@...red.net>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Kieran Bingham <kieran@...uared.org.uk>
Subject: Re: [PATCH v3] scripts/gdb: make lx-dmesg command work (reliably)

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>
> 
> ---
> 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>

Andrew, please include in your queue.

Jan

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