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

From: Jan Kiszka
Date: Fri May 26 2017 - 07:30:34 EST


On 2017-05-26 13:22, Andrà 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: Andrà Draszik <git@xxxxxxxxxx>
>
> ---
> 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@xxxxxxxxxxx>

Andrew, please include in your queue.

Jan

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