[PATCH 4.9 17/45] scripts/gdb: make lx-dmesg command work (reliably)

Greg Kroah-Hartman
2021-01-11T14:04:55Z

From: André Draszik <git@andred.net>

commit d6c9708737c2107c38bd75f133d14d5801b8d6d5 upstream

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

Link: http://lkml.kernel.org/r/20170526112222.3414-1-git@andred.net
Signed-off-by: André Draszik <git@andred.net>
Tested-by: Kieran Bingham <kieran@bingham.xyz>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 f9b92ece78343..5afd1098e33a1 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
--
2.27.0