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From: "André Draszik" <git@andred.net>
To: linux-kernel@vger.kernel.org
Cc: Jan Kiszka <jan.kiszka@siemens.com>,
	Kieran Bingham <kieran@ksquared.org.uk>
Subject: [PATCH v3] scripts/gdb: make lx-dmesg command work (reliably)
Date: Fri, 26 May 2017 12:22:22 +0100
Message-ID: <20170526112222.3414-1-git@andred.net> (raw)
In-Reply-To: <20170526110454.1119-1-git@andred.net>

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@andred.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
-- 
2.11.0

  parent reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-05-26 11:04 [PATCH] " André Draszik
2017-05-26 11:12 ` Jan Kiszka
2017-05-27 14:01   ` Andy Shevchenko
2017-05-28 16:29     ` Jan Kiszka
2017-05-26 11:13 ` [PATCH v2] " André Draszik
2017-05-26 11:22 ` André Draszik [this message]
2017-05-26 11:28   ` [PATCH v3] " Jan Kiszka
2017-05-31  5:10     ` Kieran Bingham

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