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* [PATCH] scripts/gdb: make lx-dmesg command work (reliably)
@ 2017-05-26 11:04 André Draszik
  2017-05-26 11:12 ` Jan Kiszka
                   ` (2 more replies)
  0 siblings, 3 replies; 8+ messages in thread
From: André Draszik @ 2017-05-26 11:04 UTC (permalink / raw)
  To: linux-kernel; +Cc: Jan Kiszka, Kieran Bingham

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)
+        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

^ permalink raw reply related	[flat|nested] 8+ messages in thread

end of thread, other threads:[~2017-05-31  5:10 UTC | newest]

Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2017-05-26 11:04 [PATCH] scripts/gdb: make lx-dmesg command work (reliably) 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 ` [PATCH v3] " André Draszik
2017-05-26 11:28   ` Jan Kiszka
2017-05-31  5:10     ` Kieran Bingham

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