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 --- 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