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/[email protected]
Signed-off-by: André Draszik <[email protected]>
Tested-by: Kieran Bingham <[email protected]>
Acked-by: Jan Kiszka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
André Draszik
d6c9708737