scripts/gdb: lx-dmesg: use explicit encoding=utf8 errors=replace

Use errors=replace because it is never desirable for lx-dmesg to fail on
string decoding errors, not even if the log buffer is corrupt and we
show incorrect info.

The kernel will sometimes print utf8, for example the copyright symbol
from jffs2.  In order to make this work specify 'utf8' everywhere
because python2 otherwise defaults to 'ascii'.

In theory the second errors='replace' is not be required because
everything that can be decoded as utf8 should also be encodable back to
utf8.  But it's better to be extra safe here.  It's worth noting that
this is definitely not true for encoding='ascii', unknown characters are
replaced with U+FFFD REPLACEMENT CHARACTER and they fail to encode back
to ascii.

Link: http://lkml.kernel.org/r/acee067f3345954ed41efb77b80eebdc038619c6.1498481469.git.leonard.crestez@nxp.com
Signed-off-by: Leonard Crestez <[email protected]>
Acked-by: Jan Kiszka <[email protected]>
Cc: Jason Wessel <[email protected]>
Cc: Kieran Bingham <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

scripts/gdb/linux/dmesg.py

@@ -12,6 +12,7 @@
 #
 
 import gdb
+import sys
 
 from linux import utils
 
@@ -52,13 +53,19 @@ def invoke(self, arg, from_tty):
                 continue
 
             text_len = utils.read_u16(log_buf[pos + 10:pos + 12])
-            text = log_buf[pos + 16:pos + 16 + text_len].decode()
+            text = log_buf[pos + 16:pos + 16 + text_len].decode(
+                encoding='utf8', errors='replace')
             time_stamp = utils.read_u64(log_buf[pos:pos + 8])
 
             for line in text.splitlines():
-                gdb.write("[{time:12.6f}] {line}\n".format(
+                msg = u"[{time:12.6f}] {line}\n".format(
                     time=time_stamp / 1000000000.0,
-                    line=line))
+                    line=line)
+                # With python2 gdb.write will attempt to convert unicode to
+                # ascii and might fail so pass an utf8-encoded str instead.
+                if sys.hexversion < 0x03000000:
+                    msg = msg.encode(encoding='utf8', errors='replace')
+                gdb.write(msg)
 
             pos += length