scripts/gdb: fix lx-dmesg when CONFIG_PRINTK_CALLER is set

When CONFIG_PRINTK_CALLER is set, struct printk_log contains an
additional member caller_id.  This affects the offset of the log text.
Account for this by using the type information from gdb to determine all
the offsets instead of using hardcoded values.

This fixes following error:

  (gdb) lx-dmesg
  Python Exception <class 'ValueError'> embedded null character:
  Error occurred in Python command: embedded null character

The read_u* utility functions now take an offset argument to make them
easier to use.

Link: http://lkml.kernel.org/r/20191011142500.2339-1-joel.colledge@linbit.com
Signed-off-by: Joel Colledge <joel.colledge@linbit.com>
Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/gdb/linux/dmesg.py

@@ -16,6 +16,8 @@ import sys
 
 from linux import utils
 
+printk_log_type = utils.CachedType("struct printk_log")
+
 
 class LxDmesg(gdb.Command):
     """Print Linux kernel log buffer."""
@@ -42,9 +44,14 @@ class LxDmesg(gdb.Command):
             b = utils.read_memoryview(inf, log_buf_addr, log_next_idx)
             log_buf = a.tobytes() + b.tobytes()
 
+        length_offset = printk_log_type.get_type()['len'].bitpos // 8
+        text_len_offset = printk_log_type.get_type()['text_len'].bitpos // 8
+        time_stamp_offset = printk_log_type.get_type()['ts_nsec'].bitpos // 8
+        text_offset = printk_log_type.get_type().sizeof
+
         pos = 0
         while pos < log_buf.__len__():
-            length = utils.read_u16(log_buf[pos + 8:pos + 10])
+            length = utils.read_u16(log_buf, pos + length_offset)
             if length == 0:
                 if log_buf_2nd_half == -1:
                     gdb.write("Corrupted log buffer!\n")
@@ -52,10 +59,11 @@ class LxDmesg(gdb.Command):
                 pos = log_buf_2nd_half
                 continue
 
-            text_len = utils.read_u16(log_buf[pos + 10:pos + 12])
-            text = log_buf[pos + 16:pos + 16 + text_len].decode(
+            text_len = utils.read_u16(log_buf, pos + text_len_offset)
+            text_start = pos + text_offset
+            text = log_buf[text_start:text_start + text_len].decode(
                 encoding='utf8', errors='replace')
-            time_stamp = utils.read_u64(log_buf[pos:pos + 8])
+            time_stamp = utils.read_u64(log_buf, pos + time_stamp_offset)
 
             for line in text.splitlines():
                 msg = u"[{time:12.6f}] {line}\n".format(

scripts/gdb/linux/utils.py

@@ -92,15 +92,16 @@ def read_memoryview(inf, start, length):
     return memoryview(inf.read_memory(start, length))
 
 
-def read_u16(buffer):
+def read_u16(buffer, offset):
+    buffer_val = buffer[offset:offset + 2]
     value = [0, 0]
 
-    if type(buffer[0]) is str:
-        value[0] = ord(buffer[0])
-        value[1] = ord(buffer[1])
+    if type(buffer_val[0]) is str:
+        value[0] = ord(buffer_val[0])
+        value[1] = ord(buffer_val[1])
     else:
-        value[0] = buffer[0]
-        value[1] = buffer[1]
+        value[0] = buffer_val[0]
+        value[1] = buffer_val[1]
 
     if get_target_endianness() == LITTLE_ENDIAN:
         return value[0] + (value[1] << 8)
@@ -108,18 +109,18 @@ def read_u16(buffer):
         return value[1] + (value[0] << 8)
 
 
-def read_u32(buffer):
+def read_u32(buffer, offset):
     if get_target_endianness() == LITTLE_ENDIAN:
-        return read_u16(buffer[0:2]) + (read_u16(buffer[2:4]) << 16)
+        return read_u16(buffer, offset) + (read_u16(buffer, offset + 2) << 16)
     else:
-        return read_u16(buffer[2:4]) + (read_u16(buffer[0:2]) << 16)
+        return read_u16(buffer, offset + 2) + (read_u16(buffer, offset) << 16)
 
 
-def read_u64(buffer):
+def read_u64(buffer, offset):
     if get_target_endianness() == LITTLE_ENDIAN:
-        return read_u32(buffer[0:4]) + (read_u32(buffer[4:8]) << 32)
+        return read_u32(buffer, offset) + (read_u32(buffer, offset + 4) << 32)
     else:
-        return read_u32(buffer[4:8]) + (read_u32(buffer[0:4]) << 32)
+        return read_u32(buffer, offset + 4) + (read_u32(buffer, offset) << 32)
 
 
 target_arch = None