[PATCH] scripts/gdb: fix 'lx-dmesg' on 32 bits arch

[Antonio Borneo]

The type atomic_long_t can have size 4 or 8 bytes, depending on
CONFIG_64BIT; it's only content, the field 'counter', is either an
int or a s64 value.

Current code incorrectly uses the fixed size utils.read_u64() to
read the field 'counter' inside atomic_long_t.

On 32 bits architectures reading the last element 'tail_id' of the
struct prb_desc_ring:
	struct prb_desc_ring {
		...
		atomic_long_t tail_id;
	};
causes the utils.read_u64() to access outside the boundary of the
struct and the gdb command 'lx-dmesg' exits with error:
	Python Exception <class 'IndexError'>: index out of range
	Error occurred in Python: index out of range

Query the really used atomic_long_t counter type size.

Fixes: e60768311af8 ("scripts/gdb: update for lockless printk ringbuffer")
Signed-off-by: Antonio Borneo <antonio.borneo@xxxxxxxxxxx>
[pmladek@xxxxxxxx: Query the really used atomic_long_t counter type size]
Link: https://lore.kernel.org/r/20220617143758.137307-1-antonio.borneo@xxxxxxxxxxx
---
 scripts/gdb/linux/dmesg.py |  9 +++------
 scripts/gdb/linux/utils.py | 14 ++++++++++++--
 2 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/scripts/gdb/linux/dmesg.py b/scripts/gdb/linux/dmesg.py
index d5983cf3db7d..c771831eb077 100644
--- a/scripts/gdb/linux/dmesg.py
+++ b/scripts/gdb/linux/dmesg.py
@@ -22,7 +22,6 @@ prb_desc_type = utils.CachedType("struct prb_desc")
 prb_desc_ring_type = utils.CachedType("struct prb_desc_ring")
 prb_data_ring_type = utils.CachedType("struct prb_data_ring")
 printk_ringbuffer_type = utils.CachedType("struct printk_ringbuffer")
-atomic_long_type = utils.CachedType("atomic_long_t")
 
 class LxDmesg(gdb.Command):
     """Print Linux kernel log buffer."""
@@ -68,8 +67,6 @@ class LxDmesg(gdb.Command):
         off = prb_data_ring_type.get_type()['data'].bitpos // 8
         text_data_addr = utils.read_ulong(text_data_ring, off)
 
-        counter_off = atomic_long_type.get_type()['counter'].bitpos // 8
-
         sv_off = prb_desc_type.get_type()['state_var'].bitpos // 8
 
         off = prb_desc_type.get_type()['text_blk_lpos'].bitpos // 8
@@ -89,9 +86,9 @@ class LxDmesg(gdb.Command):
 
         # read in tail and head descriptor ids
         off = prb_desc_ring_type.get_type()['tail_id'].bitpos // 8
-        tail_id = utils.read_u64(desc_ring, off + counter_off)
+        tail_id = utils.read_atomic_long(desc_ring, off)
         off = prb_desc_ring_type.get_type()['head_id'].bitpos // 8
-        head_id = utils.read_u64(desc_ring, off + counter_off)
+        head_id = utils.read_atomic_long(desc_ring, off)
 
         did = tail_id
         while True:
@@ -102,7 +99,7 @@ class LxDmesg(gdb.Command):
             desc = utils.read_memoryview(inf, desc_addr + desc_off, desc_sz).tobytes()
 
             # skip non-committed record
-            state = 3 & (utils.read_u64(desc, sv_off + counter_off) >> desc_flags_shift)
+            state = 3 & (utils.read_atomic_long(desc, sv_off) >> desc_flags_shift)
             if state != desc_committed and state != desc_finalized:
                 if did == head_id:
                     break
diff --git a/scripts/gdb/linux/utils.py b/scripts/gdb/linux/utils.py
index ff7c1799d588..1553f68716cc 100644
--- a/scripts/gdb/linux/utils.py
+++ b/scripts/gdb/linux/utils.py
@@ -35,13 +35,12 @@ class CachedType:
 
 
 long_type = CachedType("long")
-
+atomic_long_type = CachedType("atomic_long_t")
 
 def get_long_type():
     global long_type
     return long_type.get_type()
 
-
 def offset_of(typeobj, field):
     element = gdb.Value(0).cast(typeobj)
     return int(str(element[field].address).split()[0], 16)
@@ -129,6 +128,17 @@ def read_ulong(buffer, offset):
     else:
         return read_u32(buffer, offset)
 
+atomic_long_counter_offset = atomic_long_type.get_type()['counter'].bitpos
+atomic_long_counter_sizeof = atomic_long_type.get_type()['counter'].type.sizeof
+
+def read_atomic_long(buffer, offset):
+    global atomic_long_counter_offset
+    global atomic_long_counter_sizeof
+
+    if atomic_long_counter_sizeof == 8:
+        return read_u64(buffer, offset + atomic_long_counter_offset)
+    else:
+        return read_u32(buffer, offset + atomic_long_counter_offset)
 
 target_arch = None
 
-- 
2.35.3