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  1. MariaDB Server
  2. MDEV-27115

10.4.22 segfault at SELECT RELEASE_LOCK() in ull_get_key (bad MDL_ticket)



    • Bug
    • Status: Closed (View Workflow)
    • Major
    • Resolution: Fixed
    • 10.4.22
    • 10.4.28
    • Galera
    • None


      This morning: a crash in ull_get_key because $rax (return value from MDL_ticket::get_key) held a bad pointer.

      This is the first time I've seen this particular crash. And this is on the first nodes we have running with 10.4.22.


      galera-4 - 26.4.9-bionic
      galera-arbitrator-4 - 26.4.9-bionic
      libmariadb3:amd64 - 1:10.4.22+maria~bionic
      mariadb-backup - 1:10.4.22+maria~bionic
      mariadb-client-10.4 - 1:10.4.22+maria~bionic
      mariadb-client-core-10.4 - 1:10.4.22+maria~bionic
      mariadb-common - 1:10.4.22+maria~bionic
      mariadb-server - 1:10.4.22+maria~bionic
      mariadb-server-10.4 - 1:10.4.22+maria~bionic
      mariadb-server-core-10.4 - 1:10.4.22+maria~bionic

      Short backtrace:

      (gdb) bt
      #0  ull_get_key (ptr=<optimized out>, length=0x7fdfb66c5938, not_used=<optimized out>) at ./sql/item_func.cc:4002
      #1  0x000055657d7b3413 in my_hash_key (first=1 '\001', length=0x7fdfb66c5938, record=<optimized out>, hash=0x7fde94002e58) at ./mysys/hash.c:196
      #2  hashcmp (pos=0x7fde9431ea38, length=43, key=0x7fdfb66c59a8 "\bb28527f1559cc629795224a389d14a07ba41cb80", hash=0x7fde94002e58) at ./mysys/hash.c:371
      #3  my_hash_first_from_hash_value (hash=hash@entry=0x7fde94002e58, hash_value=<optimized out>, key=key@entry=0x7fdfb66c59a8 "\bb28527f1559cc629795224a389d14a07ba41cb80", length=43, current_record=current_record@entry=0x7fdfb66c598c)
          at ./mysys/hash.c:288
      #4  0x000055657d7b3538 in my_hash_first (hash=hash@entry=0x7fde94002e58, key=key@entry=0x7fdfb66c59a8 "\bb28527f1559cc629795224a389d14a07ba41cb80", length=<optimized out>, current_record=current_record@entry=0x7fdfb66c598c)
          at ./mysys/hash.c:262
      #5  0x000055657d7b3551 in my_hash_search (hash=hash@entry=0x7fde94002e58, key=key@entry=0x7fdfb66c59a8 "\bb28527f1559cc629795224a389d14a07ba41cb80", length=<optimized out>) at ./mysys/hash.c:235
      #6  0x000055657d2afc4f in Item_func_release_lock::val_int (this=0x7fde94010800) at ./sql/item_func.cc:4243

      Segfault due to invalid memory access:

      (gdb) disassemble 
      Dump of assembler code for function ull_get_key(unsigned char const*, unsigned long*, char):
         0x000055657d2ab4e0 <+0>:	push   %rbp
         0x000055657d2ab4e1 <+1>:	mov    %rsp,%rbp
         0x000055657d2ab4e4 <+4>:	push   %rbx
         0x000055657d2ab4e5 <+5>:	mov    %rsi,%rbx
         0x000055657d2ab4e8 <+8>:	sub    $0x8,%rsp
         0x000055657d2ab4ec <+12>:	mov    (%rdi),%rdi
         0x000055657d2ab4ef <+15>:	callq  0x55657d122be0 <MDL_ticket::get_key() const>
      => 0x000055657d2ab4f4 <+20>:	movzwl (%rax),%edx
         0x000055657d2ab4f7 <+23>:	add    $0x8,%rax

      (gdb) x $rax 
      0x7fde00707063:	Cannot access memory at address 0x7fde00707063

      (That's a "cpp" in there.)

      /** Extract a hash key from User_level_lock. */
      uchar *ull_get_key(const uchar *ptr, size_t *length,
                         my_bool not_used __attribute__((unused)))
        User_level_lock *ull = (User_level_lock*) ptr;
        MDL_key *key = ull->lock->get_key();
        *length= key->length();
        return (uchar*) key->ptr();

      class User_level_lock
        MDL_ticket *lock;
        int refs;

      So, ull might be good or bad, but the MDL_ticket lock is pointing to something that is not a lock (anymore).

      (gdb) disassemble  0x55657d122be0
      Dump of assembler code for function MDL_ticket::get_key() const:
         0x000055657d122be0 <+0>:	push   %rbp
         0x000055657d122be1 <+1>:	mov    0x38(%rdi),%rax
         0x000055657d122be5 <+5>:	mov    %rsp,%rbp
         0x000055657d122be8 <+8>:	pop    %rbp
         0x000055657d122be9 <+9>:	retq   
      End of assembler dump.

      So, I would conclude that ull (ptr at entry) is 0x00007fde94311008 (0x8(%r10),%rdi in my_hash_first_from_hash_value) and ull->lock is then 0x00007fde9425bc10:

      (gdb) x/g $r10+8
      0x7fde9431ea40:	0x00007fde94311008
      (gdb) x 0x00007fde94311008
      0x7fde94311008:	0x00007fde9425bc10

      Examining that memory:

      (gdb) print *(User_level_lock*)0x00007fde94311008
      $9 = {lock = 0x7fde9425bc10, refs = 1}
      (gdb) print *((User_level_lock*)0x00007fde94311008)->lock
      $11 = {<MDL_wait_for_subgraph> = {_vptr.MDL_wait_for_subgraph = 0x7fde9404dd40},
             next_in_context = 0x622f746f62646c69, prev_in_context = 0x2f746f62646c6975,
             next_in_lock = 0x61672f646c697562, prev_in_lock = 0x6372732f6172656c, 
             m_type = 1920169775, m_ctx = 0x2e72656469766f72, m_lock = 0x7fde00707063}

      That last one is definitely garbage:

      (gdb) print sizeof(MDL_ticket)
      $12 = 64
      (gdb) x/8gx 0x7fde9425bc10
      0x7fde9425bc10:	0x00007fde9404dd40	0x622f746f62646c69
      0x7fde9425bc20:	0x2f746f62646c6975	0x61672f646c697562
      0x7fde9425bc30:	0x6372732f6172656c	0x705f70657273772f
      0x7fde9425bc40:	0x2e72656469766f72	0x00007fde00707063
      (gdb) print (char*)0x7fde9425bc10+8
      $13 = 0x7fde9425bc18 "ildbot/buildbot/build/galera/src/wsrep_provider.cpp"

      So, when calling get_key on the lock 0x7fde9425bc10 we got:

      MDL_key *MDL_ticket::get_key() const
              return &m_lock->key;

      That is:

      (gdb) print &((MDL_ticket*)0x7fde9425bc10)->m_lock.key
      $15 = (MDL_key *) 0x7fde00707063


      • ull->refs looks okay, so ULL is possibly still good;
      • ull->lock (MDL_ticket) is received in Item_func_get_lock::val_int at one point;
      • now the MDL_ticket memory is overwritten.

      Relevant SQL:

      SELECT RELEASE_LOCK('b28527f1559cc629795224a389d14a07ba41cb80')

      Full backtrace attached.

      Is there anything I can get you? I have the core dump, so if you're interested in other threads, I can take a peek.

      Walter Doekes
      OSSO B.V.


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              jplindst Jan Lindström (Inactive)
              wdoekes Walter Doekes
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