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memdebug.c
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1 /*-------------------------------------------------------------------------
2  *
3  * memdebug.c
4  * Declarations used in memory context implementations, not part of the
5  * public API of the memory management subsystem.
6  *
7  *
8  * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
9  * Portions Copyright (c) 1994, Regents of the University of California
10  *
11  * src/backend/utils/mmgr/memdebug.c
12  *
13  *
14  * About CLOBBER_FREED_MEMORY:
15  *
16  * If this symbol is defined, all freed memory is overwritten with 0x7F's.
17  * This is useful for catching places that reference already-freed memory.
18  *
19  * About MEMORY_CONTEXT_CHECKING:
20  *
21  * Since we usually round request sizes up to the next power of 2, there
22  * is often some unused space immediately after a requested data area.
23  * Thus, if someone makes the common error of writing past what they've
24  * requested, the problem is likely to go unnoticed ... until the day when
25  * there *isn't* any wasted space, perhaps because of different memory
26  * alignment on a new platform, or some other effect. To catch this sort
27  * of problem, the MEMORY_CONTEXT_CHECKING option stores 0x7E just beyond
28  * the requested space whenever the request is less than the actual chunk
29  * size, and verifies that the byte is undamaged when the chunk is freed.
30  *
31  *
32  * About USE_VALGRIND and Valgrind client requests:
33  *
34  * Valgrind provides "client request" macros that exchange information with
35  * the host Valgrind (if any). Under !USE_VALGRIND, memdebug.h stubs out
36  * currently-used macros.
37  *
38  * When running under Valgrind, we want a NOACCESS memory region both before
39  * and after the allocation. The chunk header is tempting as the preceding
40  * region, but mcxt.c expects to able to examine the standard chunk header
41  * fields. Therefore, we use, when available, the requested_size field and
42  * any subsequent padding. requested_size is made NOACCESS before returning
43  * a chunk pointer to a caller. However, to reduce client request traffic,
44  * it is kept DEFINED in chunks on the free list.
45  *
46  * The rounded-up capacity of the chunk usually acts as a post-allocation
47  * NOACCESS region. If the request consumes precisely the entire chunk,
48  * there is no such region; another chunk header may immediately follow. In
49  * that case, Valgrind will not detect access beyond the end of the chunk.
50  *
51  * See also the cooperating Valgrind client requests in mcxt.c.
52  *
53  *-------------------------------------------------------------------------
54  */
55 
56 #include "postgres.h"
57 
58 #include "utils/memdebug.h"
59 
60 #ifdef RANDOMIZE_ALLOCATED_MEMORY
61 
62 /*
63  * Fill a just-allocated piece of memory with "random" data. It's not really
64  * very random, just a repeating sequence with a length that's prime. What
65  * we mainly want out of it is to have a good probability that two palloc's
66  * of the same number of bytes start out containing different data.
67  *
68  * The region may be NOACCESS, so make it UNDEFINED first to avoid errors as
69  * we fill it. Filling the region makes it DEFINED, so make it UNDEFINED
70  * again afterward. Whether to finally make it UNDEFINED or NOACCESS is
71  * fairly arbitrary. UNDEFINED is more convenient for SlabRealloc(), and
72  * other callers have no preference.
73  */
74 void
75 randomize_mem(char *ptr, size_t size)
76 {
77  static int save_ctr = 1;
78  size_t remaining = size;
79  int ctr;
80 
81  ctr = save_ctr;
82  VALGRIND_MAKE_MEM_UNDEFINED(ptr, size);
83  while (remaining-- > 0)
84  {
85  *ptr++ = ctr;
86  if (++ctr > 251)
87  ctr = 1;
88  }
89  VALGRIND_MAKE_MEM_UNDEFINED(ptr - size, size);
90  save_ctr = ctr;
91 }
92 
93 #endif /* RANDOMIZE_ALLOCATED_MEMORY */
int remaining
Definition: informix.c:667
#define VALGRIND_MAKE_MEM_UNDEFINED(addr, size)
Definition: memdebug.h:28