diff options
Diffstat (limited to 'windows/utils/minefield.c')
| -rw-r--r-- | windows/utils/minefield.c | 228 |
1 files changed, 228 insertions, 0 deletions
diff --git a/windows/utils/minefield.c b/windows/utils/minefield.c new file mode 100644 index 00000000..c5262ae6 --- /dev/null +++ b/windows/utils/minefield.c @@ -0,0 +1,228 @@ +/* + * 'Minefield' - a crude Windows memory debugger, similar in concept + * to the old Unix 'Electric Fence'. The main difference is that + * Electric Fence can be imposed on a program from outside, via + * LD_PRELOAD, whereas this has to be included in the program at + * compile time with its own cooperation. + * + * This module provides the Minefield allocator. Actually enabling it + * is done by a #define in force when the main utils/memory.c is + * compiled. + */ + +#include "putty.h" +#include "puttymem.h" + +#define PAGESIZE 4096 + +/* + * Design: + * + * We start by reserving as much virtual address space as Windows + * will sensibly (or not sensibly) let us have. We flag it all as + * invalid memory. + * + * Any allocation attempt is satisfied by committing one or more + * pages, with an uncommitted page on either side. The returned + * memory region is jammed up against the _end_ of the pages. + * + * Freeing anything causes instantaneous decommitment of the pages + * involved, so stale pointers are caught as soon as possible. + */ + +static int minefield_initialised = 0; +static void *minefield_region = NULL; +static long minefield_size = 0; +static long minefield_npages = 0; +static long minefield_curpos = 0; +static unsigned short *minefield_admin = NULL; +static void *minefield_pages = NULL; + +static void minefield_admin_hide(int hide) +{ + int access = hide ? PAGE_NOACCESS : PAGE_READWRITE; + VirtualProtect(minefield_admin, minefield_npages * 2, access, NULL); +} + +static void minefield_init(void) +{ + int size; + int admin_size; + int i; + + for (size = 0x40000000; size > 0; size = ((size >> 3) * 7) & ~0xFFF) { + minefield_region = VirtualAlloc(NULL, size, + MEM_RESERVE, PAGE_NOACCESS); + if (minefield_region) + break; + } + minefield_size = size; + + /* + * Firstly, allocate a section of that to be the admin block. + * We'll need a two-byte field for each page. + */ + minefield_admin = minefield_region; + minefield_npages = minefield_size / PAGESIZE; + admin_size = (minefield_npages * 2 + PAGESIZE - 1) & ~(PAGESIZE - 1); + minefield_npages = (minefield_size - admin_size) / PAGESIZE; + minefield_pages = (char *) minefield_region + admin_size; + + /* + * Commit the admin region. + */ + VirtualAlloc(minefield_admin, minefield_npages * 2, + MEM_COMMIT, PAGE_READWRITE); + + /* + * Mark all pages as unused (0xFFFF). + */ + for (i = 0; i < minefield_npages; i++) + minefield_admin[i] = 0xFFFF; + + /* + * Hide the admin region. + */ + minefield_admin_hide(1); + + minefield_initialised = 1; +} + +static void minefield_bomb(void) +{ + div(1, *(int *) minefield_pages); +} + +static void *minefield_alloc(int size) +{ + int npages; + int pos, lim, region_end, region_start; + int start; + int i; + + npages = (size + PAGESIZE - 1) / PAGESIZE; + + minefield_admin_hide(0); + + /* + * Search from current position until we find a contiguous + * bunch of npages+2 unused pages. + */ + pos = minefield_curpos; + lim = minefield_npages; + while (1) { + /* Skip over used pages. */ + while (pos < lim && minefield_admin[pos] != 0xFFFF) + pos++; + /* Count unused pages. */ + start = pos; + while (pos < lim && pos - start < npages + 2 && + minefield_admin[pos] == 0xFFFF) + pos++; + if (pos - start == npages + 2) + break; + /* If we've reached the limit, reset the limit or stop. */ + if (pos >= lim) { + if (lim == minefield_npages) { + /* go round and start again at zero */ + lim = minefield_curpos; + pos = 0; + } else { + minefield_admin_hide(1); + return NULL; + } + } + } + + minefield_curpos = pos - 1; + + /* + * We have npages+2 unused pages starting at start. We leave + * the first and last of these alone and use the rest. + */ + region_end = (start + npages + 1) * PAGESIZE; + region_start = region_end - size; + /* FIXME: could align here if we wanted */ + + /* + * Update the admin region. + */ + for (i = start + 2; i < start + npages + 1; i++) + minefield_admin[i] = 0xFFFE; /* used but no region starts here */ + minefield_admin[start + 1] = region_start % PAGESIZE; + + minefield_admin_hide(1); + + VirtualAlloc((char *) minefield_pages + region_start, size, + MEM_COMMIT, PAGE_READWRITE); + return (char *) minefield_pages + region_start; +} + +static void minefield_free(void *ptr) +{ + int region_start, i, j; + + minefield_admin_hide(0); + + region_start = (char *) ptr - (char *) minefield_pages; + i = region_start / PAGESIZE; + if (i < 0 || i >= minefield_npages || + minefield_admin[i] != region_start % PAGESIZE) + minefield_bomb(); + for (j = i; j < minefield_npages && minefield_admin[j] != 0xFFFF; j++) { + minefield_admin[j] = 0xFFFF; + } + + VirtualFree(ptr, j * PAGESIZE - region_start, MEM_DECOMMIT); + + minefield_admin_hide(1); +} + +static int minefield_get_size(void *ptr) +{ + int region_start, i, j; + + minefield_admin_hide(0); + + region_start = (char *) ptr - (char *) minefield_pages; + i = region_start / PAGESIZE; + if (i < 0 || i >= minefield_npages || + minefield_admin[i] != region_start % PAGESIZE) + minefield_bomb(); + for (j = i; j < minefield_npages && minefield_admin[j] != 0xFFFF; j++); + + minefield_admin_hide(1); + + return j * PAGESIZE - region_start; +} + +void *minefield_c_malloc(size_t size) +{ + if (!minefield_initialised) + minefield_init(); + return minefield_alloc(size); +} + +void minefield_c_free(void *p) +{ + if (!minefield_initialised) + minefield_init(); + minefield_free(p); +} + +/* + * realloc _always_ moves the chunk, for rapid detection of code + * that assumes it won't. + */ +void *minefield_c_realloc(void *p, size_t size) +{ + size_t oldsize; + void *q; + if (!minefield_initialised) + minefield_init(); + q = minefield_alloc(size); + oldsize = minefield_get_size(p); + memcpy(q, p, (oldsize < size ? oldsize : size)); + minefield_free(p); + return q; +} |