1 #include <limits.h>
2 #include <stdint.h>
3 #include <errno.h>
4 #include <sys/mman.h>
5 #include "libc.h"
6 #include "syscall.h"
7 #include "malloc_impl.h"
8
9 /* This function returns true if the interval [old,new]
10 * intersects the 'len'-sized interval below &libc.auxv
11 * (interpreted as the main-thread stack) or below &b
12 * (the current stack). It is used to defend against
13 * buggy brk implementations that can cross the stack. */
14
traverses_stack_p(uintptr_t old,uintptr_t new)15 static int traverses_stack_p(uintptr_t old, uintptr_t new)
16 {
17 const uintptr_t len = 8<<20;
18 uintptr_t a, b;
19
20 b = (uintptr_t)libc.auxv;
21 a = b > len ? b-len : 0;
22 if (new>a && old<b) return 1;
23
24 b = (uintptr_t)&b;
25 a = b > len ? b-len : 0;
26 if (new>a && old<b) return 1;
27
28 return 0;
29 }
30
31 /* Expand the heap in-place if brk can be used, or otherwise via mmap,
32 * using an exponential lower bound on growth by mmap to make
33 * fragmentation asymptotically irrelevant. The size argument is both
34 * an input and an output, since the caller needs to know the size
35 * allocated, which will be larger than requested due to page alignment
36 * and mmap minimum size rules. The caller is responsible for locking
37 * to prevent concurrent calls. */
38
__expand_heap(size_t * pn)39 void *__expand_heap(size_t *pn)
40 {
41 static uintptr_t brk;
42 static unsigned mmap_step;
43 size_t n = *pn;
44
45 if (n > SIZE_MAX/2 - PAGE_SIZE) {
46 errno = ENOMEM;
47 return 0;
48 }
49 n += -n & PAGE_SIZE-1;
50
51 if (!brk) {
52 brk = __syscall(SYS_brk, 0);
53 brk += -brk & PAGE_SIZE-1;
54 }
55
56 if (n < SIZE_MAX-brk && !traverses_stack_p(brk, brk+n)
57 && __syscall(SYS_brk, brk+n)==brk+n) {
58 *pn = n;
59 brk += n;
60 return (void *)(brk-n);
61 }
62
63 size_t min = (size_t)PAGE_SIZE << mmap_step/2;
64 if (n < min) n = min;
65 void *area = __mmap(0, n, PROT_READ|PROT_WRITE,
66 MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
67 if (area == MAP_FAILED) return 0;
68 *pn = n;
69 mmap_step++;
70 return area;
71 }
72