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 #include <sys/prctl.h>
10
11 /* This function returns true if the interval [old,new]
12 * intersects the 'len'-sized interval below &libc.auxv
13 * (interpreted as the main-thread stack) or below &b
14 * (the current stack). It is used to defend against
15 * buggy brk implementations that can cross the stack. */
16
traverses_stack_p(uintptr_t old,uintptr_t new)17 static int traverses_stack_p(uintptr_t old, uintptr_t new)
18 {
19 const uintptr_t len = 8<<20;
20 uintptr_t a, b;
21
22 b = (uintptr_t)libc.auxv;
23 a = b > len ? b-len : 0;
24 if (new>a && old<b) return 1;
25
26 b = (uintptr_t)&b;
27 a = b > len ? b-len : 0;
28 if (new>a && old<b) return 1;
29
30 return 0;
31 }
32
33 /* Expand the heap in-place if brk can be used, or otherwise via mmap,
34 * using an exponential lower bound on growth by mmap to make
35 * fragmentation asymptotically irrelevant. The size argument is both
36 * an input and an output, since the caller needs to know the size
37 * allocated, which will be larger than requested due to page alignment
38 * and mmap minimum size rules. The caller is responsible for locking
39 * to prevent concurrent calls. */
40
__expand_heap(size_t * pn)41 void *__expand_heap(size_t *pn)
42 {
43 static uintptr_t brk;
44 static unsigned mmap_step;
45 size_t n = *pn;
46
47 if (n > SIZE_MAX/2 - PAGE_SIZE) {
48 errno = ENOMEM;
49 return 0;
50 }
51 n += -n & PAGE_SIZE-1;
52
53 if (!brk) {
54 brk = __syscall(SYS_brk, 0);
55 brk += -brk & PAGE_SIZE-1;
56 }
57
58 if (n < SIZE_MAX-brk && !traverses_stack_p(brk, brk+n)
59 && __syscall(SYS_brk, brk+n)==brk+n) {
60 *pn = n;
61 brk += n;
62 return (void *)(brk-n);
63 }
64
65 size_t min = (size_t)PAGE_SIZE << mmap_step/2;
66 if (n < min) n = min;
67 void *area = __mmap(0, n, PROT_READ|PROT_WRITE,
68 MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
69 if (area == MAP_FAILED) return 0;
70 prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, area, n, "native_heap:musl");
71
72 *pn = n;
73 mmap_step++;
74 return area;
75 }
76