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 8/* This function returns true if the interval [old,new] 9 * intersects the 'len'-sized interval below &libc.auxv 10 * (interpreted as the main-thread stack) or below &b 11 * (the current stack). It is used to defend against 12 * buggy brk implementations that can cross the stack. */ 13 14static int traverses_stack_p(uintptr_t old, uintptr_t new) 15{ 16 const uintptr_t len = 8<<20; 17 uintptr_t a, b; 18 19 b = (uintptr_t)libc.auxv; 20 a = b > len ? b-len : 0; 21 if (new>a && old<b) return 1; 22 23 b = (uintptr_t)&b; 24 a = b > len ? b-len : 0; 25 if (new>a && old<b) return 1; 26 27 return 0; 28} 29 30void *__mmap(void *, size_t, int, int, int, off_t); 31 32/* Expand the heap in-place if brk can be used, or otherwise via mmap, 33 * using an exponential lower bound on growth by mmap to make 34 * fragmentation asymptotically irrelevant. The size argument is both 35 * an input and an output, since the caller needs to know the size 36 * allocated, which will be larger than requested due to page alignment 37 * and mmap minimum size rules. The caller is responsible for locking 38 * to prevent concurrent calls. */ 39 40void *__expand_heap(size_t *pn) 41{ 42 static uintptr_t brk; 43 static unsigned mmap_step; 44 size_t n = *pn; 45 46 if (n > SIZE_MAX/2 - PAGE_SIZE) { 47 errno = ENOMEM; 48 return 0; 49 } 50 n += -n & PAGE_SIZE-1; 51 52 if (!brk) { 53 brk = __syscall(SYS_brk, 0); 54 brk += -brk & PAGE_SIZE-1; 55 } 56 57 if (n < SIZE_MAX-brk && !traverses_stack_p(brk, brk+n) 58 && __syscall(SYS_brk, brk+n)==brk+n) { 59 *pn = n; 60 brk += n; 61 return (void *)(brk-n); 62 } 63 64 size_t min = (size_t)PAGE_SIZE << mmap_step/2; 65 if (n < min) n = min; 66 void *area = __mmap(0, n, PROT_READ|PROT_WRITE, 67 MAP_PRIVATE|MAP_ANONYMOUS, -1, 0); 68 if (area == MAP_FAILED) return 0; 69 *pn = n; 70 mmap_step++; 71 return area; 72} 73