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