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72 lines
1.8 KiB
72 lines
1.8 KiB
#include <limits.h>
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#include <stdint.h>
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#include <errno.h>
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#include <sys/mman.h>
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#include "libc.h"
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#include "syscall.h"
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/* This function returns true if the interval [old,new]
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* intersects the 'len'-sized interval below &libc.auxv
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* (interpreted as the main-thread stack) or below &b
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* (the current stack). It is used to defend against
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* buggy brk implementations that can cross the stack. */
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static int traverses_stack_p(uintptr_t old, uintptr_t new)
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{
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const uintptr_t len = 8<<20;
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uintptr_t a, b;
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b = (uintptr_t)libc.auxv;
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a = b > len ? b-len : 0;
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if (new>a && old<b) return 1;
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b = (uintptr_t)&b;
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a = b > len ? b-len : 0;
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if (new>a && old<b) return 1;
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return 0;
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}
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void *__mmap(void *, size_t, int, int, int, off_t);
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/* Expand the heap in-place if brk can be used, or otherwise via mmap,
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* using an exponential lower bound on growth by mmap to make
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* fragmentation asymptotically irrelevant. The size argument is both
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* an input and an output, since the caller needs to know the size
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* allocated, which will be larger than requested due to page alignment
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* and mmap minimum size rules. The caller is responsible for locking
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* to prevent concurrent calls. */
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void *__expand_heap(size_t *pn)
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{
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static uintptr_t brk;
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static unsigned mmap_step;
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size_t n = *pn;
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if (n > SIZE_MAX/2 - PAGE_SIZE) {
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errno = ENOMEM;
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return 0;
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}
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n += -n & PAGE_SIZE-1;
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if (!brk) {
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brk = __syscall(SYS_brk, 0);
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brk += -brk & PAGE_SIZE-1;
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}
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if (n < SIZE_MAX-brk && !traverses_stack_p(brk, brk+n)
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&& __syscall(SYS_brk, brk+n)==brk+n) {
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*pn = n;
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brk += n;
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return (void *)(brk-n);
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}
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size_t min = (size_t)PAGE_SIZE << mmap_step/2;
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if (n < min) n = min;
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void *area = __mmap(0, n, PROT_READ|PROT_WRITE,
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MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
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if (area == MAP_FAILED) return 0;
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*pn = n;
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mmap_step++;
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return area;
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}
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