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回复 5# hhuccpj
小伙很有前途啊, 观察问题很仔细, 从glibc代码来看, 的确是一开始映射的flags为0, 后来通过mprotect改成rw的
你如果用strace跟踪应该能抓到这个过程。
上代码:
static heap_info *
internal_function
#if __STD_C
new_heap(size_t size, size_t top_pad)
#else
new_heap(size, top_pad) size_t size, top_pad;
#endif
{
size_t page_mask = malloc_getpagesize - 1;
char *p1, *p2;
unsigned long ul;
heap_info *h;
if(size+top_pad < HEAP_MIN_SIZE)
size = HEAP_MIN_SIZE;
else if(size+top_pad <= HEAP_MAX_SIZE)
size += top_pad;
else if(size > HEAP_MAX_SIZE)
return 0;
else
size = HEAP_MAX_SIZE;
size = (size + page_mask) & ~page_mask;
/* A memory region aligned to a multiple of HEAP_MAX_SIZE is needed.
No swap space needs to be reserved for the following large
mapping (on Linux, this is the case for all non-writable mappings
anyway). */
p2 = MAP_FAILED;
if(aligned_heap_area) {
p2 = (char *)MMAP(aligned_heap_area, HEAP_MAX_SIZE, PROT_NONE,
MAP_PRIVATE|MAP_NORESERVE);
aligned_heap_area = NULL;
if (p2 != MAP_FAILED && ((unsigned long)p2 & (HEAP_MAX_SIZE-1))) {
munmap(p2, HEAP_MAX_SIZE);
p2 = MAP_FAILED;
}
}
if(p2 == MAP_FAILED) {
p1 = (char *)MMAP(0, HEAP_MAX_SIZE<<1, PROT_NONE,
MAP_PRIVATE|MAP_NORESERVE);
if(p1 != MAP_FAILED) {
p2 = (char *)(((unsigned long)p1 + (HEAP_MAX_SIZE-1))
& ~(HEAP_MAX_SIZE-1));
ul = p2 - p1;
if (ul)
munmap(p1, ul);
else
aligned_heap_area = p2 + HEAP_MAX_SIZE;
munmap(p2 + HEAP_MAX_SIZE, HEAP_MAX_SIZE - ul);
} else {
/* Try to take the chance that an allocation of only HEAP_MAX_SIZE
is already aligned. */
p2 = (char *)MMAP(0, HEAP_MAX_SIZE, PROT_NONE, MAP_PRIVATE|MAP_NORESERVE);
if(p2 == MAP_FAILED)
return 0;
if((unsigned long)p2 & (HEAP_MAX_SIZE-1)) {
munmap(p2, HEAP_MAX_SIZE);
return 0;
}
}
}
if(mprotect(p2, size, PROT_READ|PROT_WRITE) != 0) {
munmap(p2, HEAP_MAX_SIZE);
return 0;
}
h = (heap_info *)p2;
h->size = size;
h->mprotect_size = size;
THREAD_STAT(stat_n_heaps++);
return h;
} |
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