96 lines
2 KiB
C
96 lines
2 KiB
C
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/*
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* This emulates brk() and sbrk() using mmap() and mprotect().
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*
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* We reserve exactly SEGMENTSZ bytes of address space by calling
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* mmap() with PROT_NONE. Then we allocate pages in our segment by
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* calling mprotect() with PROT_READ|PROT_WRITE.
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*
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* This emulation can't resize its segment. If SEGMENTSZ is too big,
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* then programs might run out of address space for other mappings.
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*/
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#include <sys/mman.h>
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#include <errno.h>
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#include <unistd.h>
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/*
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* PAGESZ must be correct for this system!
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* SEGMENTSZ must be a multiple of PAGESZ.
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*/
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#define PAGESZ 0x1000 /* page size for i386, powerpc */
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#define SEGMENTSZ 0x20000000
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static char *segment;
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static char *cbreak; /* current break */
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static void brk_init(void)
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{
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/*
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* Try exactly once to reserve our segment. If we fail, then
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* segment == MAP_FAILED and we never try again.
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*/
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if (segment == NULL) {
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segment = mmap(NULL, SEGMENTSZ, PROT_NONE,
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MAP_PRIVATE|MAP_ANON, -1, 0);
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cbreak = segment;
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}
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}
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static int brk1(char *nbreak)
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{
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size_t sz;
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char *new, *old;
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sz = (segment == MAP_FAILED) ? 0 : SEGMENTSZ;
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if (nbreak < segment || nbreak > segment + sz) {
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errno = ENOMEM;
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return -1;
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}
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/* Round up to page size. */
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old = (char *)(((size_t)cbreak + (PAGESZ-1)) & ~(PAGESZ-1));
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new = (char *)(((size_t)nbreak + (PAGESZ-1)) & ~(PAGESZ-1));
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if (new > old) {
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/* Allocate pages by unprotecting them. */
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if (mprotect(old, new - old, PROT_READ|PROT_WRITE) < 0) {
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errno = ENOMEM;
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return -1;
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}
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} else if (new < old) {
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/*
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* Free pages by using MAP_FIXED to replace the
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* mapping. Ignore errors.
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*/
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mmap(new, old - new, PROT_NONE,
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MAP_PRIVATE|MAP_ANON|MAP_FIXED, -1, 0);
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}
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cbreak = nbreak;
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return 0;
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}
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int brk(void *addr)
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{
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brk_init();
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return brk1(addr);
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}
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void *sbrk(int incr)
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{
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char *base, *nbreak;
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brk_init();
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base = cbreak;
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nbreak = base + incr;
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/* Did base + incr overflow? */
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if ((incr < 0 && nbreak > base) ||
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(incr > 0 && nbreak < base)) {
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errno = ENOMEM;
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return (void*)-1;
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}
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if (brk1(nbreak) < 0)
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return (void*)-1;
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return base;
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}
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