libc-top-half/musl/src/malloc/lite_malloc.c

   1 #include <stdlib.h>
   2 #include <stdint.h>
   3 #include <limits.h>
   4 #include <errno.h>
   5 #include <sys/mman.h>
   6 #include "libc.h"
   7 #include "lock.h"
   8 #include "syscall.h"
   9
  10 #define ALIGN 16
  11
  12 /* This function returns true if the interval [old,new]
  13  * intersects the 'len'-sized interval below &libc.auxv
  14  * (interpreted as the main-thread stack) or below &b
  15  * (the current stack). It is used to defend against
  16  * buggy brk implementations that can cross the stack. */
  17
  18 static int traverses_stack_p(uintptr_t old, uintptr_t new)
  19 {
  20         const uintptr_t len = 8<<20;
  21         uintptr_t a, b;
  22
  23         b = (uintptr_t)libc.auxv;
  24         a = b > len ? b-len : 0;
  25         if (new>a && old<b) return 1;
  26
  27         b = (uintptr_t)&b;
  28         a = b > len ? b-len : 0;
  29         if (new>a && old<b) return 1;
  30
  31         return 0;
  32 }
  33
  34 static void *__simple_malloc(size_t n)
  35 {
  36         static uintptr_t brk, cur, end;
  37         static volatile int lock[1];
  38         static unsigned mmap_step;
  39         size_t align=1;
  40         void *p;
  41
  42         if (n > SIZE_MAX/2) {
  43                 errno = ENOMEM;
  44                 return 0;
  45         }
  46
  47         if (!n) n++;
  48         while (align<n && align<ALIGN)
  49                 align += align;
  50
  51         LOCK(lock);
  52
  53         cur += -cur & align-1;
  54
  55         if (n > end-cur) {
  56                 size_t req = n - (end-cur) + PAGE_SIZE-1 & -PAGE_SIZE;
  57
  58                 if (!cur) {
  59                         brk = __syscall(SYS_brk, 0);
  60                         brk += -brk & PAGE_SIZE-1;
  61                         cur = end = brk;
  62                 }
  63
  64                 if (brk == end && req < SIZE_MAX-brk
  65                     && !traverses_stack_p(brk, brk+req)
  66                     && __syscall(SYS_brk, brk+req)==brk+req) {
  67                         brk = end += req;
  68                 } else {
  69                         int new_area = 0;
  70                         req = n + PAGE_SIZE-1 & -PAGE_SIZE;
  71                         /* Only make a new area rather than individual mmap
  72                          * if wasted space would be over 1/8 of the map. */
  73                         if (req-n > req/8) {
  74                                 /* Geometric area size growth up to 64 pages,
  75                                  * bounding waste by 1/8 of the area. */
  76                                 size_t min = PAGE_SIZE<<(mmap_step/2);
  77                                 if (min-n > end-cur) {
  78                                         if (req < min) {
  79                                                 req = min;
  80                                                 if (mmap_step < 12)
  81                                                         mmap_step++;
  82                                         }
  83                                         new_area = 1;
  84                                 }
  85                         }
  86                         void *mem = __mmap(0, req, PROT_READ|PROT_WRITE,
  87                                 MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
  88                         if (mem == MAP_FAILED || !new_area) {
  89                                 UNLOCK(lock);
  90                                 return mem==MAP_FAILED ? 0 : mem;
  91                         }
  92                         cur = (uintptr_t)mem;
  93                         end = cur + req;
  94                 }
  95         }
  96
  97         p = (void *)cur;
  98         cur += n;
  99         UNLOCK(lock);
 100         return p;
 101 }
 102
 103 weak_alias(__simple_malloc, malloc);