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 #include "fork_impl.h"
  10
  11 #define ALIGN 16
  12
  13 /* This function returns true if the interval [old,new]
  14  * intersects the 'len'-sized interval below &libc.auxv
  15  * (interpreted as the main-thread stack) or below &b
  16  * (the current stack). It is used to defend against
  17  * buggy brk implementations that can cross the stack. */
  18
  19 static int traverses_stack_p(uintptr_t old, uintptr_t new)
  20 {
  21         const uintptr_t len = 8<<20;
  22         uintptr_t a, b;
  23
  24         b = (uintptr_t)libc.auxv;
  25         a = b > len ? b-len : 0;
  26         if (new>a && old<b) return 1;
  27
  28         b = (uintptr_t)&b;
  29         a = b > len ? b-len : 0;
  30         if (new>a && old<b) return 1;
  31
  32         return 0;
  33 }
  34
  35 static volatile int lock[1];
  36 volatile int *const __bump_lockptr = lock;
  37
  38 static void *__simple_malloc(size_t n)
  39 {
  40         static uintptr_t brk, cur, end;
  41         static unsigned mmap_step;
  42         size_t align=1;
  43         void *p;
  44
  45         if (n > SIZE_MAX/2) {
  46                 errno = ENOMEM;
  47                 return 0;
  48         }
  49
  50         if (!n) n++;
  51         while (align<n && align<ALIGN)
  52                 align += align;
  53
  54         LOCK(lock);
  55
  56         cur += -cur & align-1;
  57
  58         if (n > end-cur) {
  59                 size_t req = n - (end-cur) + PAGE_SIZE-1 & -PAGE_SIZE;
  60
  61                 if (!cur) {
  62                         brk = __syscall(SYS_brk, 0);
  63                         brk += -brk & PAGE_SIZE-1;
  64                         cur = end = brk;
  65                 }
  66
  67                 if (brk == end && req < SIZE_MAX-brk
  68                     && !traverses_stack_p(brk, brk+req)
  69                     && __syscall(SYS_brk, brk+req)==brk+req) {
  70                         brk = end += req;
  71                 } else {
  72                         int new_area = 0;
  73                         req = n + PAGE_SIZE-1 & -PAGE_SIZE;
  74                         /* Only make a new area rather than individual mmap
  75                          * if wasted space would be over 1/8 of the map. */
  76                         if (req-n > req/8) {
  77                                 /* Geometric area size growth up to 64 pages,
  78                                  * bounding waste by 1/8 of the area. */
  79                                 size_t min = PAGE_SIZE<<(mmap_step/2);
  80                                 if (min-n > end-cur) {
  81                                         if (req < min) {
  82                                                 req = min;
  83                                                 if (mmap_step < 12)
  84                                                         mmap_step++;
  85                                         }
  86                                         new_area = 1;
  87                                 }
  88                         }
  89                         void *mem = __mmap(0, req, PROT_READ|PROT_WRITE,
  90                                 MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
  91                         if (mem == MAP_FAILED || !new_area) {
  92                                 UNLOCK(lock);
  93                                 return mem==MAP_FAILED ? 0 : mem;
  94                         }
  95                         cur = (uintptr_t)mem;
  96                         end = cur + req;
  97                 }
  98         }
  99
 100         p = (void *)cur;
 101         cur += n;
 102         UNLOCK(lock);
 103         return p;
 104 }
 105
 106 weak_alias(__simple_malloc, __libc_malloc_impl);
 107
 108 void *__libc_malloc(size_t n)
 109 {
 110         return __libc_malloc_impl(n);
 111 }
 112
 113 static void *default_malloc(size_t n)
 114 {
 115         return __libc_malloc_impl(n);
 116 }
 117
 118 weak_alias(default_malloc, malloc);