this extends the brk/stack collision protection added to full malloc
in commit
276904c2f6bde3a31a24ebfa201482601d18b4f9 to also protect the
__simple_malloc function used in static-linked programs that don't
reference the free function.
it also extends support for using mmap when brk fails, which full
malloc got in commit
5446303328adf4b4e36d9fba21848e6feb55fab4, to
__simple_malloc.
since __simple_malloc may expand the heap by arbitrarily large
increments, the stack collision detection is enhanced to detect
interval overlap rather than just proximity of a single address to the
stack. code size is increased a bit, but this is partly offset by the
sharing of code between the two malloc implementations, which due to
linking semantics, both get linked in a program that needs the full
malloc with realloc/free support.
--- /dev/null
+#include <limits.h>
+#include <stdint.h>
+#include <errno.h>
+#include <sys/mman.h>
+#include "libc.h"
+#include "syscall.h"
+
+/* This function returns true if the interval [old,new]
+ * intersects the 'len'-sized interval below &libc.auxv
+ * (interpreted as the main-thread stack) or below &b
+ * (the current stack). It is used to defend against
+ * buggy brk implementations that can cross the stack. */
+
+static int traverses_stack_p(uintptr_t old, uintptr_t new)
+{
+ const uintptr_t len = 8<<20;
+ uintptr_t a, b;
+
+ b = (uintptr_t)libc.auxv;
+ a = b > len ? b-len : 0;
+ if (new>a && old<b) return 1;
+
+ b = (uintptr_t)&b;
+ a = b > len ? b-len : 0;
+ if (new>a && old<b) return 1;
+
+ return 0;
+}
+
+void *__mmap(void *, size_t, int, int, int, off_t);
+
+/* Expand the heap in-place if brk can be used, or otherwise via mmap,
+ * using an exponential lower bound on growth by mmap to make
+ * fragmentation asymptotically irrelevant. The size argument is both
+ * an input and an output, since the caller needs to know the size
+ * allocated, which will be larger than requested due to page alignment
+ * and mmap minimum size rules. The caller is responsible for locking
+ * to prevent concurrent calls. */
+
+void *__expand_heap(size_t *pn)
+{
+ static uintptr_t brk;
+ static unsigned mmap_step;
+ size_t n = *pn;
+
+ if (n > SIZE_MAX/2 - PAGE_SIZE) {
+ errno = ENOMEM;
+ return 0;
+ }
+ n += -n & PAGE_SIZE-1;
+
+ if (!brk) {
+ brk = __syscall(SYS_brk, 0);
+ brk += -brk & PAGE_SIZE-1;
+ }
+
+ if (n < SIZE_MAX-brk && !traverses_stack_p(brk, brk+n)
+ && __syscall(SYS_brk, brk+n)==brk+n) {
+ *pn = n;
+ brk += n;
+ return (void *)(brk-n);
+ }
+
+ size_t min = (size_t)PAGE_SIZE << mmap_step/2;
+ if (n < min) n = min;
+ void *area = __mmap(0, n, PROT_READ|PROT_WRITE,
+ MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+ if (area == MAP_FAILED) return 0;
+ *pn = n;
+ mmap_step++;
+ return area;
+}
#include <errno.h>
#include "libc.h"
-uintptr_t __brk(uintptr_t);
-
#define ALIGN 16
+void *__expand_heap(size_t *);
+
void *__simple_malloc(size_t n)
{
- static uintptr_t cur, brk;
- uintptr_t base, new;
+ static char *cur, *end;
static volatile int lock[2];
- size_t align=1;
+ size_t align=1, pad;
+ void *p;
if (!n) n++;
- if (n > SIZE_MAX/2) goto toobig;
-
while (align<n && align<ALIGN)
align += align;
- n = n + align - 1 & -align;
LOCK(lock);
- if (!cur) cur = brk = __brk(0)+16;
- base = cur + align-1 & -align;
- if (n > SIZE_MAX - PAGE_SIZE - base) goto fail;
- if (base+n > brk) {
- new = base+n + PAGE_SIZE-1 & -PAGE_SIZE;
- if (__brk(new) != new) goto fail;
- brk = new;
- }
- cur = base+n;
- UNLOCK(lock);
- return (void *)base;
+ pad = -(uintptr_t)cur & align-1;
+
+ if (n <= SIZE_MAX/2 + ALIGN) n += pad;
+
+ if (n > end-cur) {
+ size_t m = n;
+ char *new = __expand_heap(&m);
+ if (!new) {
+ UNLOCK(lock);
+ return 0;
+ }
+ if (new != end) {
+ cur = new;
+ n -= pad;
+ pad = 0;
+ }
+ end = new + m;
+ }
-fail:
+ p = cur + pad;
+ cur += n;
UNLOCK(lock);
-toobig:
- errno = ENOMEM;
- return 0;
+ return p;
}
weak_alias(__simple_malloc, malloc);
#define inline inline __attribute__((always_inline))
#endif
-uintptr_t __brk(uintptr_t);
void *__mmap(void *, size_t, int, int, int, off_t);
int __munmap(void *, size_t);
void *__mremap(void *, size_t, size_t, int, ...);
};
static struct {
- uintptr_t brk;
- size_t *heap;
volatile uint64_t binmap;
struct bin bins[64];
- volatile int brk_lock[2];
volatile int free_lock[2];
- unsigned mmap_step;
} mal;
}
#endif
-static int is_near_stack(uintptr_t b)
-{
- const uintptr_t c = 8<<20;
- uintptr_t a = (uintptr_t)libc.auxv;
- uintptr_t d = (uintptr_t)&b;
- return a-b<=c || d-b<=c;
-}
+void *__expand_heap(size_t *);
static struct chunk *expand_heap(size_t n)
{
- static int init;
+ static int heap_lock[2];
+ static void *end;
+ void *p;
struct chunk *w;
- uintptr_t new;
-
- lock(mal.brk_lock);
- if (!init) {
- mal.brk = __brk(0);
-#ifdef SHARED
- mal.brk = mal.brk + PAGE_SIZE-1 & -PAGE_SIZE;
-#endif
- mal.brk = mal.brk + 2*SIZE_ALIGN-1 & -SIZE_ALIGN;
- mal.heap = (void *)mal.brk;
- init = 1;
- }
+ /* The argument n already accounts for the caller's chunk
+ * overhead needs, but if the heap can't be extended in-place,
+ * we need room for an extra zero-sized sentinel chunk. */
+ n += SIZE_ALIGN;
- if (n > SIZE_MAX - mal.brk - 2*PAGE_SIZE) goto fail;
- new = mal.brk + n + SIZE_ALIGN + PAGE_SIZE - 1 & -PAGE_SIZE;
- n = new - mal.brk;
+ lock(heap_lock);
- if (is_near_stack(mal.brk) || __brk(new) != new) {
- size_t min = (size_t)PAGE_SIZE << mal.mmap_step/2;
- n += -n & PAGE_SIZE-1;
- if (n < min) n = min;
- void *area = __mmap(0, n, PROT_READ|PROT_WRITE,
- MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
- if (area == MAP_FAILED) goto fail;
+ p = __expand_heap(&n);
+ if (!p) {
+ unlock(heap_lock);
+ return 0;
+ }
- mal.mmap_step++;
- area = (char *)area + SIZE_ALIGN - OVERHEAD;
- w = area;
+ /* If not just expanding existing space, we need to make a
+ * new sentinel chunk below the allocated space. */
+ if (p != end) {
+ /* Valid/safe because of the prologue increment. */
n -= SIZE_ALIGN;
+ p = (char *)p + SIZE_ALIGN;
+ w = MEM_TO_CHUNK(p);
w->psize = 0 | C_INUSE;
- w->csize = n | C_INUSE;
- w = NEXT_CHUNK(w);
- w->psize = n | C_INUSE;
- w->csize = 0 | C_INUSE;
-
- unlock(mal.brk_lock);
-
- return area;
}
- w = MEM_TO_CHUNK(mal.heap);
- w->psize = 0 | C_INUSE;
-
- w = MEM_TO_CHUNK(new);
+ /* Record new heap end and fill in footer. */
+ end = (char *)p + n;
+ w = MEM_TO_CHUNK(end);
w->psize = n | C_INUSE;
w->csize = 0 | C_INUSE;
- w = MEM_TO_CHUNK(mal.brk);
+ /* Fill in header, which may be new or may be replacing a
+ * zero-size sentinel header at the old end-of-heap. */
+ w = MEM_TO_CHUNK(p);
w->csize = n | C_INUSE;
- mal.brk = new;
-
- unlock(mal.brk_lock);
+
+ unlock(heap_lock);
return w;
-fail:
- unlock(mal.brk_lock);
- errno = ENOMEM;
- return 0;
}
static int adjust_size(size_t *n)
projects / oweals / musl.git / commitdiff