reintroduce hardening against partially-replaced allocator

commit 618b18c78e33acfe54a4434e91aa57b8e171df89 removed the previous
detection and hardening since it was incorrect. commit
72141795d4edd17f88da192447395a48444afa10 already handled all that
remained for hardening the static-linked case. in the dynamic-linked
case, have the dynamic linker check whether malloc was replaced and
make that information available.

with these changes, the properties documented in commit
c9f415d7ea2dace5bf77f6518b6afc36bb7a5732 are restored: if calloc is
not provided, it will behave as malloc+memset, and any of the
memalign-family functions not provided will fail with ENOMEM.

ldso/dynlink.c

@@ -133,6 +133,9 @@ static struct dso *const nodeps_dummy;
 
 struct debug *_dl_debug_addr = &debug;
 
+__attribute__((__visibility__("hidden")))
+extern int __malloc_replaced;
+
 __attribute__((__visibility__("hidden")))
 void (*const __init_array_start)(void)=0, (*const __fini_array_start)(void)=0;
 
@@ -1691,6 +1694,12 @@ _Noreturn void __dls3(size_t *sp)
 	if (ldso_fail) _exit(127);
 	if (ldd_mode) _exit(0);
 
+	/* Determine if malloc was interposed by a replacement implementation
+	 * so that calloc and the memalign family can harden against the
+	 * possibility of incomplete replacement. */
+	if (find_sym(head, "malloc", 1).dso != &ldso)
+		__malloc_replaced = 1;
+
 	/* Switch to runtime mode: any further failures in the dynamic
 	 * linker are a reportable failure rather than a fatal startup
 	 * error. */

src/internal/malloc_impl.h

@@ -39,4 +39,7 @@ struct bin {
 __attribute__((__visibility__("hidden")))
 void __bin_chunk(struct chunk *);
 
+__attribute__((__visibility__("hidden")))
+extern int __malloc_replaced;
+
 #endif

src/malloc/malloc.c

@@ -20,6 +20,8 @@ static struct {
 	volatile int free_lock[2];
 } mal;
 
+int __malloc_replaced;
+
 /* Synchronization tools */
 
 static inline void lock(volatile int *lk)
@@ -356,10 +358,13 @@ void *calloc(size_t m, size_t n)
 	}
 	n *= m;
 	void *p = malloc(n);
-	if (!p || IS_MMAPPED(MEM_TO_CHUNK(p)))
-		return p;
-	if (n >= PAGE_SIZE)
-		n = mal0_clear(p, PAGE_SIZE, n);
+	if (!p) return p;
+	if (!__malloc_replaced) {
+		if (IS_MMAPPED(MEM_TO_CHUNK(p)))
+			return p;
+		if (n >= PAGE_SIZE)
+			n = mal0_clear(p, PAGE_SIZE, n);
+	}
 	return memset(p, 0, n);
 }
 

src/malloc/memalign.c

@@ -13,7 +13,7 @@ void *__memalign(size_t align, size_t len)
 		return 0;
 	}
 
-	if (len > SIZE_MAX - align) {
+	if (len > SIZE_MAX - align || __malloc_replaced) {
 		errno = ENOMEM;
 		return 0;
 	}