[PATCH v2 7/9] tools/nolibc/stdlib: Implement `malloc()`, `calloc()`, `realloc()` and `free()`

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

Implement basic dynamic allocator functions. These functions are
currently only available on architectures that have nolibc mmap()
syscall implemented. These are not a super-fast memory allocator,
but at least they can satisfy basic needs for having heap without
libc.

Cc: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
===

@@ Changelog:

   Link v1: https://lore.gnuweeb.org/gwml/20220324073039.140946-10-ammarfaizi2@gnuweeb.org
   v1 -> v2:
    * No changes *

   Link v2: https://lore.kernel.org/lkml/20220322102115.186179-7-ammarfaizi2@gnuweeb.org
   RFC v2 -> v1:
    - Round up the malloc() allocation to 4096 (comment from David).
    - Don't realloc() if we still have enough memory to contain the
      requested new size (comment from David).
    - Fix conflict with getenv() fix (after rebase).

   Link v1: https://lore.kernel.org/lkml/20220320093750.159991-6-ammarfaizi2@gnuweeb.org
   RFC v1 -> RFC v2:
    - Move container_of() and offsetof() macro to types.h with a
      separate patch (comment from Willy).
---
 tools/include/nolibc/stdlib.h | 81 +++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)

diff --git a/tools/include/nolibc/stdlib.h b/tools/include/nolibc/stdlib.h
index 8a07e263f0d0..8fd32eaf8037 100644
--- a/tools/include/nolibc/stdlib.h
+++ b/tools/include/nolibc/stdlib.h
@@ -11,7 +11,12 @@
 #include "arch.h"
 #include "types.h"
 #include "sys.h"
+#include "string.h"
 
+struct nolibc_heap {
+	size_t	len;
+	char	user_p[] __attribute__((__aligned__));
+};
 
 /* Buffer used to store int-to-ASCII conversions. Will only be implemented if
  * any of the related functions is implemented. The area is large enough to
@@ -60,6 +65,18 @@ int atoi(const char *s)
 	return atol(s);
 }
 
+static __attribute__((unused))
+void free(void *ptr)
+{
+	struct nolibc_heap *heap;
+
+	if (!ptr)
+		return;
+
+	heap = container_of(ptr, struct nolibc_heap, user_p);
+	munmap(heap, heap->len);
+}
+
 /* getenv() tries to find the environment variable named <name> in the
  * environment array pointed to by global variable "environ" which must be
  * declared as a char **, and must be terminated by a NULL (it is recommended
@@ -91,6 +108,70 @@ char *getenv(const char *name)
 	return _getenv(name, environ);
 }
 
+static __attribute__((unused))
+void *malloc(size_t len)
+{
+	struct nolibc_heap *heap;
+
+	/* Always allocate memory with size multiple of 4096. */
+	len  = sizeof(*heap) + len;
+	len  = (len + 4095UL) & -4096UL;
+	heap = mmap(NULL, len, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE,
+		    -1, 0);
+	if (__builtin_expect(heap == MAP_FAILED, 0))
+		return NULL;
+
+	heap->len = len;
+	return heap->user_p;
+}
+
+static __attribute__((unused))
+void *calloc(size_t size, size_t nmemb)
+{
+	void *orig;
+	size_t res = 0;
+
+	if (__builtin_expect(__builtin_mul_overflow(nmemb, size, &res), 0)) {
+		SET_ERRNO(ENOMEM);
+		return NULL;
+	}
+
+	/*
+	 * No need to zero the heap, the MAP_ANONYMOUS in malloc()
+	 * already does it.
+	 */
+	return malloc(res);
+}
+
+static __attribute__((unused))
+void *realloc(void *old_ptr, size_t new_size)
+{
+	struct nolibc_heap *heap;
+	size_t user_p_len;
+	void *ret;
+
+	if (!old_ptr)
+		return malloc(new_size);
+
+	heap = container_of(old_ptr, struct nolibc_heap, user_p);
+	user_p_len = heap->len - sizeof(*heap);
+	/*
+	 * Don't realloc() if @user_p_len >= @new_size, this block of
+	 * memory is still enough to handle the @new_size. Just return
+	 * the same pointer.
+	 */
+	if (user_p_len >= new_size)
+		return old_ptr;
+
+	ret = malloc(new_size);
+	if (__builtin_expect(!ret, 0))
+		return NULL;
+
+	memcpy(ret, heap->user_p, heap->len);
+	munmap(heap, heap->len);
+	return ret;
+}
+
 /* Converts the unsigned long integer <in> to its hex representation into
  * buffer <buffer>, which must be long enough to store the number and the
  * trailing zero (17 bytes for "ffffffffffffffff" or 9 for "ffffffff"). The
-- 
Ammar Faizi