1 files changed, 90 insertions, 0 deletions
diff --git a/src/libnm-std-aux/nm-std-utils.c b/src/libnm-std-aux/nm-std-utils.c
new file mode 100644
index 00000000..18692b19
--- /dev/null
+++ b/src/libnm-std-aux/nm-std-utils.c
@@ -0,0 +1,90 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "nm-default-std.h"
+
+#include "nm-std-utils.h"
+
+#include <stdint.h>
+#include <assert.h>
+#include <limits.h>
+
+/*****************************************************************************/
+
+size_t
+nm_utils_get_next_realloc_size(bool true_realloc, size_t requested)
+{
+    size_t n, x;
+
+    /* https://doc.qt.io/qt-5/containers.html#growth-strategies */
+
+    if (requested <= 40) {
+        /* small allocations. Increase in small steps of 8 bytes.
+         *
+         * We get thus sizes of 8, 16, 32, 40. */
+        if (requested <= 8)
+            return 8;
+        if (requested <= 16)
+            return 16;
+        if (requested <= 32)
+            return 32;
+
+        /* The return values for < 104 are essentially hard-coded, and the choice here is
+         * made without very strong reasons.
+         *
+         * We want to stay 24 bytes below the power-of-two border 64. Hence, return 40 here.
+         * However, the next step then is already 104 (128 - 24). It's a larger gap than in
+         * the steps before.
+         *
+         * It's not clear whether some of the steps should be adjusted (or how exactly). */
+        return 40;
+    }
+
+    if (requested <= 0x2000u - 24u || NM_UNLIKELY(!true_realloc)) {
+        /* mid sized allocations. Return next power of two, minus 24 bytes extra space
+         * at the beginning.
+         * That means, we double the size as we grow.
+         *
+         * With !true_realloc, it means that the caller does not intend to call
+         * realloc() but instead clone the buffer. This is for example the case, when we
+         * want to nm_explicit_bzero() the old buffer. In that case we really want to grow
+         * the buffer exponentially every time and not increment in page sizes of 4K (below).
+         *
+         * We get thus sizes of 104, 232, 488, 1000, 2024, 4072, 8168... */
+
+        if (NM_UNLIKELY(requested > SIZE_MAX / 2u - 24u))
+            goto out_huge;
+
+        x = requested + 24u;
+        n = 128u;
+        while (n < x) {
+            n <<= 1;
+            nm_assert(n > 128u);
+        }
+
+        nm_assert(n > 24u && n - 24u >= requested);
+        return n - 24u;
+    }
+
+    if (NM_UNLIKELY(requested > SIZE_MAX - 0x1000u - 24u)) {
+        /* overflow happened. */
+        goto out_huge;
+    }
+
+    /* For large allocations (with !true_realloc) we allocate memory in chunks of
+     * 4K (- 24 bytes extra), assuming that the memory gets mmapped and thus
+     * realloc() is efficient by just reordering pages. */
+    n = ((requested + (0x0FFFu + 24u)) & ~((size_t) 0x0FFFu)) - 24u;
+    nm_assert(n >= requested);
+    return n;
+
+out_huge:
+    if (sizeof(size_t) > 4u) {
+        /* on s390x (64 bit), gcc with LTO can complain that the size argument to
+         * malloc must not be larger than 9223372036854775807.
+         *
+         * Work around that by returning SSIZE_MAX. It should be plenty still! */
+        assert(requested <= (size_t) SSIZE_MAX);
+        return (size_t) SSIZE_MAX;
+    }
+    return SIZE_MAX;
+}