From 2e9bb0a47635479914532cc77160549fca985589 Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Sun, 6 Sep 2026 14:51:04 +0200
Subject: [PATCH] feat: add pageAlignDown and pageAlignUp

---
 src/mmap.c | 161 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 161 insertions(+)
 create mode 100644 src/mmap.c

diff --git a/src/mmap.c b/src/mmap.c
new file mode 100644
index 0000000..ebbd741
--- /dev/null
+++ b/src/mmap.c
@@ -0,0 +1,161 @@
+#include "mmap.h"
+#include "sched.h"
+#include "elf.h"
+#include "paging.h"
+#include "pmm.h"
+#include "vfs.h"
+#include "string.h"
+
+/*
+ * Real demand-paged mmap(2)/munmap(2) on top of this kernel's single
+ * shared page table: a Vma only ever RESERVES address space (mmapReserve)
+ * -- no physical frame is allocated and no file byte is read until the
+ * very first access to a given page actually traps into isr.c's #PF
+ * handler, which resolves it here (mmapFaultHandler). This is the same
+ * "lazy" contract pagingMapUserRange's own eager sibling does NOT have
+ * (that one maps and zeroes every page up front, for an ELF image/stack
+ * that is about to be fully used anyway) -- mmap's whole point is a
+ * mapping the caller may only ever touch a fraction of.
+ *
+ * Every mapping is placed inside the calling thread's own elfSlotSize
+ * slot (elf.h), specifically the sub-range [userSlotBase +
+ * elfSlotStackOffset, userSlotBase + elfSlotSize) -- everything from the
+ * top of the small, fixed-size user stack up to the end of the slot, 128
+ * MiB that no ELF LOAD segment or brk-growth ever reaches in this
+ * kernel's own binaries (elfSlotBrkOffset caps brk's own growth at 64
+ * MiB in, elfSlotStackOffset itself at 128 MiB in -- see elf.h). There is
+ * no address-space-wide mmap arena the way a real kernel's own mm_struct
+ * has one: with a single page table shared by every thread in the
+ * system, "the whole address space" is not a per-thread concept to begin
+ * with, so mmap here is scoped exactly the way brk/the stack already are.
+ */
+
+static uint64 pageAlignDown(uint64 x)
+{
+    return x & ~(uint64)(frameSize - 1);
+}
+
+static uint64 pageAlignUp(uint64 x)
+{
+    return (x + frameSize - 1) & ~(uint64)(frameSize - 1);
+}
+
+static uint64 mmapRegionStart(struct Thread* thread)
+{
+    return thread->userSlotBase + elfSlotStackOffset;
+}
+
+static uint64 mmapRegionEnd(struct Thread* thread)
+{
+    return thread->userSlotBase + elfSlotSize;
+}
+
+static bool rangesOverlap(uint64 aStart, uint64 aEnd, uint64 bStart, uint64 bEnd)
+{
+    return aStart < bEnd && bStart < aEnd;
+}
+
+Vma* mmapFind(struct Thread* thread, uint64 addr)
+{
+    for (int i = 0; i < maxVmasPerThread; i++) {
+        Vma* v = &thread->vmas[i];
+        if (v->used && addr >= v->start && addr < v->end)
+            return v;
+    }
+    return nullptr;
+}
+
+static Vma* findFreeVmaSlot(struct Thread* thread)
+{
+    for (int i = 0; i < maxVmasPerThread; i++)
+        if (!thread->vmas[i].used)
+            return &thread->vmas[i];
+    return nullptr;
+}
+
+static bool regionFree(struct Thread* thread, uint64 start, uint64 end)
+{
+    for (int i = 0; i < maxVmasPerThread; i++) {
+        Vma* v = &thread->vmas[i];
+        if (v->used && rangesOverlap(start, end, v->start, v->end))
+            return false;
+    }
+    return true;
+}
+
+/* First-fit search for `length` free bytes inside thread's own mmap
+   region: candidate gaps are the region's own start plus every existing
+   Vma's own end, sorted into address order (a plain insertion sort --
+   maxVmasPerThread's small fixed bound makes anything fancier pointless),
+   then the first candidate `length` bytes actually fits at wins. */
+static uint64 findFreeRegion(struct Thread* thread, uint64 length)
+{
+    uint64 regionStart = mmapRegionStart(thread);
+    uint64 regionEnd = mmapRegionEnd(thread);
+
+    uint64 candidates[maxVmasPerThread + 1];
+    int candidateCount = 0;
+    candidates[candidateCount++] = regionStart;
+    for (int i = 0; i < maxVmasPerThread; i++)
+        if (thread->vmas[i].used)
+            candidates[candidateCount++] = thread->vmas[i].end;
+
+    for (int i = 1; i < candidateCount; i++) {
+        uint64 key = candidates[i];
+        int j = i - 1;
+        while (j >= 0 && candidates[j] > key) {
+            candidates[j + 1] = candidates[j];
+            j--;
+        }
+        candidates[j + 1] = key;
+    }
+
+    for (int i = 0; i < candidateCount; i++) {
+        uint64 candidate = candidates[i];
+        if (candidate < regionStart)
+            continue;
+        if (candidate + length < candidate || candidate + length > regionEnd)
+            continue;
+        if (regionFree(thread, candidate, candidate + length))
+            return candidate;
+    }
+    return 0;
+}
+
+bool mmapReserve(struct Thread* thread, uint64 addr, uint64 length, uint32 prot,
+                  bool anonymous, int fd, uint64 fileOffset, uint64* outAddr)
+{
+    if (length == 0)
+        return false;
+    if (addr % frameSize != 0 || length % frameSize != 0)
+        return false;
+    if (!anonymous && fileOffset % frameSize != 0)
+        return false;
+
+    uint64 start = addr;
+    if (start == 0) {
+        start = findFreeRegion(thread, length);
+        if (start == 0)
+            return false;
+    }
+
+    uint64 end = start + length;
+    if (end < start)
+        return false;
+    if (start < mmapRegionStart(thread) || end > mmapRegionEnd(thread))
+        return false;
+    if (!regionFree(thread, start, end))
+        return false;
+
+    Vma* slot = findFreeVmaSlot(thread);
+    if (!slot)
+        return false;
+
+    slot->used = true;
+    slot->start = start;
+    slot->end = end;
+    slot->prot = prot;
+    slot->anonymous = anonymous;
+    slot->fd = anonymous ? -1 : fd;
+    slot->fileOffset = anonymous ? 0 : fileOffset;
+