linux-user: Rewrite mmap_reserve

Use 'last' variables instead of 'end' variables; be careful
about avoiding overflow.  Assert that the mmap succeeded.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230707204054.8792-21-richard.henderson@linaro.org>

linux-user/mmap.c

@@ -722,47 +722,63 @@ abi_long target_mmap(abi_ulong start, abi_ulong len, int target_prot,
     return -1;
 }
 
-static void mmap_reserve(abi_ulong start, abi_ulong size)
+static void mmap_reserve(abi_ulong start, abi_ulong len)
 {
     abi_ulong real_start;
-    abi_ulong real_end;
-    abi_ulong addr;
-    abi_ulong end;
+    abi_ulong real_last;
+    abi_ulong real_len;
+    abi_ulong last;
+    abi_ulong a;
+    void *host_start, *ptr;
     int prot;
 
+    last = start + len - 1;
     real_start = start & qemu_host_page_mask;
-    real_end = HOST_PAGE_ALIGN(start + size);
-    end = start + size;
-    if (start > real_start) {
-        /* handle host page containing start */
+    real_last = HOST_PAGE_ALIGN(last) - 1;
+
+    /*
+     * If guest pages remain on the first or last host pages,
+     * adjust the deallocation to retain those guest pages.
+     * The single page special case is required for the last page,
+     * lest real_start overflow to zero.
+     */
+    if (real_last - real_start < qemu_host_page_size) {
         prot = 0;
-        for (addr = real_start; addr < start; addr += TARGET_PAGE_SIZE) {
-            prot |= page_get_flags(addr);
+        for (a = real_start; a < start; a += TARGET_PAGE_SIZE) {
+            prot |= page_get_flags(a);
         }
-        if (real_end == real_start + qemu_host_page_size) {
-            for (addr = end; addr < real_end; addr += TARGET_PAGE_SIZE) {
-                prot |= page_get_flags(addr);
-            }
-            end = real_end;
+        for (a = last; a < real_last; a += TARGET_PAGE_SIZE) {
+            prot |= page_get_flags(a + 1);
+        }
+        if (prot != 0) {
+            return;
+        }
+    } else {
+        for (prot = 0, a = real_start; a < start; a += TARGET_PAGE_SIZE) {
+            prot |= page_get_flags(a);
         }
         if (prot != 0) {
             real_start += qemu_host_page_size;
         }
-    }
-    if (end < real_end) {
-        prot = 0;
-        for (addr = end; addr < real_end; addr += TARGET_PAGE_SIZE) {
-            prot |= page_get_flags(addr);
+
+        for (prot = 0, a = last; a < real_last; a += TARGET_PAGE_SIZE) {
+            prot |= page_get_flags(a + 1);
         }
         if (prot != 0) {
-            real_end -= qemu_host_page_size;
+            real_last -= qemu_host_page_size;
+        }
+
+        if (real_last < real_start) {
+            return;
         }
     }
-    if (real_start != real_end) {
-        mmap(g2h_untagged(real_start), real_end - real_start, PROT_NONE,
-                 MAP_FIXED | MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE,
-                 -1, 0);
-    }
+
+    real_len = real_last - real_start + 1;
+    host_start = g2h_untagged(real_start);
+
+    ptr = mmap(host_start, real_len, PROT_NONE,
+               MAP_FIXED | MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE, -1, 0);
+    assert(ptr == host_start);
 }
 
 int target_munmap(abi_ulong start, abi_ulong len)