[Qemu-devel] [PATCH v2] linux-user: Fix 32-on-64 mmap for x86_64

[Alexander Graf <agraf@suse.de>]

When running a 32 bit guest on a 64 bit host, we can run into trouble while
calling the host's mmap() because it could potentially give us a 64 bit
return value which the guest can't interpret.

There are 2 ways of dealing with this:

  1) Only do MAP_FIXED mmap calls and implement our own vm management in QEMU
  2) Tell the kernel that we only want mappings in the lower 32 bits

Way 1 is very involved and hard to do. It's been advocated forever now but
nobody sat down to actually implement it.

Way 2 is easy. It's what this patch does. However, it only works on x86_64
because that's the only platform implementing the MAP_32BIT flag. Since most
people are on x86_64 though, I think it's a good enough compromise for now
though

Signed-off-by: Alexander Graf <agraf@suse.de>

---

v1 -> v2:

  - make prettier by just wrapping mmap in linux-user/mmap.c
---
 linux-user/mmap.c |   46 +++++++++++++++++++++++++++++++++-------------
 1 files changed, 33 insertions(+), 13 deletions(-)

diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index 994c02b..b6f0fbf 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -33,6 +33,26 @@
 
 //#define DEBUG_MMAP
 
+/*
+ * On x86_64 we can tell mmap that we only want to map within the first 32
+ * bits to not get pointers that potentially exceed the return size. Without
+ * this flag set mmap will eventually break for users when running 32-on-64.
+ *
+ * However, Linux doesn't implement this for non-x86_64 systems. So we have
+ * to safeguard the flag and just ignore it on other architectures. At least
+ * we fixed the "common case" this way :).
+ *
+ * - agraf
+ */
+static void *qemu_mmap(void *addr, size_t length, int prot, int flags, int fd,
+                       off_t offset)
+{
+#if defined(MAP_32BIT) && defined(__x86_64__) && (TARGET_LONG_BITS == 32)
+    flags |= MAP_32BIT;
+#endif
+    return mmap(addr, length, prot, flags, fd, offset);
+}
+
 #if defined(CONFIG_USE_NPTL)
 static pthread_mutex_t mmap_mutex = PTHREAD_MUTEX_INITIALIZER;
 static __thread int mmap_lock_count;
@@ -168,8 +188,8 @@ static int mmap_frag(abi_ulong real_start,
 
     if (prot1 == 0) {
         /* no page was there, so we allocate one */
-        void *p = mmap(host_start, qemu_host_page_size, prot,
-                       flags | MAP_ANONYMOUS, -1, 0);
+        void *p = qemu_mmap(host_start, qemu_host_page_size, prot,
+                            flags | MAP_ANONYMOUS, -1, 0);
         if (p == MAP_FAILED)
             return -1;
         prot1 = prot;
@@ -291,8 +311,8 @@ abi_ulong mmap_find_vma(abi_ulong start, abi_ulong size)
          *  - mremap() with MREMAP_FIXED flag
          *  - shmat() with SHM_REMAP flag
          */
-        ptr = mmap(g2h(addr), size, PROT_NONE,
-                   MAP_ANONYMOUS|MAP_PRIVATE|MAP_NORESERVE, -1, 0);
+        ptr = qemu_mmap(g2h(addr), size, PROT_NONE,
+                        MAP_ANONYMOUS|MAP_PRIVATE|MAP_NORESERVE, -1, 0);
 
         /* ENOMEM, if host address space has no memory */
         if (ptr == MAP_FAILED) {
@@ -453,15 +473,15 @@ abi_long target_mmap(abi_ulong start, abi_ulong len, int prot,
         /* Note: we prefer to control the mapping address. It is
            especially important if qemu_host_page_size >
            qemu_real_host_page_size */
-        p = mmap(g2h(mmap_start),
-                 host_len, prot, flags | MAP_FIXED | MAP_ANONYMOUS, -1, 0);
+        p = qemu_mmap(g2h(mmap_start),
+                      host_len, prot, flags | MAP_FIXED | MAP_ANONYMOUS, -1, 0);
         if (p == MAP_FAILED)
             goto fail;
         /* update start so that it points to the file position at 'offset' */
         host_start = (unsigned long)p;
         if (!(flags & MAP_ANONYMOUS)) {
-            p = mmap(g2h(mmap_start), len, prot, 
-                     flags | MAP_FIXED, fd, host_offset);
+            p = qemu_mmap(g2h(mmap_start), len, prot,
+                          flags | MAP_FIXED, fd, host_offset);
             host_start += offset - host_offset;
         }
         start = h2g(host_start);
@@ -546,8 +566,8 @@ abi_long target_mmap(abi_ulong start, abi_ulong len, int prot,
                 offset1 = 0;
             else
                 offset1 = offset + real_start - start;
-            p = mmap(g2h(real_start), real_end - real_start,
-                     prot, flags, fd, offset1);
+            p = qemu_mmap(g2h(real_start), real_end - real_start,
+                          prot, flags, fd, offset1);
             if (p == MAP_FAILED)
                 goto fail;
         }
@@ -602,9 +622,9 @@ static void mmap_reserve(abi_ulong start, abi_ulong size)
             real_end -= qemu_host_page_size;
     }
     if (real_start != real_end) {
-        mmap(g2h(real_start), real_end - real_start, PROT_NONE,
-                 MAP_FIXED | MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE,
-                 -1, 0);
+        qemu_mmap(g2h(real_start), real_end - real_start, PROT_NONE,
+                  MAP_FIXED | MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE,
+                  -1, 0);
     }
 }
 
-- 
1.6.0.2