Don't try to mmap past EOF

On Mac OS, mmap would succeed, returning a valid pointer, but trying to
read from it would result in a SIGBUS.

By adding this check we commit to a safe cross-platform behavior users
can depend on.

Reviewed-by: Thiago Macieira

1 files changed, 6 insertions, 0 deletions

diff --git a/src/corelib/io/qfsfileengine_unix.cpp b/src/corelib/io/qfsfileengine_unix.cpp
index 6af5674..7824520 100644
--- a/src/corelib/io/qfsfileengine_unix.cpp
+++ b/src/corelib/io/qfsfileengine_unix.cpp
@@ -1250,6 +1250,12 @@ uchar *QFSFileEnginePrivate::map(qint64 offset, qint64 size, QFile::MemoryMapFla
         return 0;
     }
 
+    // If we know the mapping will extend beyond EOF, fail early to avoid
+    // undefined behavior. Otherwise, let mmap have its say.
+    if (doStat()
+            && (QT_OFF_T(size) > st.st_size - QT_OFF_T(offset)))
+        return 0;
+
     int access = 0;
     if (openMode & QIODevice::ReadOnly) access |= PROT_READ;
     if (openMode & QIODevice::WriteOnly) access |= PROT_WRITE;