Android application sandbox
Android is built upon the Linux kernel. Linux is an open source operating system that runs on devices, from something as small as a mobile phone (in the case of Android), through to the largest of super computers.Under Linux, processes belong to users, where each user has a unique user ID (UID). The operating system prevents a process belonging to one UID from reading the files and data belonging to another UID, unless the second UID has explicitly shared the data with the first UID.Android takes this further. Rather than each UID belonging to a distinct person, e.g. multiple users logged into a server, in Android each app is given a unique UID.From the perspective of the Linux kernel, each Android app corresponds to a distinct ‘user’. This makes sense as each Android device only has one real physical user (mobile phones don’t have multiple login accounts and simultaneous users!).
Note for Nerds: two apps that are signed with the same key (i.e. from the same developer) can share a UID if, at installation, they both request this. Apps that share a UID can read and write to each other’s files.
Note for Nerds: Dalvik (Android’s VM 1), unlike JVMs or the .NET runtime, the Dalvik VM is not a security boundary as Dalvik can interoperate with native code. The security boundary is the Android Application Sandbox, i.e. security is enforced by the Linux kernel, not by Dalvik.
- Since Android 5.0 Dalvik has been replaced by the Android Runtime (ART), which is a new virtual machine with Ahead-of-Time (AOT) compilation, and improved garbage collection. ↩
Secure Android App Development
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