Forkserver Does Not Work On Android

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Introduction

The forkserver is a crucial component in the AFL (American Fuzzy Lop) fuzzing framework, responsible for creating a new process to execute the target binary. However, users have reported issues with the forkserver not working on Android devices. In this article, we will delve into the reasons behind this problem and provide a step-by-step guide to resolve it.

Understanding the Forkserver

The forkserver is a mechanism that allows AFL to create a new process to execute the target binary. This process is essential for fuzzing, as it enables AFL to test the target binary with various inputs and identify potential vulnerabilities. The forkserver is typically implemented in C, but in recent versions of AFL, it has been rewritten in Rust.

The Issue with Forkserver on Android

Users have reported that the forkserver does not work on Android devices. This issue is likely due to the fact that the forkserver is not properly configured to work on Android. The forkserver relies on certain system calls, such as shmat, which are not available on Android.

The Role of shmat in Forkserver

shmat is a system call that allows a process to attach to a shared memory segment. In the context of the forkserver, shmat is used to attach to the shared memory segment created by the parent process. However, on Android, shmat is not available, which causes the forkserver to fail.

The Patch for shmat

In a previous patch, the shmat issue was addressed by modifying the libc crate to include a custom implementation of shmat. However, this patch is not applicable to the Rust version of the forkserver.

The Need for a Wrapper in Rust

As mentioned earlier, the forkserver has been rewritten in Rust. To resolve the issue with the forkserver on Android, a wrapper needs to be added to the Rust version of the forkserver to handle the shmat system call.

Adding a Wrapper to the Rust Forkserver

To add a wrapper to the Rust forkserver, you need to create a new file called wrapper.rs in the forkserver directory. In this file, you need to implement the shmat system call using the libc crate.

use libc::{c_int, c_void, shmat};

pub fn shmat(id: c_int, addr: *mut c_void, flags: c_int) -> *mut c_void {
    // Implement the shmat system call here
    unimplemented!()
}

Configuring the Forkserver on Android

Once you have added the wrapper to the Rust forkserver, you need to configure the forkserver to use the wrapper. To do this, you need to modify the Cargo.toml file to include the wrapper module.

[lib]
name = "forkserver"
crate-type = ["cdylib"]

[dependencies]
libc = "0.2.93"

Building the Forkserver on Android

To build the forkserver on Android, you need to use the cargo command to build the `forkserver crate.

cargo build --target aarch64-linux-android

Conclusion

In conclusion, the forkserver does not work on Android due to the lack of the shmat system call. To resolve this issue, a wrapper needs to be added to the Rust version of the forkserver to handle the shmat system call. By following the steps outlined in this article, you should be able to configure the forkserver to work on Android devices.

Troubleshooting

If you encounter any issues while building the forkserver on Android, you can refer to the following troubleshooting guide:

  • Error: shmat not found: This error occurs when the shmat system call is not available on the Android device. To resolve this issue, you need to add the wrapper to the Rust forkserver.
  • Error: wrapper module not found: This error occurs when the wrapper module is not included in the Cargo.toml file. To resolve this issue, you need to modify the Cargo.toml file to include the wrapper module.

References

Q&A: Forkserver on Android

Q: What is the forkserver and why is it important? A: The forkserver is a crucial component in the AFL (American Fuzzy Lop) fuzzing framework, responsible for creating a new process to execute the target binary. It is essential for fuzzing, as it enables AFL to test the target binary with various inputs and identify potential vulnerabilities.

Q: Why does the forkserver not work on Android? A: The forkserver does not work on Android due to the lack of the shmat system call. shmat is a system call that allows a process to attach to a shared memory segment, which is necessary for the forkserver to function.

Q: What is the role of shmat in the forkserver? A: shmat is used to attach to the shared memory segment created by the parent process. In the context of the forkserver, shmat is used to attach to the shared memory segment and execute the target binary.

Q: How can I resolve the issue with the forkserver on Android? A: To resolve the issue with the forkserver on Android, you need to add a wrapper to the Rust version of the forkserver to handle the shmat system call. This involves creating a new file called wrapper.rs in the forkserver directory and implementing the shmat system call using the libc crate.

Q: What are the steps to add a wrapper to the Rust forkserver? A: To add a wrapper to the Rust forkserver, you need to:

  1. Create a new file called wrapper.rs in the forkserver directory.
  2. Implement the shmat system call using the libc crate in the wrapper.rs file.
  3. Modify the Cargo.toml file to include the wrapper module.
  4. Build the forkserver using the cargo command.

Q: How can I configure the forkserver to use the wrapper? A: To configure the forkserver to use the wrapper, you need to modify the Cargo.toml file to include the wrapper module. This involves adding the following line to the Cargo.toml file:

[lib]
name = "forkserver"
crate-type = ["cdylib"]

[dependencies]
libc = "0.2.93"

Q: How can I build the forkserver on Android? A: To build the forkserver on Android, you need to use the cargo command to build the forkserver crate. This involves running the following command:

cargo build --target aarch64-linux-android

Q: What are the common issues that may occur while building the forkserver on Android? A: The common issues that may occur while building the forkserver on Android include:

  • Error: shmat not found: This error occurs when the shmat system call is not available on the Android device. To resolve this issue, you need to add the wrapper to the Rust forkserver.
  • Error: wrapper module not found: This error occurs when the wrapper module is not included in the Cargo.toml file. To resolve this issue, you need to modify the Cargo.toml file to include the wrapper module.

Q: Where can I find more information about the forkserver and AFL? A: You can find more information about the forkserver and AFL on the following websites: