novdso

About vDSO

vDSO is a shared library provided by the kernel that exports a subset of frequently used system calls (e.g., gettimeofday(2)) as user space functions. By using this technique, the need for costly context switches can be avoided. On the other hand, vDSO can complicate debugging since these vsyscalls are invisible to ptrace.

The AT_SYSINFO_EHDR field in the auxiliary vector of a process points to the vDSO memory area mapped by the kernel.

man 7 vdso

About novdso

novdso is a small wrapper programm that forces the wrapped process to not use the vDSO lib by overwriting the AT_SYSINFO_EHDR tag in its auxiliary vector. Afterwards, the wrapped process can be seized by another program (e.g., lldb or strace) for further debugging.

Compatible with amd64 machines.

Build

make

Usage

$ novdso
usage: novdso FILE [ARGV]...
example: novdso /bin/ls /bin/ls -l -i -s -a

Example

Assume one wanted to evaluate the syscalls issued by ls with vDSO disabled, one would run novdso as follows:

novdso /bin/ls /bin/ls -l -i -s -a

Once the following output occurs, the process is SIGSTOPed and novdso waits for the process state to change:

--- Process paused and detached. PID: 25215 ---

The process can now be seized by strace:

$ strace -p 25215
strace: Process 25215 attached
--- stopped by SIGSTOP ---

Finally, in order to continue the paused process, a SIGCONT has to be raised:

kill -SIGCONT 25215

The process is now unpaused and can be traced as usual.

Yama ptrace scope

On a number of modern Linux distributions, e.g., Ubuntu 22.04, the Yama security module limits which processes may attach to a process via ptrace to its parent process or privileged processes. This ptrace scope can be disabled for the process wrapped with novdso by setting the NOVDSO_PTRACER_ANY environment variable:

NOVDSO_PTRACER_ANY= novdso /bin/ls /bin/ls -l -i -s -a

By doing so, the wrapped process can now be traced by any process running under the same UID.