Workflow for Using strace to Trap System Calls

When you want to trace and analyze system calls made by a process, strace is a powerful tool. Below is a step-by-step workflow for trapping system calls using strace:

1. Install strace

  • Ensure strace is installed on your system. On most Linux distributions, you can install it using:

sudo apt-get install strace # For Debian/Ubuntu
sudo yum install strace # For CentOS/RHEL

2. Basic Usage of strace

  • To trace all system calls made by a program, run:

strace <command>

Example:

strace ls -l

This will display all system calls made by the ls -l command.

3. Trap Specific System Calls

  • Use the -e option to filter specific system calls. For example, to trace only open and read system calls:

strace -e trace=open,read <command>

Example:

strace -e trace=open,read cat /etc/passwd

4. Attach to a Running Process

  • If you want to trace system calls of an already running process, use the -p option with the process ID (PID):

strace -p <PID>

Example:

strace -p 1234

5. Save Output to a File

  • Redirect the output of strace to a file for later analysis:

strace -o output.txt <command>

Example:

strace -o trace.log ls -l

6. Trace System Calls with Timestamps

  • Add timestamps to the output to see when each system call occurs:

strace -tt <command>

Example:

strace -tt ls -l

7. Trace Child Processes

  • Use the -f option to trace child processes spawned by the main process:

strace -f <command>

Example:

strace -f ./my_script.sh

8. Analyze System Call Statistics

  • Use the -c option to get a summary of system calls made by the program:

strace -c <command>

Example:

strace -c ls -l

9. Advanced Filtering

  • Combine filters to narrow down the output. For example, trace only open system calls that fail:

strace -e trace=open -e fail=open <command>

Example:

strace -e trace=open -e fail=open cat /nonexistent_file

10. Exit on First Error

  • Use the -e option with inject to exit when a specific system call fails:

strace -e inject=open:error=ENOENT <command>

Example:

strace -e inject=open:error=ENOENT cat /nonexistent_file

Example Workflow

  1. Start a program and trace its system calls:

strace -o trace.log -tt -f ./my_program