Archive for March, 2011

A hack to strace -f

By chys on March 25th, 2011

I have a multithreaded program which I would like to strace for debugging purpose. My program sometimes calls (fork and exec) an external program, which in turn calls a setuid program.

Because my program is multithreaded, I cannot omit the “-f” flag (also trace child threads and processes) when using strace. And because all children, including the setuid program, are traced, setuid fails. (Yes, I am aware that strace claims it is possible to trace setuid programs, but the trick does not work for me, probably because the setuid program is not directly executed by strace.)

Fortunately, the clone system call has many useful flags. It works fine for me when I substitute calls to fork() with:

(pid_t) syscall (__NR_clone, CLONE_UNTRACED|SIGCHLD, NULL);

(Yes, SIGCHLD, not CLONE_SIGCHLD. It’s not a typo.)

I guess there may be better solutions, without modifying the program being traced?

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