If the tracee calls fork() after PTRACE_SINGLESTEP, the forked child starts with TIF_SINGLESTEP/X86_EFLAGS_TF bits copied from ptraced parent. This is not right, especially when the new child is not auto-attaced: in this case it is killed by SIGTRAP. Change copy_process() to call user_disable_single_step() if TIF_SINGLESTEP is set. Tested on x86, hopefully this is correct on any architecture. Test-case: #include <stdio.h> #include <unistd.h> #include <signal.h> #include <sys/ptrace.h> #include <sys/wait.h> #include <assert.h> int main(void) { int pid, status; if (!(pid = fork())) { assert(ptrace(PTRACE_TRACEME) == 0); kill(getpid(), SIGSTOP); if (!fork()) { /* kernel bug: this child will be killed by SIGTRAP */ printf("Hello world\n"); return 43; } wait(&status); return WEXITSTATUS(status); } for (;;) { assert(pid == wait(&status)); if (WIFEXITED(status)) break; assert(ptrace(PTRACE_SINGLESTEP, pid, 0,0) == 0); } assert(WEXITSTATUS(status) == 43); return 0; } Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx> --- kernel/fork.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) --- V1/kernel/fork.c~FORK_DISABLE_STEP 2009-10-09 19:52:23.000000000 +0200 +++ V1/kernel/fork.c 2009-11-07 22:15:15.000000000 +0100 @@ -1199,10 +1199,14 @@ static struct task_struct *copy_process( p->sas_ss_sp = p->sas_ss_size = 0; /* - * Syscall tracing should be turned off in the child regardless - * of CLONE_PTRACE. + * Syscall tracing and stepping hould be turned off in the + * child regardless of CLONE_PTRACE. */ clear_tsk_thread_flag(p, TIF_SYSCALL_TRACE); +#ifdef TIF_SINGLESTEP + if (unlikely(test_tsk_thread_flag(p, TIF_SINGLESTEP))) + user_disable_single_step(p); +#endif #ifdef TIF_SYSCALL_EMU clear_tsk_thread_flag(p, TIF_SYSCALL_EMU); #endif -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html