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(). Tested on x86.
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 | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
--- V1/kernel/fork.c~FORK_DISABLE_STEP 2009-10-09 19:52:23.000000000 +0200
+++ V1/kernel/fork.c 2009-11-10 00:45:12.000000000 +0100
@@ -1199,9 +1199,10 @@ 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 should be turned off in the
+ * child regardless of CLONE_PTRACE.
*/
+ user_disable_single_step(p);
clear_tsk_thread_flag(p, TIF_SYSCALL_TRACE);
#ifdef TIF_SYSCALL_EMU
clear_tsk_thread_flag(p, TIF_SYSCALL_EMU);
--
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