Thanks a lot Oleg.
Question - shouldn't ptrace tests be put in
tools/testing/selftests/ptrace/ in the kernel tree nowadays?
Thanks,
Pedro Alves
On 11/03/2014 08:13 PM, Oleg Nesterov wrote:
> When the TIF_SINGLESTEP tracee dequeues a signal, handle_signal()
> clears TIF_FORCED_TF and X86_EFLAGS_TF but leaves TIF_SINGLESTEP set.
>
> If the tracer does PTRACE_SINGLESTEP again, enable_single_step() sets
> X86_EFLAGS_TF but not TIF_FORCED_TF. This means that the subsequent
> PTRACE_CONT doesn't not clear X86_EFLAGS_TF, and the tracee gets the
> wrong SIGTRAP.
>
> Test-case (needs -O2 to avoid prologue insns in signal handler):
>
> #include <unistd.h>
> #include <stdio.h>
> #include <sys/ptrace.h>
> #include <sys/wait.h>
> #include <sys/user.h>
> #include <assert.h>
> #include <stddef.h>
>
> void handler(int n)
> {
> asm("nop");
> }
>
> int child(void)
> {
> assert(ptrace(PTRACE_TRACEME, 0,0,0) == 0);
> signal(SIGALRM, handler);
> kill(getpid(), SIGALRM);
> return 0x23;
> }
>
> void *getip(int pid)
> {
> return (void*)ptrace(PTRACE_PEEKUSER, pid,
> offsetof(struct user, regs.rip), 0);
> }
>
> int main(void)
> {
> int pid, status;
>
> pid = fork();
> if (!pid)
> return child();
>
> assert(wait(&status) == pid);
> assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGALRM);
>
> assert(ptrace(PTRACE_SINGLESTEP, pid, 0, SIGALRM) == 0);
> assert(wait(&status) == pid);
> assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP);
> assert((getip(pid) - (void*)handler) == 0);
>
> assert(ptrace(PTRACE_SINGLESTEP, pid, 0, SIGALRM) == 0);
> assert(wait(&status) == pid);
> assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP);
> assert((getip(pid) - (void*)handler) == 1);
>
> assert(ptrace(PTRACE_CONT, pid, 0,0) == 0);
> assert(wait(&status) == pid);
> assert(WIFEXITED(status) && WEXITSTATUS(status) == 0x23);
>
> return 0;
> }
>
> The last assert() fails because PTRACE_CONT wrongly triggers another
> single-step and X86_EFLAGS_TF can't be cleared by debugger until the
> tracee does sys_rt_sigreturn().
>
> Change handle_signal() to do user_disable_single_step() if stepping,
> we do not need to preserve TIF_SINGLESTEP because we are going to do
> ptrace_notify(), and it is simply wrong to leak this bit.
>
> While at it, change the comment to explain why we also need to clear
> TF unconditionally after setup_rt_frame().
>
> Note: in the longer term we should probably change setup_sigcontext()
> to use get_flags() and then just remove this user_disable_single_step().
> And, the state of TIF_FORCED_TF can be wrong after restore_sigcontext()
> which can set/clear TF, this needs another fix.
>
> Reported-by: Evan Teran <ete...@alum.rit.edu>
> Reported-by: Pedro Alves <pal...@redhat.com>
> Signed-off-by: Oleg Nesterov <o...@redhat.com>
> ---
> arch/x86/kernel/signal.c | 22 +++++++++++-----------
> 1 files changed, 11 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index ed37a76..9d3a15b 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -629,7 +629,8 @@ setup_rt_frame(struct ksignal *ksig, struct pt_regs *regs)
> static void
> handle_signal(struct ksignal *ksig, struct pt_regs *regs)
> {
> - bool failed;
> + bool stepping, failed;
> +
> /* Are we from a system call? */
> if (syscall_get_nr(current, regs) >= 0) {
> /* If so, check system call restarting.. */
> @@ -653,12 +654,13 @@ handle_signal(struct ksignal *ksig, struct pt_regs
> *regs)
> }
>
> /*
> - * If TF is set due to a debugger (TIF_FORCED_TF), clear the TF
> - * flag so that register information in the sigcontext is correct.
> + * If TF is set due to a debugger (TIF_FORCED_TF), clear TF now
> + * so that register information in the sigcontext is correct and
> + * then notify the tracer before entering the signal handler.
> */
> - if (unlikely(regs->flags & X86_EFLAGS_TF) &&
> - likely(test_and_clear_thread_flag(TIF_FORCED_TF)))
> - regs->flags &= ~X86_EFLAGS_TF;
> + stepping = test_thread_flag(TIF_SINGLESTEP);
> + if (stepping)
> + user_disable_single_step(current);
>
> failed = (setup_rt_frame(ksig, regs) < 0);
> if (!failed) {
> @@ -669,10 +671,8 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
> * it might disable possible debug exception from the
> * signal handler.
> *
> - * Clear TF when entering the signal handler, but
> - * notify any tracer that was single-stepping it.
> - * The tracer may want to single-step inside the
> - * handler too.
> + * Clear TF for the case when it wasn't set by debugger to
> + * avoid the recursive send_sigtrap() in SIGTRAP handler.
> */
> regs->flags &= ~(X86_EFLAGS_DF|X86_EFLAGS_RF|X86_EFLAGS_TF);
> /*
> @@ -681,7 +681,7 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
> if (used_math())
> drop_init_fpu(current);
> }
> - signal_setup_done(failed, ksig, test_thread_flag(TIF_SINGLESTEP));
> + signal_setup_done(failed, ksig, stepping);
> }
>
> #ifdef CONFIG_X86_32
>
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