ptrace: Remove arch_ptrace_attach

The last remaining implementation of arch_ptrace_attach is ia64's
ptrace_attach_sync_user_rbs which was added at the end of 2007 in
commit aa91a2e90044 ("[IA64] Synchronize RBS on PTRACE_ATTACH").

Reading the comments and examining the code ptrace_attach_sync_user_rbs
has the sole purpose of saving registers to the stack when ptrace_attach
changes TASK_STOPPED to TASK_TRACED.  In all other cases arch_ptrace_stop
takes care of the register saving.

In commit d79fdd6d96f4 ("ptrace: Clean transitions between TASK_STOPPED and TRACED")
modified ptrace_attach to wake up the thread and enter ptrace_stop normally even
when the thread starts out stopped.

This makes ptrace_attach_sync_user_rbs completely unnecessary.  So just
remove it.

I read through the code to verify that ptrace_attach_sync_user_rbs is
unnecessary.  What I found is that the code is quite dead.

Reading ptrace_attach_sync_user_rbs it is easy to see that the it does
nothing unless __state == TASK_STOPPED.

Calling arch_ptrace_attach (aka ptrace_attach_sync_user_rbs) after
ptrace_traceme it is easy to see that because we are talking about the
current process the value of __state is TASK_RUNNING.  Which means
ptrace_attach_sync_user_rbs does nothing.

The only other call of arch_ptrace_attach (aka
ptrace_attach_sync_user_rbs) is after ptrace_attach.

If the task is running (and PTRACE_SEIZE is not specified), a SIGSTOP
is sent which results in do_signal_stop setting JOBCTL_TRAP_STOP on
the target task (as it is ptraced) and the target task stopping
in ptrace_stop with __state == TASK_TRACED.

If the task was already stopped then ptrace_attach sets
JOBCTL_TRAPPING and JOBCTL_TRAP_STOP, wakes it out of __TASK_STOPPED,
and waits until the JOBCTL_TRAPPING_BIT is clear.  At which point
the task stops in ptrace_stop.

In both cases there are a couple of funning excpetions such as if the
traced task receiveds a SIGCONT, or is set a fatal signal.

However in all of those cases the tracee never stops in __state
TASK_STOPPED.  Which is a long way of saying that ptrace_attach_sync_user_rbs
is guaranteed never to do anything.

Cc: linux-ia64@vger.kernel.org
Tested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lkml.kernel.org/r/20220505182645.497868-4-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 0 insertions, 18 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index ccc4b465775b..da30dcd477a0 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -1285,10 +1285,6 @@ int ptrace_request(struct task_struct *child, long request,
 	return ret;
 }
 
-#ifndef arch_ptrace_attach
-#define arch_ptrace_attach(child)	do { } while (0)
-#endif
-
 SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
 		unsigned long, data)
 {
@@ -1297,8 +1293,6 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
 
 	if (request == PTRACE_TRACEME) {
 		ret = ptrace_traceme();
-		if (!ret)
-			arch_ptrace_attach(current);
 		goto out;
 	}
 
@@ -1310,12 +1304,6 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
 
 	if (request == PTRACE_ATTACH || request == PTRACE_SEIZE) {
 		ret = ptrace_attach(child, request, addr, data);
-		/*
-		 * Some architectures need to do book-keeping after
-		 * a ptrace attach.
-		 */
-		if (!ret)
-			arch_ptrace_attach(child);
 		goto out_put_task_struct;
 	}
 
@@ -1455,12 +1443,6 @@ COMPAT_SYSCALL_DEFINE4(ptrace, compat_long_t, request, compat_long_t, pid,
 
 	if (request == PTRACE_ATTACH || request == PTRACE_SEIZE) {
 		ret = ptrace_attach(child, request, addr, data);
-		/*
-		 * Some architectures need to do book-keeping after
-		 * a ptrace attach.
-		 */
-		if (!ret)
-			arch_ptrace_attach(child);
 		goto out_put_task_struct;
 	}