Re: [RFC PATCH] x86 single-step (TF) vs system calls & traps

[Andrew Morton <akpm@osdl.org>]

Linus Torvalds <torvalds@osdl.org> wrote:
>
>  On Mon, 28 Jun 2004, Roland McGrath wrote:
>  >
>  > > And I refuse to make the fast-path slower just because of this. 
>  > 
>  > You are talking about the int $0x80 system call path here?
>  > That is the only non-exception path touched by my changes.
> 
>  That's still the fast path on any machine where this matters.

Davide's patch (which has been in -mm for 6-7 weeks) doesn't add
fastpath overhead.


diff -puN arch/i386/kernel/entry.S~really-ptrace-single-step-2 arch/i386/kernel/entry.S
--- 25/arch/i386/kernel/entry.S~really-ptrace-single-step-2	2004-06-24 13:19:51.721958784 -0700
+++ 25-akpm/arch/i386/kernel/entry.S	2004-06-24 13:19:51.728957720 -0700
@@ -375,7 +375,7 @@ syscall_trace_entry:
 	# perform syscall exit tracing
 	ALIGN
 syscall_exit_work:
-	testb $(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT), %cl
+	testb $(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_SINGLESTEP), %cl
 	jz work_pending
 	sti				# could let do_syscall_trace() call
 					# schedule() instead
diff -puN arch/i386/kernel/ptrace.c~really-ptrace-single-step-2 arch/i386/kernel/ptrace.c
--- 25/arch/i386/kernel/ptrace.c~really-ptrace-single-step-2	2004-06-24 13:19:51.723958480 -0700
+++ 25-akpm/arch/i386/kernel/ptrace.c	2004-06-24 13:19:51.729957568 -0700
@@ -147,6 +147,7 @@ void ptrace_disable(struct task_struct *
 { 
 	long tmp;
 
+	clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 	tmp = get_stack_long(child, EFL_OFFSET) & ~TRAP_FLAG;
 	put_stack_long(child, EFL_OFFSET, tmp);
 }
@@ -370,6 +371,7 @@ asmlinkage int sys_ptrace(long request, 
 		else {
 			clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
 		}
+		clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 		child->exit_code = data;
 	/* make sure the single step bit is not set. */
 		tmp = get_stack_long(child, EFL_OFFSET) & ~TRAP_FLAG;
@@ -391,6 +393,7 @@ asmlinkage int sys_ptrace(long request, 
 		if (child->state == TASK_ZOMBIE)	/* already dead */
 			break;
 		child->exit_code = SIGKILL;
+		clear_tsk_thread_flag(child, TIF_SINGLESTEP);
 		/* make sure the single step bit is not set. */
 		tmp = get_stack_long(child, EFL_OFFSET) & ~TRAP_FLAG;
 		put_stack_long(child, EFL_OFFSET, tmp);
@@ -411,6 +414,7 @@ asmlinkage int sys_ptrace(long request, 
 		}
 		tmp = get_stack_long(child, EFL_OFFSET) | TRAP_FLAG;
 		put_stack_long(child, EFL_OFFSET, tmp);
+		set_tsk_thread_flag(child, TIF_SINGLESTEP);
 		child->exit_code = data;
 		/* give it a chance to run. */
 		wake_up_process(child);
@@ -535,7 +539,8 @@ void do_syscall_trace(struct pt_regs *re
 			audit_syscall_exit(current, regs->eax);
 	}
 
-	if (!test_thread_flag(TIF_SYSCALL_TRACE))
+	if (!test_thread_flag(TIF_SYSCALL_TRACE) &&
+	    !test_thread_flag(TIF_SINGLESTEP))
 		return;
 	if (!(current->ptrace & PT_PTRACED))
 		return;
diff -puN include/asm-i386/thread_info.h~really-ptrace-single-step-2 include/asm-i386/thread_info.h
--- 25/include/asm-i386/thread_info.h~really-ptrace-single-step-2	2004-06-24 13:19:51.724958328 -0700
+++ 25-akpm/include/asm-i386/thread_info.h	2004-06-24 13:19:51.729957568 -0700
@@ -157,7 +157,7 @@ static inline unsigned long current_stac
 
 /* work to do on interrupt/exception return */
 #define _TIF_WORK_MASK \
-  (0x0000FFFF & ~(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT))
+  (0x0000FFFF & ~(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_SINGLESTEP))
 #define _TIF_ALLWORK_MASK	0x0000FFFF	/* work to do on any return to u-space */
 
 /*
_