: bug fix, remove partial zero out

partial-zero-out a struct is very dangerous, we should zero out field by field directly when need.

partial-zero-out for struct trace_iterator exists when ftrace was first introduced into mainline kernel. But in this few years, the code of ftrace is changed a lot, and:

1) partial-zero-out for struct trace_iterator has a bug now, cpumask_var_t started should not be zeroed out.

2) I viewed the codes and found that fields below "/* The below is zeroed out in pipe_read */" don't need to be zeroed out or initialized now.

So, we remove the code of "partial zero out"

Signed-off-by Lai Jiangshan <[email protected]>

---

include/linux/ftrace_event.h +0 -1

index 3ca9485..c6d0e1a 100644
@@ -54,7 +54,6 @@ struct trace_iterator {
 	struct ring_buffer_iter	*buffer_iter[NR_CPUS];
 	unsigned long		iter_flags;
 
-	/* The below is zeroed out in pipe_read */
 	struct trace_seq	seq;
 	struct trace_entry	*ent;
 	int			leftover;

kernel/trace/trace.c +0 -0

index 5314c90..27fecf8 100644

Quoted 35 lines · Lai Jiangshan

partial-zero-out a struct is very dangerous, we should zero out field by field directly when need.

partial-zero-out for struct trace_iterator exists when ftrace was first introduced into mainline kernel. But in this few years, the code of ftrace is changed a lot, and:

1) partial-zero-out for struct trace_iterator has a bug now, cpumask_var_t started should not be zeroed out.

2) I viewed the codes and found that fields below "/* The below is zeroed out in pipe_read */" don't need to be zeroed out or initialized now.

So, we remove the code of "partial zero out"

Signed-off-by Lai Jiangshan <[email protected]>

---

include/linux/ftrace_event.h +0 -1

index 3ca9485..c6d0e1a 100644
@@ -54,7 +54,6 @@ struct trace_iterator {
 	struct ring_buffer_iter	*buffer_iter[NR_CPUS];
 	unsigned long		iter_flags;
 
-	/* The below is zeroed out in pipe_read */
 	struct trace_seq	seq;
 	struct trace_entry	*ent;
 	int			leftover;

kernel/trace/trace.c +0 -0

index 5314c90..27fecf8 100644

kernel/trace/trace.c waitagain: · line 3132

Quoted 9 lines

kernel/trace/trace.c +0 -6

@@ -3124,12 +3124,6 @@ waitagain:
 	if (cnt >= PAGE_SIZE)
 		cnt = PAGE_SIZE - 1;
 
-	/* reset all but tr, trace, and overruns */
-	memset(&iter->seq, 0,
-	       sizeof(struct trace_iterator) -
-	       offsetof(struct trace_iterator, seq));
-	iter->pos = -1;
-

I'm not sure exaclty why we needed to zero the seq here. We already reset it in trace_seq_init().

We might do it again on waitagain. I lost track how we could ever need to goto waitagain. It was about a tricky bug to fix but I'm don't remember exactly the details.

That said, if trace_seq_to_user returns -EBUSY, we re-init the seq buffer, so it should be fine I guess.

But concerning the need of setting iter->pos to -1, I'm not sure we need to remove it. Shouldn't it be set to 0 btw?

Quoted 51 lines · Frederic Weisbecker

Quoted 35 lines · Lai Jiangshan

partial-zero-out a struct is very dangerous, we should zero out field by field directly when need.

partial-zero-out for struct trace_iterator exists when ftrace was first introduced into mainline kernel. But in this few years, the code of ftrace is changed a lot, and:

1) partial-zero-out for struct trace_iterator has a bug now, cpumask_var_t started should not be zeroed out.

2) I viewed the codes and found that fields below "/* The below is zeroed out in pipe_read */" don't need to be zeroed out or initialized now.

So, we remove the code of "partial zero out"

Signed-off-by Lai Jiangshan <[email protected]>

---

include/linux/ftrace_event.h +0 -1

index 3ca9485..c6d0e1a 100644
@@ -54,7 +54,6 @@ struct trace_iterator {
 	struct ring_buffer_iter	*buffer_iter[NR_CPUS];
 	unsigned long		iter_flags;
 
-	/* The below is zeroed out in pipe_read */
 	struct trace_seq	seq;
 	struct trace_entry	*ent;
 	int			leftover;

kernel/trace/trace.c +0 -0

index 5314c90..27fecf8 100644

kernel/trace/trace.c waitagain: · line 3132

Quoted 9 lines

kernel/trace/trace.c +0 -6

@@ -3124,12 +3124,6 @@ waitagain:
 	if (cnt >= PAGE_SIZE)
 		cnt = PAGE_SIZE - 1;
 
-	/* reset all but tr, trace, and overruns */
-	memset(&iter->seq, 0,
-	       sizeof(struct trace_iterator) -
-	       offsetof(struct trace_iterator, seq));
-	iter->pos = -1;
-

I'm not sure exaclty why we needed to zero the seq here. We already reset it in trace_seq_init().

Quoted 6 lines

We might do it again on waitagain. I lost track how we could ever need to goto waitagain. It was about a tricky bug to fix but I'm don't remember exactly the details.

That said, if trace_seq_to_user returns -EBUSY, we re-init the seq buffer, so it should be fine I guess.

Yes, -EBUSY is strange here.
but any way, trace_seq_init() is called.

Quoted 1 line

Quoted 3 lines

But concerning the need of setting iter->pos to -1, I'm not sure we need to remove it. Shouldn't it be set to 0 btw?

->pos is not used here, ->idx is just increased here, so we don't need to initialize them.