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[PATCH 1/4] tracing/histogram: Don't use strlen to find length of stacktrace variables

Tom Zanussi posted 4 patches 2 years, 7 months ago
[PATCH 1/4] tracing/histogram: Don't use strlen to find length of stacktrace variables
Posted by Tom Zanussi 2 years, 7 months ago 
Because stacktraces are saved in dynamic strings,
trace_event_raw_event_synth() uses strlen to determine the length of
the stack.  Stacktraces may contain 0-bytes, though, in the saved
addresses, so the length found and passed to reserve() will be too
small.

Fix this by using the first unsigned long in the stack variables to
store the actual number of elements in the stack and have
trace_event_raw_event_synth() use that to determine the length of the
stack.

Signed-off-by: Tom Zanussi <zanussi@kernel.org>
---
 kernel/trace/trace_events_hist.c  | 12 ++++++++----
 kernel/trace/trace_events_synth.c |  7 ++++++-
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index 888b7a394ce5..76bd105988c6 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -3135,13 +3135,15 @@ static inline void __update_field_vars(struct tracing_map_elt *elt,
 				size = min(val->size, STR_VAR_LEN_MAX);
 				strscpy(str, val_str, size);
 			} else {
+				char *stack_start = str + sizeof(unsigned long);
 				int e;
 
-				e = stack_trace_save((void *)str,
+				e = stack_trace_save((void *)stack_start,
 						     HIST_STACKTRACE_DEPTH,
 						     HIST_STACKTRACE_SKIP);
 				if (e < HIST_STACKTRACE_DEPTH - 1)
-					((unsigned long *)str)[e] = 0;
+					((unsigned long *)stack_start)[e] = 0;
+				*((unsigned long *)str) = e;
 			}
 			var_val = (u64)(uintptr_t)str;
 		}
@@ -5133,13 +5135,15 @@ static void hist_trigger_elt_update(struct hist_trigger_data *hist_data,
 					size = min(hist_field->size, STR_VAR_LEN_MAX);
 					strscpy(str, val_str, size);
 				} else {
+					char *stack_start = str + sizeof(unsigned long);
 					int e;
 
-					e = stack_trace_save((void *)str,
+					e = stack_trace_save((void *)stack_start,
 							     HIST_STACKTRACE_DEPTH,
 							     HIST_STACKTRACE_SKIP);
 					if (e < HIST_STACKTRACE_DEPTH - 1)
-						((unsigned long *)str)[e] = 0;
+						((unsigned long *)stack_start)[e] = 0;
+					*((unsigned long *)str) = e;
 				}
 				hist_val = (u64)(uintptr_t)str;
 			}
diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
index d458d7a0dfd7..6209b23c863f 100644
--- a/kernel/trace/trace_events_synth.c
+++ b/kernel/trace/trace_events_synth.c
@@ -538,7 +538,12 @@ static notrace void trace_event_raw_event_synth(void *__data,
 		val_idx = var_ref_idx[field_pos];
 		str_val = (char *)(long)var_ref_vals[val_idx];
 
-		len = kern_fetch_store_strlen((unsigned long)str_val);
+		if (event->dynamic_fields[i]->is_stack) {
+			len = *((unsigned long *)str_val);
+			len *= sizeof(unsigned long);
+		} else {
+			len = kern_fetch_store_strlen((unsigned long)str_val);
+		}
 
 		fields_size += len;
 	}
-- 
2.34.1
Re: [PATCH 1/4] tracing/histogram: Don't use strlen to find length of stacktrace variables
Posted by Masami Hiramatsu (Google) 2 years, 7 months ago 
On Fri, 10 Feb 2023 15:33:03 -0600
Tom Zanussi <zanussi@kernel.org> wrote:

> Because stacktraces are saved in dynamic strings,
> trace_event_raw_event_synth() uses strlen to determine the length of
> the stack.  Stacktraces may contain 0-bytes, though, in the saved
> addresses, so the length found and passed to reserve() will be too
> small.

Good catch!

> 
> Fix this by using the first unsigned long in the stack variables to
> store the actual number of elements in the stack and have
> trace_event_raw_event_synth() use that to determine the length of the
> stack.
> 
> Signed-off-by: Tom Zanussi <zanussi@kernel.org>
> ---
>  kernel/trace/trace_events_hist.c  | 12 ++++++++----
>  kernel/trace/trace_events_synth.c |  7 ++++++-
>  2 files changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> index 888b7a394ce5..76bd105988c6 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -3135,13 +3135,15 @@ static inline void __update_field_vars(struct tracing_map_elt *elt,
>  				size = min(val->size, STR_VAR_LEN_MAX);
>  				strscpy(str, val_str, size);
>  			} else {
> +				char *stack_start = str + sizeof(unsigned long);
>  				int e;
>  
> -				e = stack_trace_save((void *)str,
> +				e = stack_trace_save((void *)stack_start,
>  						     HIST_STACKTRACE_DEPTH,
>  						     HIST_STACKTRACE_SKIP);

BTW, the size of "str" is enough to store HIST_STACKTRACE_DEPTH?
In string case, 

size = min(val->size, STR_VAR_LEN_MAX);

will limit the max size.

Thank you,

>  				if (e < HIST_STACKTRACE_DEPTH - 1)
> -					((unsigned long *)str)[e] = 0;
> +					((unsigned long *)stack_start)[e] = 0;
> +				*((unsigned long *)str) = e;
>  			}
>  			var_val = (u64)(uintptr_t)str;
>  		}
> @@ -5133,13 +5135,15 @@ static void hist_trigger_elt_update(struct hist_trigger_data *hist_data,
>  					size = min(hist_field->size, STR_VAR_LEN_MAX);
>  					strscpy(str, val_str, size);
>  				} else {
> +					char *stack_start = str + sizeof(unsigned long);
>  					int e;
>  
> -					e = stack_trace_save((void *)str,
> +					e = stack_trace_save((void *)stack_start,
>  							     HIST_STACKTRACE_DEPTH,
>  							     HIST_STACKTRACE_SKIP);
>  					if (e < HIST_STACKTRACE_DEPTH - 1)
> -						((unsigned long *)str)[e] = 0;
> +						((unsigned long *)stack_start)[e] = 0;
> +					*((unsigned long *)str) = e;
>  				}
>  				hist_val = (u64)(uintptr_t)str;
>  			}
> diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
> index d458d7a0dfd7..6209b23c863f 100644
> --- a/kernel/trace/trace_events_synth.c
> +++ b/kernel/trace/trace_events_synth.c
> @@ -538,7 +538,12 @@ static notrace void trace_event_raw_event_synth(void *__data,
>  		val_idx = var_ref_idx[field_pos];
>  		str_val = (char *)(long)var_ref_vals[val_idx];
>  
> -		len = kern_fetch_store_strlen((unsigned long)str_val);
> +		if (event->dynamic_fields[i]->is_stack) {
> +			len = *((unsigned long *)str_val);
> +			len *= sizeof(unsigned long);
> +		} else {
> +			len = kern_fetch_store_strlen((unsigned long)str_val);
> +		}
>  
>  		fields_size += len;
>  	}
> -- 
> 2.34.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>
Re: [PATCH 1/4] tracing/histogram: Don't use strlen to find length of stacktrace variables
Posted by Steven Rostedt 2 years, 7 months ago 
On Tue, 14 Feb 2023 00:24:18 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> > -				e = stack_trace_save((void *)str,
> > +				e = stack_trace_save((void *)stack_start,
> >  						     HIST_STACKTRACE_DEPTH,
> >  						     HIST_STACKTRACE_SKIP);  
> 
> BTW, the size of "str" is enough to store HIST_STACKTRACE_DEPTH?
> In string case, 
> 
> size = min(val->size, STR_VAR_LEN_MAX);
> 
> will limit the max size.

Well, we have:

#define HIST_STACKTRACE_DEPTH  16

And 16 * 8 = 128

#define STR_VAR_LEN_MAX              MAX_FILTER_STR_VAL
#define MAX_FILTER_STR_VAL 256U

So 128 < 256, with plenty of room. I wouldn't do this runtime, but we
should add here:

	BUILD_BUG_ON((HIST_STACKTRACE_DEPTH + 1) * sizeof(long) >= STR_VAR_LEN_MAX);

-- Steve

Patchew 2.1-devel-b67e4c2

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