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Subject[PATCH v2 3/3] tracing/filters: Support filtering for char * strings
Usually, char * entries are dangerous in traces because the string
can be released whereas a pointer to it can still wait to be read from
the ring buffer.

But sometimes we can assume it's safe, like in case of RO data
(eg: __file__ or __line__, used in bkl trace event). If these RO data
are in a module and so is the call to the trace event, then it's safe,
because the ring buffer will be flushed once this module get unloaded.

To allow char * to be treated as a string:

TRACE_EVENT(...,

TP_STRUCT__entry(
__field_ext(const char *, name, FILTER_PTR_STRING)
...
)

...
);

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
---
include/linux/ftrace_event.h | 1 +
kernel/trace/trace_events_filter.c | 26 +++++++++++++++++++++++---
2 files changed, 24 insertions(+), 3 deletions(-)
diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
index f034e8f..fd5f329 100644
--- a/include/linux/ftrace_event.h
+++ b/include/linux/ftrace_event.h
@@ -140,6 +140,7 @@ enum {
FILTER_OTHER = 0,
FILTER_STATIC_STRING,
FILTER_DYN_STRING,
+ FILTER_PTR_STRING,
};

extern int trace_define_field(struct ftrace_event_call *call, char *type,
diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 5e7f031..b16923e 100644
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -163,6 +163,20 @@ static int filter_pred_string(struct filter_pred *pred, void *event,
return match;
}

+/* Filter predicate for char * pointers */
+static int filter_pred_pchar(struct filter_pred *pred, void *event,
+ int val1, int val2)
+{
+ char **addr = (char **)(event + pred->offset);
+ int cmp, match;
+
+ cmp = strncmp(*addr, pred->str_val, pred->str_len);
+
+ match = (!cmp) ^ pred->not;
+
+ return match;
+}
+
/*
* Filter predicate for dynamic sized arrays of characters.
* These are implemented through a list of strings at the end
@@ -489,7 +503,8 @@ int filter_assign_type(const char *type)
static bool is_string_field(struct ftrace_event_field *field)
{
return field->filter_type == FILTER_DYN_STRING ||
- field->filter_type == FILTER_STATIC_STRING;
+ field->filter_type == FILTER_STATIC_STRING ||
+ field->filter_type == FILTER_PTR_STRING;
}

static int is_legal_op(struct ftrace_event_field *field, int op)
@@ -579,11 +594,16 @@ static int filter_add_pred(struct filter_parse_state *ps,
}

if (is_string_field(field)) {
+ pred->str_len = field->size;
+
if (field->filter_type == FILTER_STATIC_STRING)
fn = filter_pred_string;
- else
+ else if (field->filter_type == FILTER_DYN_STRING)
fn = filter_pred_strloc;
- pred->str_len = field->size;
+ else {
+ fn = filter_pred_pchar;
+ pred->str_len = strlen(pred->str_val);
+ }
} else {
if (field->is_signed)
ret = strict_strtoll(pred->str_val, 0, &val);
--
1.6.3


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