[PATCH] tracing: Fix filter string testing

[Steven Rostedt]

From: Steven Rostedt <rostedt@xxxxxxxxxxx>

The filter string testing uses strncpy_from_kernel/user_nofault() to
retrieve the string to test the filter against. The if() statement was
incorrect as it considered 0 as a fault, when it is only negative that it
faulted.

Link: https://lore.kernel.org/all/CAEf4BzbVPQ=BjWztmEwBPRKHUwNfKBkS3kce-Rzka6zvbQeVpg@xxxxxxxxxxxxxx/

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 77360f9bbc7e5 ("tracing: Add test for user space strings when filtering on string pointers")
Reported-by: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
Reported-by: Mykyta Yatsenko <mykyta.yatsenko5@xxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
 kernel/trace/trace_events_filter.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 0993dfc1c5c1..2048560264bb 100644
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -808,7 +808,7 @@ static __always_inline char *test_string(char *str)
 	kstr = ubuf->buffer;
 
 	/* For safety, do not trust the string pointer */
-	if (!strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE))
+	if (strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE) < 0)
 		return NULL;
 	return kstr;
 }
@@ -827,7 +827,7 @@ static __always_inline char *test_ustring(char *str)
 
 	/* user space address? */
 	ustr = (char __user *)str;
-	if (!strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE))
+	if (strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE) < 0)
 		return NULL;
 
 	return kstr;
-- 
2.47.2