[PATCH v2] perf/core: use strndup_user() instead of buggy open-coded version

[Jann Horn]

The first version of this method was missing the check for
`ret == PATH_MAX`; then such a check was added, but it didn't call kfree()
on error, so there was still a small memory leak in the error case.
Fix it by using strndup_user() instead of open-coding it.

Fixes: 0eadcc7a7bc0 ("perf/core: Fix perf_uprobe_init()")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
v2:
 - be compatible with existing error codes (Masami Hiramatsu)

 kernel/trace/trace_event_perf.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
index 76217bbef815..4629a6104474 100644
--- a/kernel/trace/trace_event_perf.c
+++ b/kernel/trace/trace_event_perf.c
@@ -299,15 +299,13 @@ int perf_uprobe_init(struct perf_event *p_event,
 
 	if (!p_event->attr.uprobe_path)
 		return -EINVAL;
-	path = kzalloc(PATH_MAX, GFP_KERNEL);
-	if (!path)
-		return -ENOMEM;
-	ret = strncpy_from_user(
-		path, u64_to_user_ptr(p_event->attr.uprobe_path), PATH_MAX);
-	if (ret == PATH_MAX)
-		return -E2BIG;
-	if (ret < 0)
-		goto out;
+
+	path = strndup_user(u64_to_user_ptr(p_event->attr.uprobe_path),
+			    PATH_MAX);
+	if (IS_ERR(path)) {
+		ret = PTR_ERR(path);
+		return (ret == -EINVAL) ? -E2BIG : ret;
+	}
 	if (path[0] == '\0') {
 		ret = -EINVAL;
 		goto out;
-- 
2.21.0.rc0.258.g878e2cd30e-goog