[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 <ja...@google.com>
---
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