bpf: Fix unpopulated path_size when uprobe_multi fields unset

Previously when retrieving `bpf_link_info.uprobe_multi` with `path` and
`path_size` fields unset, the `path_size` field is not populated
(remains 0). This behavior was inconsistent with how other input/output
string buffer fields work, as the field should be populated in cases
when:
- both buffer and length are set (currently works as expected)
- both buffer and length are unset (not working as expected)

This patch now fills the `path_size` field when `path` and `path_size`
are unset.

Fixes: e56fdbfb06e2 ("bpf: Add link_info support for uprobe multi link")
Signed-off-by: Tyrone Wu <wudevelops@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241011000803.681190-1-wudevelops@gmail.com

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index a582cd2..3bd402f 100644 (file)
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -3133,7 +3133,8 @@ static int bpf_uprobe_multi_link_fill_link_info(const struct bpf_link *link,
        struct bpf_uprobe_multi_link *umulti_link;
        u32 ucount = info->uprobe_multi.count;
        int err = 0, i;
-       long left;
+       char *p, *buf;
+       long left = 0;
 
        if (!upath ^ !upath_size)
                return -EINVAL;
@@ -3147,26 +3148,23 @@ static int bpf_uprobe_multi_link_fill_link_info(const struct bpf_link *link,
        info->uprobe_multi.pid = umulti_link->task ?
                                 task_pid_nr_ns(umulti_link->task, task_active_pid_ns(current)) : 0;
 
-       if (upath) {
-               char *p, *buf;
-
-               upath_size = min_t(u32, upath_size, PATH_MAX);
-
-               buf = kmalloc(upath_size, GFP_KERNEL);
-               if (!buf)
-                       return -ENOMEM;
-               p = d_path(&umulti_link->path, buf, upath_size);
-               if (IS_ERR(p)) {
-                       kfree(buf);
-                       return PTR_ERR(p);
-               }
-               upath_size = buf + upath_size - p;
-               left = copy_to_user(upath, p, upath_size);
+       upath_size = upath_size ? min_t(u32, upath_size, PATH_MAX) : PATH_MAX;
+       buf = kmalloc(upath_size, GFP_KERNEL);
+       if (!buf)
+               return -ENOMEM;
+       p = d_path(&umulti_link->path, buf, upath_size);
+       if (IS_ERR(p)) {
                kfree(buf);
-               if (left)
-                       return -EFAULT;
-               info->uprobe_multi.path_size = upath_size;
+               return PTR_ERR(p);
        }
+       upath_size = buf + upath_size - p;
+
+       if (upath)
+               left = copy_to_user(upath, p, upath_size);
+       kfree(buf);
+       if (left)
+               return -EFAULT;
+       info->uprobe_multi.path_size = upath_size;
 
        if (!uoffsets && !ucookies && !uref_ctr_offsets)
                return 0;