Previously when retrieving `bpf_link_info.perf_event` for
kprobe/uprobe/tracepoint, the `name_len` field was not populated by the
kernel, leaving it to reflect the value initially set by the user. This
behavior was inconsistent with how other input/output string buffer
fields function (e.g. `raw_tracepoint.tp_name_len`).

This patch fills `name_len` with the actual size of the string name.

Link: 
https://lore.kernel.org/bpf/cabvu1kxwqxhqqge0rtrr7eegtm6svw_kayzby16-yb0snzt...@mail.gmail.com/
Fixes: 1b715e1b0ec5 ("bpf: Support ->fill_link_info for perf_event")
Signed-off-by: tyrone-wu <wudevel...@gmail.com>
Acked-by: Jiri Olsa <jo...@kernel.org>
---
V4 -> V5:
Link: 
https://lore.kernel.org/bpf/caloahbc5xm7cbfhau3z5x2pquhihecnwaptjcwiovqtkmdz...@mail.gmail.com/
- Check that buf is not NULL before retrieving/using its length

V3 -> V4:
Link: https://lore.kernel.org/bpf/Zv_PP6Gs5cq3W2Ey@krava/
- Split patch into separate kernel and selftest change

V2 -> V3:
Link: https://lore.kernel.org/bpf/Zv7sISV0yEyGlEM3@krava/
- Use clearer variable name for user set/inputted name len (name_len -> 
input_len)
- Change (name_len -> input_len) type from size_t to u32 since it's only 
received and used as u32

V1 -> V2:
Link: https://lore.kernel.org/bpf/Zv0wl-S13WJnIkb_@krava/
- Use user set *ulen in bpf_copy_to_user before overwriting *ulen

 kernel/bpf/syscall.c | 38 ++++++++++++++++++++++++--------------
 1 file changed, 24 insertions(+), 14 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index a8f1808a1ca5..3df192a6bdcc 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3565,32 +3565,35 @@ static void bpf_perf_link_dealloc(struct bpf_link *link)
 }
 
 static int bpf_perf_link_fill_common(const struct perf_event *event,
-                                    char __user *uname, u32 ulen,
+                                    char __user *uname, u32 *ulen,
                                     u64 *probe_offset, u64 *probe_addr,
                                     u32 *fd_type, unsigned long *missed)
 {
        const char *buf;
-       u32 prog_id;
+       u32 prog_id, input_len;
        size_t len;
        int err;
 
-       if (!ulen ^ !uname)
+       if (!(*ulen) ^ !uname)
                return -EINVAL;
 
        err = bpf_get_perf_event_info(event, &prog_id, fd_type, &buf,
                                      probe_offset, probe_addr, missed);
        if (err)
                return err;
-       if (!uname)
-               return 0;
+
        if (buf) {
+               input_len = *ulen;
                len = strlen(buf);
-               err = bpf_copy_to_user(uname, buf, ulen, len);
-               if (err)
-                       return err;
-       } else {
-               char zero = '\0';
+               *ulen = len + 1;
 
+               if (uname) {
+                       err = bpf_copy_to_user(uname, buf, input_len, len);
+                       if (err)
+                               return err;
+               }
+       } else if (uname) {
+               char zero = '\0';
                if (put_user(zero, uname))
                        return -EFAULT;
        }
@@ -3609,7 +3612,7 @@ static int bpf_perf_link_fill_kprobe(const struct 
perf_event *event,
 
        uname = u64_to_user_ptr(info->perf_event.kprobe.func_name);
        ulen = info->perf_event.kprobe.name_len;
-       err = bpf_perf_link_fill_common(event, uname, ulen, &offset, &addr,
+       err = bpf_perf_link_fill_common(event, uname, &ulen, &offset, &addr,
                                        &type, &missed);
        if (err)
                return err;
@@ -3617,7 +3620,7 @@ static int bpf_perf_link_fill_kprobe(const struct 
perf_event *event,
                info->perf_event.type = BPF_PERF_EVENT_KRETPROBE;
        else
                info->perf_event.type = BPF_PERF_EVENT_KPROBE;
-
+       info->perf_event.kprobe.name_len = ulen;
        info->perf_event.kprobe.offset = offset;
        info->perf_event.kprobe.missed = missed;
        if (!kallsyms_show_value(current_cred()))
@@ -3639,7 +3642,7 @@ static int bpf_perf_link_fill_uprobe(const struct 
perf_event *event,
 
        uname = u64_to_user_ptr(info->perf_event.uprobe.file_name);
        ulen = info->perf_event.uprobe.name_len;
-       err = bpf_perf_link_fill_common(event, uname, ulen, &offset, &addr,
+       err = bpf_perf_link_fill_common(event, uname, &ulen, &offset, &addr,
                                        &type, NULL);
        if (err)
                return err;
@@ -3648,6 +3651,7 @@ static int bpf_perf_link_fill_uprobe(const struct 
perf_event *event,
                info->perf_event.type = BPF_PERF_EVENT_URETPROBE;
        else
                info->perf_event.type = BPF_PERF_EVENT_UPROBE;
+       info->perf_event.uprobe.name_len = ulen;
        info->perf_event.uprobe.offset = offset;
        info->perf_event.uprobe.cookie = event->bpf_cookie;
        return 0;
@@ -3673,12 +3677,18 @@ static int bpf_perf_link_fill_tracepoint(const struct 
perf_event *event,
 {
        char __user *uname;
        u32 ulen;
+       int err;
 
        uname = u64_to_user_ptr(info->perf_event.tracepoint.tp_name);
        ulen = info->perf_event.tracepoint.name_len;
+       err = bpf_perf_link_fill_common(event, uname, &ulen, NULL, NULL, NULL, 
NULL);
+       if (err)
+               return err;
+
        info->perf_event.type = BPF_PERF_EVENT_TRACEPOINT;
+       info->perf_event.tracepoint.name_len = ulen;
        info->perf_event.tracepoint.cookie = event->bpf_cookie;
-       return bpf_perf_link_fill_common(event, uname, ulen, NULL, NULL, NULL, 
NULL);
+       return 0;
 }
 
 static int bpf_perf_link_fill_perf_event(const struct perf_event *event,
-- 
2.43.0


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