Re: [PATCH bpf v2] bpf: fix unpopulated name_len field in perf_event link info

[Jiri Olsa]

On Wed, Oct 02, 2024 at 09:38:39PM +0000, tyrone-wu wrote:
> 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. The
>  relevant selftests have also been updated to assert that `name_len`
> contains the correct size rather than 0.
> 
> 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>
> ---
> 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                          | 29 +++++++++++++------
>  .../selftests/bpf/prog_tests/fill_link_info.c |  6 ++--
>  2 files changed, 23 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index a8f1808a1ca5..26cc18693924 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3565,27 +3565,31 @@ 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;
> -     size_t len;
> +     size_t len, name_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;
> +
> +     name_len = *ulen;
> +     len = strlen(buf);
> +     *ulen = len + 1;
>       if (!uname)
>               return 0;
> +
>       if (buf) {
> -             len = strlen(buf);
> -             err = bpf_copy_to_user(uname, buf, ulen, len);
> +             err = bpf_copy_to_user(uname, buf, name_len, len);
>               if (err)
>                       return err;
>       } else {

small nit.. up to you but I'd suggest bit different naming like below,
otherwise looks good

Acked-by: Jiri Olsa <jo...@kernel.org>

thanks,
jirka


---
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index a8f1808a1ca5..b637e9dced5a 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3565,26 +3565,28 @@ 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 *ulenp,
                                     u64 *probe_offset, u64 *probe_addr,
                                     u32 *fd_type, unsigned long *missed)
 {
        const char *buf;
-       u32 prog_id;
+       u32 prog_id, ulen;
        size_t len;
        int err;
 
-       if (!ulen ^ !uname)
+       if (!(*ulenp) ^ !uname)
                return -EINVAL;
 
        err = bpf_get_perf_event_info(event, &prog_id, fd_type, &buf,
                                      probe_offset, probe_addr, missed);
        if (err)
                return err;
+       ulen = *ulenp;
+       len = strlen(buf);
+       *ulenp = len + 1;
        if (!uname)
                return 0;
        if (buf) {
-               len = strlen(buf);
                err = bpf_copy_to_user(uname, buf, ulen, len);
                if (err)
                        return err;