Re: [PATCH bpf v6 1/2] bpf: fix unpopulated name_len field in perf_event link info

[Andrii Nakryiko]

On Mon, Oct 7, 2024 at 11:29 AM Tyrone Wu <wudevelops@xxxxxxxxx> wrote:
>
> From: tyrone-wu <wudevelops@xxxxxxxxx>
>
> 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-yb0Snztmtg@xxxxxxxxxxxxxx/
> Fixes: 1b715e1b0ec5 ("bpf: Support ->fill_link_info for perf_event")
> Signed-off-by: Tyrone Wu <wudevelops@xxxxxxxxx>
> Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> Acked-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> ---
> V5 -> V6:
> Link: https://lore.kernel.org/bpf/ZwOWs_XrBtlTGE24@krava/
> - Use simpler buf check while keeping V4
> - Fix netdev/checkpatch warning for 80 cols exceeded
> - Fix Signed-off-by to use real name instead of git username
>
> V4 -> V5:
> Link: https://lore.kernel.org/bpf/CALOAHbC5xm7Cbfhau3z5X2PqUhiHECNWAPtJCWiOVqTKmdZp-Q@xxxxxxxxxxxxxx/
> - 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/

Please drop all the Link: tags, our scripts will accumulate all of
them in the final commit and it becomes very confusing. Our script
will add the final Link: for your latest applied version anyways.

Also, git still shows:

Author: tyrone-wu <wudevelops@xxxxxxxxx>

So please try to fix that.

But generally, looks good and I almost applied, but see one small
issue below, which I think needs fixing.

pw-bot: cr

> - Use user set *ulen in bpf_copy_to_user before overwriting *ulen
>
>  kernel/bpf/syscall.c | 32 +++++++++++++++++++++++---------
>  1 file changed, 23 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index a8f1808a1ca5..b5a7e428ac16 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3565,27 +3565,33 @@ 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,

nit: I'd do what Jiri suggested, use `u32 *ulenp` here

>                                      u64 *probe_offset, u64 *probe_addr,
>                                      u32 *fd_type, unsigned long *missed)
>  {
>         const char *buf;
> -       u32 prog_id;
> +       u32 prog_id, input_len;

and then just `u32 ulen;` here.

>         size_t len;
>         int err;
>

start with just

ulen = *ulenp;

> -       if (!ulen ^ !uname)
> +       if (!(*ulen) ^ !uname)

and remove all the `*ulen` uses except the final assignment

>                 return -EINVAL;
>
>         err = bpf_get_perf_event_info(event, &prog_id, fd_type, &buf,
>                                       probe_offset, probe_addr, missed);
>         if (err)
>                 return err;
> +
> +       if (buf) {
> +               input_len = *ulen;
> +               len = strlen(buf);
> +               *ulen = len + 1;
> +       }

don't we need

} else {
    *ulen = 1;
}

for cases when we don't have buf returned from
bpf_get_perf_event_info()? Though I don't think it can happen
currently, existing code is clearly ready to handle that case, so
let's keep it consistent.

>         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, input_len, len);
>                 if (err)
>                         return err;
>         } else {
> @@ -3609,7 +3615,7 @@ 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 +3623,7 @@ 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 +3645,7 @@ 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 +3654,7 @@ 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 +3680,19 @@ 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);

nit: keep it on a single line

> +       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
>