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

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Sep 30, 2024 at 11:59:20PM +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-yb0Snztmtg@mail.gmail.com/
> Fixes: 1b715e1b0ec5 ("bpf: Support ->fill_link_info for perf_event")
> Signed-off-by: tyrone-wu <wudevelops@gmail.com>
> ---
>  kernel/bpf/syscall.c                          | 25 +++++++++++++------
>  .../selftests/bpf/prog_tests/fill_link_info.c |  6 ++---
>  2 files changed, 20 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index a8f1808a1ca5..90b6add4d0c9 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3565,7 +3565,7 @@ 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)
>  {
> @@ -3574,18 +3574,20 @@ static int bpf_perf_link_fill_common(const struct perf_event *event,
>  	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;
> +
> +	len = strlen(buf);
> +	*ulen = len + 1;

I think before you overwrite *ulen with actual name lenth you should
store its value and use it in bpf_copy_to_user, otherwise we could
overwrite user space memory that we are not supposed to

jirka

>  	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, *ulen, len);
>  		if (err)
>  			return err;
>  	} else {
> @@ -3609,7 +3611,7 @@ 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;

SNIP