Re: [PATCH bpf v3] bpf: fix unpopulated name_len field in perf_event link info

[Jiri Olsa]

On Thu, Oct 03, 2024 at 08:23:00PM +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@xxxxxxxxxxxxxx/
> Fixes: 1b715e1b0ec5 ("bpf: Support ->fill_link_info for perf_event")
> Signed-off-by: tyrone-wu <wudevelops@xxxxxxxxx>
> ---
> 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

looks good, I checked with Daniel and it's better to separate
kernel and selftest changes, so it's easier to get this applied
in stable branches

could you split the change in 2 changes and repost? please keep my ack

thanks,
jirka

> 
> 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..56c556fcf325 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;
> +	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;
> +
> +	input_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, input_len, len);
>  		if (err)
>  			return err;
>  	} else {
> @@ -3609,7 +3613,7 @@ 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 +3621,7 @@ 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 +3643,7 @@ 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 +3652,7 @@ 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 +3678,18 @@ 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,
> diff --git a/tools/testing/selftests/bpf/prog_tests/fill_link_info.c b/tools/testing/selftests/bpf/prog_tests/fill_link_info.c
> index f3932941bbaa..59077f260404 100644
> --- a/tools/testing/selftests/bpf/prog_tests/fill_link_info.c
> +++ b/tools/testing/selftests/bpf/prog_tests/fill_link_info.c
> @@ -67,8 +67,8 @@ static int verify_perf_link_info(int fd, enum bpf_perf_event_type type, long add
>  
>  		ASSERT_EQ(info.perf_event.kprobe.cookie, PERF_EVENT_COOKIE, "kprobe_cookie");
>  
> +		ASSERT_EQ(info.perf_event.kprobe.name_len, strlen(KPROBE_FUNC) + 1, "name_len");
>  		if (!info.perf_event.kprobe.func_name) {
> -			ASSERT_EQ(info.perf_event.kprobe.name_len, 0, "name_len");
>  			info.perf_event.kprobe.func_name = ptr_to_u64(&buf);
>  			info.perf_event.kprobe.name_len = sizeof(buf);
>  			goto again;
> @@ -79,8 +79,8 @@ static int verify_perf_link_info(int fd, enum bpf_perf_event_type type, long add
>  		ASSERT_EQ(err, 0, "cmp_kprobe_func_name");
>  		break;
>  	case BPF_PERF_EVENT_TRACEPOINT:
> +		ASSERT_EQ(info.perf_event.tracepoint.name_len, strlen(TP_NAME) + 1, "name_len");
>  		if (!info.perf_event.tracepoint.tp_name) {
> -			ASSERT_EQ(info.perf_event.tracepoint.name_len, 0, "name_len");
>  			info.perf_event.tracepoint.tp_name = ptr_to_u64(&buf);
>  			info.perf_event.tracepoint.name_len = sizeof(buf);
>  			goto again;
> @@ -96,8 +96,8 @@ static int verify_perf_link_info(int fd, enum bpf_perf_event_type type, long add
>  	case BPF_PERF_EVENT_URETPROBE:
>  		ASSERT_EQ(info.perf_event.uprobe.offset, offset, "uprobe_offset");
>  
> +		ASSERT_EQ(info.perf_event.uprobe.name_len, strlen(UPROBE_FILE) + 1, "name_len");
>  		if (!info.perf_event.uprobe.file_name) {
> -			ASSERT_EQ(info.perf_event.uprobe.name_len, 0, "name_len");
>  			info.perf_event.uprobe.file_name = ptr_to_u64(&buf);
>  			info.perf_event.uprobe.name_len = sizeof(buf);
>  			goto again;
> -- 
> 2.43.0
>