Re: [PATCH bpf v5 1/2] bpf: fix unpopulated name_len field in perf_event link info

[Jiri Olsa <olsajiri@gmail.com>]

On Sun, Oct 06, 2024 at 07:51:30PM +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.
> 
> 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>
> Acked-by: Jiri Olsa <jolsa@kernel.org>
> ---
> V4 -> V5:
> Link: https://lore.kernel.org/bpf/CALOAHbC5xm7Cbfhau3z5X2PqUhiHECNWAPtJCWiOVqTKmdZp-Q@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;
>  	}

hm, why not just simple check buf for and keep the rest? seems less complicated..

jirka


---
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index a8f1808a1ca5..e393b94b90ec 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;
+	if (buf) {
+		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 {