Re: [PATCH net-next 02/15] net: dsa: sja1105: Remove usage of iterator for list_add() after loop

[Jakob Koschel]

Hello Vladimir,

> On 8. Apr 2022, at 13:41, Vladimir Oltean <olteanv@xxxxxxxxx> wrote:
> 
> Hello Jakob,
> 
> On Thu, Apr 07, 2022 at 12:28:47PM +0200, Jakob Koschel wrote:
>> In preparation to limit the scope of a list iterator to the list
>> traversal loop, use a dedicated pointer to point to the found element [1].
>> 
>> Before, the code implicitly used the head when no element was found
>> when using &pos->list. Since the new variable is only set if an
>> element was found, the list_add() is performed within the loop
>> and only done after the loop if it is done on the list head directly.
>> 
>> Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@xxxxxxxxxxxxxx/ [1]
>> Signed-off-by: Jakob Koschel <jakobkoschel@xxxxxxxxx>
>> ---
>> drivers/net/dsa/sja1105/sja1105_vl.c | 14 +++++++++-----
>> 1 file changed, 9 insertions(+), 5 deletions(-)
>> 
>> diff --git a/drivers/net/dsa/sja1105/sja1105_vl.c b/drivers/net/dsa/sja1105/sja1105_vl.c
>> index b7e95d60a6e4..cfcae4d19eef 100644
>> --- a/drivers/net/dsa/sja1105/sja1105_vl.c
>> +++ b/drivers/net/dsa/sja1105/sja1105_vl.c
>> @@ -27,20 +27,24 @@ static int sja1105_insert_gate_entry(struct sja1105_gating_config *gating_cfg,
>> 	if (list_empty(&gating_cfg->entries)) {
>> 		list_add(&e->list, &gating_cfg->entries);
>> 	} else {
>> -		struct sja1105_gate_entry *p;
>> +		struct sja1105_gate_entry *p = NULL, *iter;
>> 
>> -		list_for_each_entry(p, &gating_cfg->entries, list) {
>> -			if (p->interval == e->interval) {
>> +		list_for_each_entry(iter, &gating_cfg->entries, list) {
>> +			if (iter->interval == e->interval) {
>> 				NL_SET_ERR_MSG_MOD(extack,
>> 						   "Gate conflict");
>> 				rc = -EBUSY;
>> 				goto err;
>> 			}
>> 
>> -			if (e->interval < p->interval)
>> +			if (e->interval < iter->interval) {
>> +				p = iter;
>> +				list_add(&e->list, iter->list.prev);
>> 				break;
>> +			}
>> 		}
>> -		list_add(&e->list, p->list.prev);
>> +		if (!p)
>> +			list_add(&e->list, gating_cfg->entries.prev);
>> 	}
>> 
>> 	gating_cfg->num_entries++;
>> -- 
>> 2.25.1
>> 
> 
> I apologize in advance if I've misinterpreted the end goal of your patch.
> I do have a vague suspicion I understand what you're trying to achieve,
> and in that case, would you mind using this patch instead of yours?

I think you are very much spot on!

> I think it still preserves the intention of the code in a clean manner.
> 
> -----------------------------[ cut here ]-----------------------------
> From 7aed740750d1bc3bff6e85fd33298f5905bb4e01 Mon Sep 17 00:00:00 2001
> From: Vladimir Oltean <vladimir.oltean@xxxxxxx>
> Date: Fri, 8 Apr 2022 13:55:14 +0300
> Subject: [PATCH] net: dsa: sja1105: avoid use of type-confused pointer in
> sja1105_insert_gate_entry()
> 
> It appears that list_for_each_entry() leaks a type-confused pointer when
> the iteration loop ends with no early break, since "*p" will no longer
> point to a "struct sja1105_gate_entry", but rather to some memory in
> front of "gating_cfg->entries".
> 
> This isn't actually a problem here, because if the element we insert has
> the highest interval, therefore we never exit the loop early, "p->list"
> (which is all that we use outside the loop) will in fact point to
> "gating_cfg->entries" even though "p" itself is invalid.
> 
> Nonetheless, there are preparations to increase the safety of
> list_for_each_entry() by making it impossible to use the encapsulating
> structure of the iterator element outside the loop. So something needs
> to change here before those preparations go in, even though this
> constitutes legitimate use.
> 
> Make it clear that we are not dereferencing members of the encapsulating
> "struct sja1105_gate_entry" outside the loop, by using the regular
> list_for_each() iterator, and dereferencing the struct sja1105_gate_entry
> only within the loop.
> 
> With list_for_each(), the iterator element at the end of the loop does
> have a sane value in all cases, and we can just use that as the "head"
> argument of list_add().
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>
> ---
> drivers/net/dsa/sja1105/sja1105_vl.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/dsa/sja1105/sja1105_vl.c b/drivers/net/dsa/sja1105/sja1105_vl.c
> index c0e45b393fde..fe93c80fe5ef 100644
> --- a/drivers/net/dsa/sja1105/sja1105_vl.c
> +++ b/drivers/net/dsa/sja1105/sja1105_vl.c
> @@ -27,9 +27,15 @@ static int sja1105_insert_gate_entry(struct sja1105_gating_config *gating_cfg,
> 	if (list_empty(&gating_cfg->entries)) {
> 		list_add(&e->list, &gating_cfg->entries);
> 	} else {
> -		struct sja1105_gate_entry *p;
> +		struct list_head *pos;
> +
> +		/* We cannot safely use list_for_each_entry()
> +		 * because we dereference "pos" after the loop
> +		 */
> +		list_for_each(pos, &gating_cfg->entries) {
> +			struct sja1105_gate_entry *p;
> 
> -		list_for_each_entry(p, &gating_cfg->entries, list) {
> +			p = list_entry(pos, struct sja1105_gate_entry, list);
> 			if (p->interval == e->interval) {
> 				NL_SET_ERR_MSG_MOD(extack,
> 						   "Gate conflict");
> @@ -40,7 +46,7 @@ static int sja1105_insert_gate_entry(struct sja1105_gating_config *gating_cfg,
> 			if (e->interval < p->interval)
> 				break;
> 		}
> -		list_add(&e->list, p->list.prev);
> +		list_add(&e->list, pos->prev);

I was actually considering doing it this way before but wasn't sure if this would be preferred.
I've done something like this in [1] and it does turn out quite well.

I'll integrate this in the v2 series.

Thanks for the suggestion.

> 	}
> 
> 	gating_cfg->num_entries++;
> -----------------------------[ cut here ]-----------------------------

[1] https://lore.kernel.org/linux-kernel/20220407102900.3086255-12-jakobkoschel@xxxxxxxxx/

	Jakob