netdev - Re: [PATCH v3 net-next 05/14] af_unix: Detect Strongly Connected Components.

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Message-ID: <20240226190741.66233-1-kuniyu@amazon.com>
Date: Mon, 26 Feb 2024 11:07:41 -0800
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <kuba@...nel.org>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <kuni1840@...il.com>,
	<kuniyu@...zon.com>, <netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH v3 net-next 05/14] af_unix: Detect Strongly Connected Components.

From: Jakub Kicinski <kuba@...nel.org>
Date: Sat, 24 Feb 2024 16:34:30 -0800
> On Fri, 23 Feb 2024 13:39:54 -0800 Kuniyuki Iwashima wrote:
> > +	list_for_each_entry(edge, &vertex->edges, vertex_entry) {
> > +		struct unix_vertex *next_vertex = edge->successor->vertex;
> > +
> > +		if (!next_vertex)
> > +			continue;
> > +
> > +		if (next_vertex->index == UNIX_VERTEX_INDEX_UNVISITED) {
> > +			list_add(&edge->stack_entry, &edge_stack);
> > +
> > +			vertex = next_vertex;
> > +			goto next_vertex;
> > +prev_vertex:
> > +			next_vertex = vertex;
> > +
> > +			edge = list_first_entry(&edge_stack, typeof(*edge), stack_entry);
> > +			list_del_init(&edge->stack_entry);
> > +
> > +			vertex = edge->predecessor->vertex;
> > +
> > +			vertex->lowlink = min(vertex->lowlink, next_vertex->lowlink);
> > +		} else if (edge->successor->vertex->on_stack) {
> > +			vertex->lowlink = min(vertex->lowlink, next_vertex->index);
> > +		}
> > +	}
> > +
> > +	if (vertex->index == vertex->lowlink) {
> > +		struct list_head scc;
> > +
> > +		__list_cut_position(&scc, &vertex_stack, &vertex->scc_entry);
> > +
> > +		list_for_each_entry_reverse(vertex, &scc, scc_entry) {
> > +			list_move_tail(&vertex->entry, &unix_visited_vertices);
> > +
> > +			vertex->on_stack = false;
> > +		}
> > +
> > +		list_del(&scc);
> > +	}
> > +
> > +	if (!list_empty(&edge_stack))
> > +		goto prev_vertex;
> 
> coccicheck says:
> 
> net/unix/garbage.c:406:17-23: ERROR: invalid reference to the index variable of the iterator on line 425
> 
> this code looks way to complicated to untangle on a quick weekend scan,
> so please LMK if this is a false positive, I'll hide the patches from
> patchwork for now ;)

Yeah, I think it's false positive :)

The code above implements recursion withtout nesting call stack
and instead uses goto jump and restores the previous in-loop
variables there.

  __unix_walk_scc(struct unix_vertex *vertex)
  {
    ...
    list_for_each_entry(edge, &vertex->edges, vertex_entry) {
      if (next_vertex->index == UNIX_VERTEX_INDEX_UNVISITED) {
        __unix_walk_scc(next_vertex);

        ^-- This is rewritten with goto next_vertex & prev_vertex.

        vertex->lowlink = min(vertex->lowlink, next_vertex->lowlink);
      } else {
        vertex->lowlink = min(vertex->lowlink, next_vertex->index);
      }
    }
    ...

Here's the original recursive pseudocode.
https://en.wikipedia.org/wiki/Tarjan%27s_strongly_connected_components_algorithm