Re: [PATCH next] i2c: Fix end of loop test in i2c_atr_find_mapping_by_addr()

[Tomi Valkeinen]

Hi,

On 23/04/2025 20:29, Dan Carpenter wrote:
> On Wed, Apr 23, 2025 at 05:25:44PM +0200, Romain Gantois wrote:
> > Hello Dan,
> >
> > On Wednesday, 23 April 2025 10:21:18 CEST Dan Carpenter wrote:
> > > When the list_for_each_entry_reverse() exits without hitting a break
> > > then the list cursor points to invalid memory.  So this check for
> > > if (c2a->fixed) is checking bogus memory.  Fix it by using a "found"
> > > variable to track if we found what we were looking for or not.
> >
> > IIUC the for loop ending condition in list_for_each_entry_reverse() is
> > "!list_entry_is_head(pos, head, member);", so even if the loop runs to
> > completion, the pointer should still be valid right?
>
> head is &chan->alias_pairs.  pos is an offset off the head.  In this
> case, the offset is zero.  So it's &chan->alias_pairs minus zero.
>
> So we exit the list with c2a = (void *)&chan->alias_pairs.
>
> If you look how struct i2c_atr_chan is declareted the next struct member
> after alias_pairs is:
>
> 	struct i2c_atr_alias_pool *alias_pool;
>
> So if (c2a->fixed) is poking around in the alias_pool pointer.  It's not
> out of bounds but it's not valid either.

Maybe it's just me, but I had hard time following that explanation. So 
here's mine:

The list head (i2c_atr_chan.alias_pairs) is not a full entry, it's just 
a struct list_head. When the for loop runs to completion, c2a doesn't 
point to a struct i2c_atr_alias_pair, so you can't access c2a->fixed.

For the patch:

Reviewed-by: Tomi Valkeinen <tomi.valkeinen@xxxxxxxxxxxxxxxx>

 Tomi