linux-kernel - Re: [PATCH next] i2c: Fix end of loop test in i2c_atr_find_mapping_by

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Message-ID: <2778486.mvXUDI8C0e@fw-rgant>
Date: Thu, 24 Apr 2025 09:10:43 +0200
From: Romain Gantois <romain.gantois@...tlin.com>
To: Dan Carpenter <dan.carpenter@...aro.org>,
 Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Luca Ceresoli <luca.ceresoli@...tlin.com>,
 Wolfram Sang <wsa+renesas@...g-engineering.com>,
 Andi Shyti <andi.shyti@...nel.org>, linux-i2c@...r.kernel.org,
 linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject:
 Re: [PATCH next] i2c: Fix end of loop test in i2c_atr_find_mapping_by_addr()

On Thursday, 24 April 2025 08:32:22 CEST Tomi Valkeinen wrote:
> 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.

Ah I see, in that case thanks for the fix Dan!

Reviewed-by: Romain Gantois <romain.gantois@...tlin.com>

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