linux-kernel - Re: [PATCH v4 02/18] of: overlay: add missing of_node

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Message-ID: <20181018170502.GB15557@bogus>
Date:   Thu, 18 Oct 2018 12:05:02 -0500
From:   Rob Herring <robh@...nel.org>
To:     frowand.list@...il.com
Cc:     Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Alan Tull <atull@...nel.org>, Moritz Fischer <mdf@...nel.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        devicetree@...r.kernel.org, linux-fpga@...r.kernel.org
Subject: Re: [PATCH v4 02/18] of: overlay: add missing of_node_put() after
 add new node to changeset

On Mon, Oct 15, 2018 at 07:37:22PM -0700, frowand.list@...il.com wrote:
> From: Frank Rowand <frank.rowand@...y.com>
> 
> The refcount of a newly added overlay node decrements to one
> (instead of zero) when the overlay changeset is destroyed.  This
> change will cause the final decrement be to zero.
> 
> After applying this patch, new validation warnings will be
> reported from the devicetree unittest during boot due to
> a pre-existing devicetree bug.  The warnings will be similar to:
> 
>   OF: ERROR: memory leak of_node_release() overlay node /testcase-data/overlay-node/test-bus/test-unittest4 before free overlay changeset

Same comment on formatting.

> 
> This pre-existing devicetree bug will also trigger a WARN_ONCE() from
> refcount_sub_and_test_checked() when an overlay changeset is
> destroyed without having first been applied.  This scenario occurs
> when an error in the overlay is detected during the overlay changeset
> creation:
> 
>   WARNING: CPU: 0 PID: 1 at lib/refcount.c:187 refcount_sub_and_test_checked+0xa8/0xbc
>   refcount_t: underflow; use-after-free.
> 
>   (unwind_backtrace) from (show_stack+0x10/0x14)
>   (show_stack) from (dump_stack+0x6c/0x8c)
>   (dump_stack) from (__warn+0xdc/0x104)
>   (__warn) from (warn_slowpath_fmt+0x44/0x6c)
>   (warn_slowpath_fmt) from (refcount_sub_and_test_checked+0xa8/0xbc)
>   (refcount_sub_and_test_checked) from (kobject_put+0x24/0x208)
>   (kobject_put) from (of_changeset_destroy+0x2c/0xb4)
>   (of_changeset_destroy) from (free_overlay_changeset+0x1c/0x9c)
>   (free_overlay_changeset) from (of_overlay_remove+0x284/0x2cc)
>   (of_overlay_remove) from (of_unittest_apply_revert_overlay_check.constprop.4+0xf8/0x1e8)
>   (of_unittest_apply_revert_overlay_check.constprop.4) from (of_unittest_overlay+0x960/0xed8)
>   (of_unittest_overlay) from (of_unittest+0x1cc4/0x2138)
>   (of_unittest) from (do_one_initcall+0x4c/0x28c)
>   (do_one_initcall) from (kernel_init_freeable+0x29c/0x378)
>   (kernel_init_freeable) from (kernel_init+0x8/0x110)
>   (kernel_init) from (ret_from_fork+0x14/0x2c)
> 
> Signed-off-by: Frank Rowand <frank.rowand@...y.com>
> ---
>  drivers/of/overlay.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
> index 1176cb4b6e4e..32cfee68f2e3 100644
> --- a/drivers/of/overlay.c
> +++ b/drivers/of/overlay.c
> @@ -379,7 +379,9 @@ static int add_changeset_node(struct overlay_changeset *ovcs,
>  		if (ret)
>  			return ret;
>  
> -		return build_changeset_next_level(ovcs, tchild, node);
> +		ret = build_changeset_next_level(ovcs, tchild, node);
> +		of_node_put(tchild);
> +		return ret;
>  	}
>  
>  	if (node->phandle && tchild->phandle)
> -- 
> Frank Rowand <frank.rowand@...y.com>
>