of: Fix crash in of_build_overlay_info

[jharvell]

Do not set of_overlay_info attribute_group name from overlay info when there is no
overlay info device

Analysis of kernel stack trace, registers and stack along with disassembly of code pointed to ovinfo->info being zero at this point in the code.

Application of this patch prevents crash though overlays fail to load for various different reasons.

[jharvell]

In case someone is interested in reproducing the issue, I saw the crash with several overlays I tried to load. One of these is univ-bbgw-00A0.dtbo. I built this from sources below:

joey@akita arm$ pwd
/home/joey/git/bb.org-overlays/src/arm
joey@akita arm$ git remote -v
origin  git@github.com:beagleboard/bb.org-overlays.git (fetch)
origin  git@github.com:beagleboard/bb.org-overlays.git (push)
joey@akita arm$ git status
On branch master
Your branch is up to date with 'origin/master'.

nothing to commit, working tree clean
joey@akita arm$ git --no-pager log -n1
commit 604c0926a4f7505dfc3d501301413c821e59febe (HEAD -> master, origin/master, origin/HEAD)
Author: Robert Nelson <robertcnelson@gmail.com>
Date:   Tue Apr 24 08:41:58 2018 -0500

    universal rewrite: spi0-xyz and spidev1x
    
    Signed-off-by: Robert Nelson <robertcnelson@gmail.com>

Before the fix in this PR, echoing univ-bbgw to /sys/devices/platform/bone_capemgr/slots resulted i a SEGV with the kernel debug info in dmesg pointing to ovinfo->info being NULL.

After the fix, I see the following when trying to apply the same overlay:

pip /lib/firmware # echo univ-bbgw > /sys/devices/platform/bone_capemgr/slots 
bash: echo: write error: Invalid argument

[  290.750632] bone_capemgr bone_capemgr: part_number 'univ-bbgw', version 'N/A'
[  290.758437] bone_capemgr bone_capemgr: slot #4: override
[  290.768093] bone_capemgr bone_capemgr: Using override eeprom data at slot 4
[  290.780720] bone_capemgr bone_capemgr: slot #4: 'Override Board Name,00A0,Override Manuf,univ-bbgw'
[  290.847781] OF: overlay: Failed to apply prop @/__symbols__/pruss
[  290.854191] OF: overlay: apply failed '/__symbols__'
[  290.863083] bone_capemgr bone_capemgr: slot #4: Failed to create overlay

[jharvell]

I did a little more analysis. This crash will occur for any device blob that has a node /symbols. See the code below. An entry in ovinfo is added for each child node of tree in the first loop. ovinfo[cnt].info is populated in that case. Then there is a search for child node with name symbols. If found, it is added as the last element ovinfo. But for this case, ovinf[cnt].info is not set. And this is what the code crashes on. So if the compiler puts a /symbols node in the tree, then the kernel will crash tyring to apply it.

I made a simple overlay with one node, and compiled it. When I do strings on that blob, there is no "symbols" string in the blob and that does not trigger this bug. Several other blobs I have tried from bb.org-overlays blobs have symbols in them and trigger this crash.

	cnt = 0;
	for_each_child_of_node(tree, node) {
		err = of_fill_overlay_info(ov, node, &ovinfo[cnt]);
		if (err == 0)
			cnt++;
	}

	node = of_get_child_by_name(tree, "__symbols__");
	if (node) {
		ovinfo[cnt].overlay = node;
		ovinfo[cnt].target = of_find_node_by_path("/__symbols__");
		ovinfo[cnt].is_symbols_node = 1;

		if (!ovinfo[cnt].target) {
			pr_err("no symbols in root of device tree.\n");
			return -EINVAL;
		}

		cnt++;
	}