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Date:   Thu, 7 May 2020 23:07:16 -0700
From:   Kees Cook <keescook@chromium.org>
To:     WeiXiong Liao <liaoweixiong@allwinnertech.com>
Cc:     Anton Vorontsov <anton@enomsg.org>,
        Colin Cross <ccross@android.com>,
        Tony Luck <tony.luck@intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Miquel Raynal <miquel.raynal@bootlin.com>,
        Richard Weinberger <richard@nod.at>,
        Vignesh Raghavendra <vigneshr@ti.com>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Rob Herring <robh@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mtd@lists.infradead.org
Subject: Re: [PATCH v3 02/11] pstore/blk: new support logger for block devices
Message-ID: <202005072301.57F73B61CC@keescook>
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On Wed, Mar 25, 2020 at 04:54:57PM +0800, WeiXiong Liao wrote:
> pstore/blk is similar to pstore/ram, but dump log to block device
> rather than persistent ram.
> [...]
> +int psblk_register_blkdev(unsigned int major, psblk_panic_write_op panic_write)
> +{
> +	struct block_device *bdev;
> +	struct psblk_device dev = {0};
> +	struct bdev_info *binfo;
> +	int ret = -ENODEV;
> +	void *holder = blkdev;
> +
> +	binfo = psblk_get_bdev_info();
> +	if (IS_ERR(binfo))
> +		return PTR_ERR(binfo);
> +
> +	/* only allow driver matching the @blkdev */
> +	if (!binfo->devt || MAJOR(binfo->devt) != major) {
> +		pr_debug("invalid major %u (expect %u)\n",
> +				major, MAJOR(binfo->devt));
> +		return -ENODEV;
> +	}
> +
> +	/* hold bdev exclusively */
> +	bdev = psblk_get_bdev(holder);
> +	if (IS_ERR(bdev)) {
> +		pr_err("failed to open '%s'!\n", blkdev);
> +		return PTR_ERR(bdev);
> +	}
> +
> +	/* psblk_bdev must be assigned before register to pstore/blk */
> +	psblk_bdev = bdev;
> +	blkdev_panic_write = panic_write;
> +
> +	dev.total_size = psblk_bdev_size(bdev);
> +	dev.panic_write = panic_write ? psblk_blk_panic_write : NULL;
> +	dev.read = psblk_generic_blk_read;
> +	dev.write = psblk_generic_blk_write;
> +
> +	ret = psblk_register_do(&dev);
> +	if (ret)
> +		goto err_put_bdev;
> +
> +	pr_info("using '%s'\n", blkdev);
> +	return 0;
> +
> +err_put_bdev:
> +	psblk_bdev = NULL;
> +	blkdev_panic_write = NULL;
> +	psblk_put_bdev(bdev, holder);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(psblk_register_blkdev);

I've gotten this series refactored on top of current pstore, and I've
been making various bikeshed changes to names, etc, and as I went to go
start testing, I realized that nothing actually uses
psblk_register_blkdev().

It seems like it should be possible to just start using this on any
block device of the user's choosing. I assume the idea is to allow for
drivers to register panic_write handlers, but even without that, it'd be
nice to just be able to test this with something like /dev/loop0.

What's your thinking on how this would happen? It seems like if
pstore/blk uses pstore/zone, and mtdpstore uses pstore/blk, there should
be a blkoops that uses pstore/blk too? I guess I need to learn a bit
more about how block device probing works so pstore/blk can notice
devices as they're brought online, etc.

-- 
Kees Cook