Subject: Re: [PATCH] block: partitions: efi: Always check for alternative GPT
 at end of drive
To: Julius Werner <jwerner@chromium.org>, Davidlohr Bueso <dave@stgolabs.net>
References: <1461632806-5946-1-git-send-email-jwerner@chromium.org>
Cc: linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-block@vger.kernel.org, Gwendal Grignou <gwendal@chromium.org>,
        Doug Anderson <dianders@chromium.org>
From: "Austin S. Hemmelgarn" <ahferroin7@gmail.com>
Message-ID: <571F7D6D.8020209@gmail.com>
Date: Tue, 26 Apr 2016 10:38:37 -0400
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On 2016-04-25 21:06, Julius Werner wrote:
> The GUID Partiton Table layout maintains two synonymous partition tables
> on a block device, one starting in sector 1 and one in the very last
> sectors of the block device. This is useful if one of the tables gets
> accidentally corrupted (e.g. through a partial write because of an
> unexpected power loss).
>
> Linux normally only boots if the primary GPT is valid. It will not even
> try to find the alternative GPT to an invalid primary one unless the
> "gpt" command line option forces more aggressive detection. This doesn't
> really make any sense... if the "gpt" option is not set, the code
> validates the protective or hybrid MBR in sector 0 anyway before it even
> starts looking for the actual GPTs. If we get to the point where a valid
> proctective or hybrid MBR was found but the primary GPT was not found
> (valid), checking the alternative GPT is our best bet: we know that this
> block device is meant to use GPT (because any other partitioning system
> would've presumably overwritten sector 0), and we know that if the
> alternative GPT is valid it should contain more accurate information
> than parsing the protective/hybrid MBR with msdos_partition() would
> yield (which would otherwise be what happens next).
At the absolute minimum, we should be logging (at least at a warning 
level) that we had to fall back the the backup GPT.  If somebody is 
dealing with a disk that had a torn write to the primary GPT, that's one 
thing, but this could also be caused by any number of other problems 
(hardware issues, malicious intent, etc), and we need to log that we 
detected corrupted data.
>
> Signed-off-by: Julius Werner <jwerner@chromium.org>
> ---
>   block/partitions/efi.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/block/partitions/efi.c b/block/partitions/efi.c
> index 26cb624..0d4ca8e 100644
> --- a/block/partitions/efi.c
> +++ b/block/partitions/efi.c
> @@ -625,7 +625,7 @@ static int find_valid_gpt(struct parsed_partitions *state, gpt_header **gpt,
>   		good_agpt = is_gpt_valid(state,
>   					 le64_to_cpu(pgpt->alternate_lba),
>   					 &agpt, &aptes);
> -        if (!good_agpt && force_gpt)
> +        if (!good_agpt)
>                   good_agpt = is_gpt_valid(state, lastlba, &agpt, &aptes);
>
>           /* The obviously unsuccessful case */
>