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Date:   Tue, 8 Nov 2022 15:48:46 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Stephen Brennan <stephen.s.brennan@oracle.com>
Cc:     Baoquan He <bhe@redhat.com>, Vivek Goyal <vgoyal@redhat.com>,
        kexec@lists.infradead.org, linux-kernel@vger.kernel.org,
        Dave Young <dyoung@redhat.com>
Subject: Re: [PATCH] vmcoreinfo: Warn if we exceed vmcoreinfo data size
Message-Id: <20221108154846.11584119794413c7682280fc@linux-foundation.org>
In-Reply-To: <20221027205008.312534-1-stephen.s.brennan@oracle.com>
References: <20221027205008.312534-1-stephen.s.brennan@oracle.com>
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On Thu, 27 Oct 2022 13:50:08 -0700 Stephen Brennan <stephen.s.brennan@oracle.com> wrote:

> Though vmcoreinfo is intended to be small, at just one page, useful
> information is still added to it, so we risk running out of space.
> Currently there is no runtime check to see whether the vmcoreinfo buffer
> has been exhausted. Add a warning for this case.
> 
> Currently, my static checking tool[1] indicates that a good upper bound
> for vmcoreinfo size is currently 3415 bytes, but the best time to add
> warnings is before the risk becomes too high.
> 
> ...
>
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -383,6 +383,9 @@ void vmcoreinfo_append_str(const char *fmt, ...)
>  	memcpy(&vmcoreinfo_data[vmcoreinfo_size], buf, r);
>  
>  	vmcoreinfo_size += r;
> +
> +	WARN_ONCE(vmcoreinfo_size == VMCOREINFO_BYTES,
> +		  "vmcoreinfo data exceeds allocated size, truncating");
>  }

Seems that vmcoreinfo_append_str() will truncate (ie: corrupt) the
final entry when limiting the overall data size to VMCOREINFO_BYTES. 
And that final entry will be missing any terminating \n or \0.

Is all this desirable, or should we be checking for (and warning about)
sufficient space _before_ appending this string?