[PATCH] vmcoreinfo: Warn if we exceed vmcoreinfo data size

[Andrew Morton]

On Thu, 27 Oct 2022 13:50:08 -0700 Stephen Brennan <stephen.s.brennan at 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?