linux-kernel - Re: [PATCHv7] x86/kdump: bugfix, make the behavior of crashkernel=X consistent with kaslr

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Message-ID: <20190131234740.GO6749@zn.tnic>
Date:   Fri, 1 Feb 2019 00:47:40 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Jerry Hoemann <jerry.hoemann@....com>
Cc:     Dave Young <dyoung@...hat.com>, x86@...nel.org,
        Baoquan He <bhe@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Pingfan Liu <kernelfans@...il.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>, yinghai@...nel.org,
        vgoyal@...hat.com
Subject: Re: [PATCHv7] x86/kdump: bugfix, make the behavior of crashkernel=X
 consistent with kaslr

On Thu, Jan 31, 2019 at 03:27:32PM -0700, Jerry Hoemann wrote:
> So even if a system administrator is diligent and tests
> that a chosen kdump configuration works, that configuration
> might not work on some random reboot 7 months in the future.

Jerry, did you read the rest of the thread where I'm *actually*
suggesting to make the allocation code more robust against such
failures?

Doesn't look like it...

Now let's look at the code:

The "high" allocation does:

                crash_base = memblock_find_in_range(CRASH_ALIGN,
                                                    high ? CRASH_ADDR_HIGH_MAX
                                                         : CRASH_ADDR_LOW_MAX,
                                                    crash_size, CRASH_ALIGN);

where high=true and CRASH_ADDR_HIGH_MAX on 64-bit is MAXMEM:

# define CRASH_ADDR_HIGH_MAX    MAXMEM

The second fallback in the suggested patch does the same:

+               /*
+                * crashkernel=X reserve below 4G fails? Try MAXMEM
+                */
+               if (!high && !crash_base)
+                       crash_base = memblock_find_in_range(CRASH_ALIGN,
+                                               CRASH_ADDR_HIGH_MAX,
+                                               crash_size, CRASH_ALIGN);

and yet I get back that falling back to "high" if the first allocation
doesn't succeed is not something we should do by default because of
reasons. But this patch *practically* *does* it.

So no, until this hasn't been done cleanly and properly explained, we'll
be in a holding pattern.

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.