linux-kernel - Re: [PATCH] /proc/kcore: Update physical address for kcore ram and text

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Message-ID: <c99e8165-1aa7-4bb3-ef6f-7d66020c57dd@redhat.com>
Date:   Wed, 25 Jan 2017 12:21:47 +0530
From:   Pratyush Anand <panand@...hat.com>
To:     Dave Young <dyoung@...hat.com>
Cc:     linux-kernel@...r.kernel.org, bhe@...hat.com, anderson@...hat.com,
        kexec@...ts.infradead.org
Subject: Re: [PATCH] /proc/kcore: Update physical address for kcore ram and
 text

Hi Dave,

On Wednesday 25 January 2017 11:59 AM, Dave Young wrote:
> Hi Pratyush
> On 01/25/17 at 10:14am, Pratyush Anand wrote:
>> Currently all the p_paddr of PT_LOAD headers are assigned to 0, which is
>> not true and could be misleading, since 0 is a valid physical address.
> I do not know the history of /proc/kcore, so a question is why the
> p_addr was set as 0, if there were some reasons and if this could cause
> some risk or breakage.
>

I do not know why it was 0, which is a valid physical address. But 
certainly, it might break some user space tools, and those need to be 
fixed. For example, see following code from kexec-tools

kexec/kexec-elf.c:build_mem_phdrs()

435                 if ((phdr->p_paddr + phdr->p_memsz) < phdr->p_paddr) {
436                         /* The memory address wraps */
437                         if (probe_debug) {
438                                 fprintf(stderr, "ELF address wrap 
around\n");
439                         }
440                         return -1;
441                 }

We do not need to perform above check for an invalid physical address.

I think, kexec-tools and makedumpfile will need fixup. I already have 
those fixup which will be sent upstream once this patch makes through.
Pro with this approach is that, it will help to calculate variable like 
page_offset, phys_base from PT_LOAD even when they are randomized and 
therefore will reduce many variable and version specific values in user 
space tools.

~Pratyush