Re: [PATCH] fixup! arm64: kdump: add kdump support

[AKASHI Takahiro <takahiro.akashi@linaro.org>]

On Wed, Aug 17, 2016 at 04:33:31PM +0100, James Morse wrote:
> copy_oldmem_page() and mmap_vmcore() provide two ways for userspace to read
> from /proc/vmcore. Neither of these check with memblock to see if the page
> they are accessing is nomap. On Seattle this causes:
> 
> [  174.393875] Unhandled fault: synchronous external abort (0x96000210) at
> 0xffffff80096b6000
> [  174.402158] Internal error: : 96000210 [#1] PREEMPT SMP
> [  174.407370] Modules linked in:
> [  174.410417] CPU: 6 PID: 2059 Comm: cp Tainted: G S      W I     4.8.0-rc1+ #4708
> [  174.417799] Hardware name: AMD Overdrive/Supercharger/Default string, BIOS
> ROD1002C 04/08/2016
> [  174.426396] task: ffffffc0fdec5780 task.stack: ffffffc0f34bc000
> [  174.432313] PC is at __arch_copy_to_user+0x180/0x280
> [  174.437274] LR is at copy_oldmem_page+0xac/0xf0
> [  174.441791] pc : [<ffffff800835e080>] lr : [<ffffff8008095b9c>] pstate: 20000145
> [  174.449173] sp : ffffffc0f34bfc90
> [  174.452474] x29: ffffffc0f34bfc90 x28: 0000000000000000
> [  174.457776] x27: 0000000008000000 x26: 000000000000d000
> [  174.463077] x25: 0000000000000001 x24: ffffff8008eb5000
> [  174.468378] x23: 0000000000000000 x22: ffffff80096b6000
> [  174.473679] x21: 0000000000000001 x20: 0000000030127000
> [  174.478979] x19: 0000000000001000 x18: 0000007ff7085d60
> [  174.484279] x17: 0000000000429358 x16: ffffff80081d9e88
> [  174.489579] x15: 0000007fae377590 x14: 0000000000000000
> 
> [  174.494880] x13: 0000000000000000 x12: ffffff8008dd1000
> [  174.500180] x11: ffffff80096b6fff x10: ffffff80096b6fff
> [  174.505480] x9 : 0000000040000000 x8 : ffffff8008db6000
> [  174.510781] x7 : ffffff80096b7000 x6 : 0000000030127000
> [  174.516082] x5 : 0000000030128000 x4 : 0000000000000000
> [  174.521382] x3 : 00e8000000000713 x2 : 0000000000000f80
> [  174.526682] x1 : ffffff80096b6000 x0 : 0000000030127000
> [  174.531982]
> [  174.533461] Process cp (pid: 2059, stack limit = 0xffffffc0f34bc020)
> 
> [  174.848448] [<ffffff800835e080>] __arch_copy_to_user+0x180/0x280
> [  174.854448] [<ffffff8008245f34>] read_from_oldmem.part.4+0xb4/0xf4
> [  174.860615] [<ffffff8008246074>] read_vmcore+0x100/0x22c
> [  174.865919] [<ffffff8008239378>] proc_reg_read+0x64/0x90
> [  174.871223] [<ffffff80081d7da8>] __vfs_read+0x28/0x108
> [  174.876348] [<ffffff80081d8ae4>] vfs_read+0x84/0x144
> [  174.881301] [<ffffff80081d9ecc>] SyS_read+0x44/0xa0
> [  174.886167] [<ffffff8008082ef0>] el0_svc_naked+0x24/0x28
> [  174.891466] Code: 00000000 00000000 00000000 00000000 (a8c12027)
> [  174.897562] ---[ end trace 00801b2e35b0cd1f ]---
> 
> When reading /proc/vmcore with cat/cp or or mmap()ing it with makedumpfile.
> 
> The fs/proc/vmcore.c code provides a hook to indicate whether oldmem pages
> are ram or not. Use this to look for our earlier handiwork in memblock.

I'm not quite sure about the background that oldmem_pfn_is_ram() was
originally introduced on x86, but I think that this feature be deserved
for fixing an issue on Xen.
See:
commit 997c136
Author: Olaf Hering <olaf@aepfle.de>
Date:   Thu May 26 16:25:54 2011 -0700

    fs/proc/vmcore.c: add hook to read_from_oldmem() to check for non-ram pages

Thanks,
-Takahiro AKASHI

> 
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> 
> Hi Pratyush,
> 
> I couldn't get makedumpfile to build, or rather it depends on elfutils which
> wouldn't build for autotools reasons. Does implementing this hook solve your
> makedumpfile issue?
> 
> With this patch I can extract a usable vmcore file using read or mmap,
> avoiding the earlier splat.
> 
> Akashi, if you agree this is the right thing to do, please consider folding
> this into patch 5. (no need to keep the commit mesage or anything).
> 
> Thanks,
> 
> James
> 
>  arch/arm64/kernel/crash_dump.c | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/arch/arm64/kernel/crash_dump.c b/arch/arm64/kernel/crash_dump.c
> index 2dc54d129be1..76c71ab42994 100644
> --- a/arch/arm64/kernel/crash_dump.c
> +++ b/arch/arm64/kernel/crash_dump.c
> @@ -12,10 +12,38 @@
>  #include <linux/crash_dump.h>
>  #include <linux/errno.h>
>  #include <linux/io.h>
> +#include <linux/init.h>
>  #include <linux/memblock.h>
>  #include <linux/uaccess.h>
>  #include <asm/memory.h>
>  
> +#ifdef CONFIG_PROC_VMCORE
> +static int oldmem_pfn_is_ram(unsigned long pfn)
> +{
> +	unsigned long addr = pfn << PAGE_SHIFT;
> +
> +	/*
> +	 * We removed oldmem from memblock.memory, then re-added some regions
> +	 * which are reserved by/for firmware as memory and nomap.
> +	 * If an address exists as memory, but is marked nomap, return false.
> +	 */
> +	if (memblock_is_memory(addr) && !memblock_is_map_memory(addr))
> +		return 0;
> +
> +	return 1;
> +}
> +
> +static int __init do_register_oldmem_pfn_is_ram(void)
> +{
> +	return register_oldmem_pfn_is_ram(&oldmem_pfn_is_ram);
> +}
> +/*
> + * vmcore_init() is called via fs_initcall, ensure we register
> + * oldmem_pfn_is_ram() before then.
> + */
> +arch_initcall(do_register_oldmem_pfn_is_ram);
> +#endif
> +
>  /**
>   * copy_oldmem_page() - copy one page from old kernel memory
>   * @pfn: page frame number to be copied
> -- 
> 2.8.0.rc3
> 

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