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Subject/proc/vmcore and wrong PAGE_OFFSET
Dear Linux folks,

I'm investigating a problem, that the crash utility fails to work with our crash dumps:

buczek@kreios:/mnt$ crash vmlinux crash.vmcore

crash 7.2.6
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crash: read error: kernel virtual address: ffff89807ff77000 type: "memory section root table"

The crash file is a copy of /dev/vmcore taken by a crashkernel after a sysctl-forced panic.

It looks to me, that 0xffff89807ff77000 is not readable, because the virtual addresses stored in the elf header of the dump file are off by 0x0000008000000000:

buczek@kreios:/mnt$ readelf -a crash.vmcore | grep LOAD | perl -lane 'printf "%s (%016x)\n",$_,hex($F[2])-hex($F[3])'
LOAD 0x000000000000d000 0xffffffff81000000 0x000001007d000000 (fffffeff04000000)
LOAD 0x0000000001c33000 0xffff880000001000 0x0000000000001000 (ffff880000000000)
LOAD 0x0000000001cc1000 0xffff880000090000 0x0000000000090000 (ffff880000000000)
LOAD 0x0000000001cd1000 0xffff880000100000 0x0000000000100000 (ffff880000000000)
LOAD 0x0000000001cd2070 0xffff880000100070 0x0000000000100070 (ffff880000000000)
LOAD 0x0000000019bd2000 0xffff880038000000 0x0000000038000000 (ffff880000000000)
LOAD 0x000000004e6a1000 0xffff88006ffff000 0x000000006ffff000 (ffff880000000000)
LOAD 0x000000004e6a2000 0xffff880100000000 0x0000000100000000 (ffff880000000000)
LOAD 0x0000001fcda22000 0xffff882080000000 0x0000002080000000 (ffff880000000000)
LOAD 0x0000003fcd9a2000 0xffff884080000000 0x0000004080000000 (ffff880000000000)
LOAD 0x0000005fcd922000 0xffff886080000000 0x0000006080000000 (ffff880000000000)
LOAD 0x0000007fcd8a2000 0xffff888080000000 0x0000008080000000 (ffff880000000000)
LOAD 0x0000009fcd822000 0xffff88a080000000 0x000000a080000000 (ffff880000000000)
LOAD 0x000000bfcd7a2000 0xffff88c080000000 0x000000c080000000 (ffff880000000000)
LOAD 0x000000dfcd722000 0xffff88e080000000 0x000000e080000000 (ffff880000000000)
LOAD 0x000000fc4d722000 0xffff88fe00000000 0x000000fe00000000 (ffff880000000000)

(Columns are File offset, Virtual Address, Physical Address and computed offset).

I would expect the offset between the virtual and the physical address to be PAGE_OFFSET, which is 0xffff88800000000 on x86_64, not 0xffff880000000000. Unlike /proc/vmcore, /proc/kcore shows the same physical memory (of the last memory section above) with a correct offset:

buczek@kreios:/mnt$ sudo readelf -a /proc/kcore | grep 0x000000fe00000000 | perl -lane 'printf "%s (%016x)\n",$_,hex($F[2])-hex($F[3])'
LOAD 0x0000097e00004000 0xffff897e00000000 0x000000fe00000000 (ffff888000000000)

The failing address 0xffff89807ff77000 happens to be at the end of the last memory section. It is the mem_section array, which crash wants to load and which is visible in the running system:

buczek@kreios:/mnt$ sudo gdb vmlinux /proc/kcore
(gdb) print mem_section
$1 = (struct mem_section **) 0xffff89807ff77000
(gdb) print *mem_section
$2 = (struct mem_section *) 0xffff88a07f37b000
(gdb) print **mem_section
$3 = {section_mem_map = 18446719884453740551, pageblock_flags = 0xffff88a07f36f040}

I can read the same information from the crash dump, if I account for the 0x0000008000000000 error:

buczek@kreios:/mnt$ gdb vmlinux crash.vmcore
(gdb) print mem_section
$1 = (struct mem_section **) 0xffff89807ff77000
(gdb) print *mem_section
Cannot access memory at address 0xffff89807ff77000
(gdb) set $t=(struct mem_section **) ((char *)mem_section - 0x0000008000000000)
(gdb) print *$t
$2 = (struct mem_section *) 0xffff88a07f37b000
(gdb) set $s=(struct mem_section *)((char *)*$t - 0x0000008000000000 )
(gdb) print *$s
$3 = {section_mem_map = 18446719884453740551, pageblock_flags = 0xffff88a07f36f040}

In the above example, the running kernel, the crashed kernel and the crashkernel are all the same 4.19.57 compilation. But I've tried with several other versions ( crashkernel 4.4, running kernel from 4.0 to linux master) with the same result.

The machine in the above example has several numa nodes (this is why there are so many LOAD headers). But I've tried this with a small kvm virtual machine and got the same result.

buczek@kreios:/mnt/linux-4.19.57-286.x86_64/build$ grep RANDOMIZE_BASE .config
buczek@kreios:/mnt/linux-4.19.57-286.x86_64/build$ grep SPARSEMEM .config
buczek@kreios:/mnt/linux-4.19.57-286.x86_64/build$ grep PAGE_TABLE_ISOLATION .config

Any ideas?


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