[Crash-utility] [RFC]: questions about section headers in ELF format vmcores
Dave Anderson
anderson at redhat.com
Fri Jun 23 15:41:18 UTC 2017
----- Original Message -----
> Hi Dave,
>
> I've got some questions for you if you don't mind. I've got a vmcore created
> from a VMWare vmss file (I think using vmss2core) and crash just does this
> when I run it on it:
>
> $ crash -d5 vmlinux vmss.core
>
> crash64 7.1.9
> Copyright (C) 2002-2016 Red Hat, Inc.
> Copyright (C) 2004, 2005, 2006, 2010 IBM Corporation
> Copyright (C) 1999-2006 Hewlett-Packard Co
> Copyright (C) 2005, 2006, 2011, 2012 Fujitsu Limited
> Copyright (C) 2006, 2007 VA Linux Systems Japan K.K.
> Copyright (C) 2005, 2011 NEC Corporation
> Copyright (C) 1999, 2002, 2007 Silicon Graphics, Inc.
> Copyright (C) 1999, 2000, 2001, 2002 Mission Critical Linux, Inc.
> This program is free software, covered by the GNU General Public License,
> and you are welcome to change it and/or distribute copies of it under
> certain conditions. Enter "help copying" to see the conditions.
> This program has absolutely no warranty. Enter "help warranty" for details.
>
>
> crash64: Elf64_Phdr pointer: 1875020 ELF header end: 1874fe4
>
> I spent a long time running gdb on crash and found that it makes some
> assumptions about the layout of ELF files that can't really be relied upon
> (Program headers have a fixed offset) and apparently no section headers. For
> the vmss.core file I have crash works out that the header size of the ELF
> and program headers is 4 bytes (which is definitely not correct).
>
> I made some changes (see below) and can get it to get this far (some of the
> later information is definitely not correct and I've got to make changes to
> is_netdump since it has a fair number of issues as well):
>
> $ crash-7.1.9/crash -d5 vmlinux vmss.core
>
> crash 7.1.9
> Copyright (C) 2002-2016 Red Hat, Inc.
> Copyright (C) 2004, 2005, 2006, 2010 IBM Corporation
> Copyright (C) 1999-2006 Hewlett-Packard Co
> Copyright (C) 2005, 2006, 2011, 2012 Fujitsu Limited
> Copyright (C) 2006, 2007 VA Linux Systems Japan K.K.
> Copyright (C) 2005, 2011 NEC Corporation
> Copyright (C) 1999, 2002, 2007 Silicon Graphics, Inc.
> Copyright (C) 1999, 2000, 2001, 2002 Mission Critical Linux, Inc.
> This program is free software, covered by the GNU General Public License,
> and you are welcome to change it and/or distribute copies of it under
> certain conditions. Enter "help copying" to see the conditions.
> This program has absolutely no warranty. Enter "help warranty" for details.
>
> vmcore_data:
> flags: c0 (KDUMP_LOCAL|KDUMP_ELF64)
> ndfd: 3
> ofp: 3b0458f040
> header_size: 352
> num_pt_load_segments: 3
> pt_load_segment[0]:
> file_offset: 4
> phys_start: 0
> phys_end: 0
> zero_fill: 218
> pt_load_segment[1]:
> file_offset: 700000001
> phys_start: fff80000
> phys_end: 1fff00000
> zero_fill: 100000000
> pt_load_segment[2]:
> file_offset: 700000001
> phys_start: 0
> phys_end: 0
> zero_fill: c0000000
> elf_header: 1376130
> elf32: 0
> notes32: 0
> load32: 0
> elf64: 1376130
> notes64: 1376170
> load64: 13761a8
> nt_prstatus: 0
> nt_prpsinfo: 0
> nt_taskstruct: 0
> task_struct: 0
> page_size: 0
> switch_stack: 0
> xen_kdump_data: (unused)
> num_prstatus_notes: 0
> num_qemu_notes: 0
> vmcoreinfo: 0
> size_vmcoreinfo: 0
> nt_prstatus_percpu:
> nt_qemu_percpu:
> backup_src_start: 0
> backup_src_size: 0
> backup_offset: 0
>
> Elf64_Ehdr:
> e_ident: \177ELF
> e_ident[EI_CLASS]: 2 (ELFCLASS64)
> e_ident[EI_DATA]: 1 (ELFDATA2LSB)
> e_ident[EI_VERSION]: 1 (EV_CURRENT)
> e_ident[EI_OSABI]: 0 (ELFOSABI_SYSV)
> e_ident[EI_ABIVERSION]: 0
> e_type: 4 (ET_CORE)
> e_machine: 62 (EM_X86_64)
> e_version: 1 (EV_CURRENT)
> e_entry: 0
> e_phoff: 80
> e_shoff: 40
> e_flags: 0
> e_ehsize: 40
> e_phentsize: 38
> e_phnum: 4
> e_shentsize: 40
> e_shnum: 1
> e_shstrndx: 0
> Elf64_Phdr:
> p_type: 0 (PT_NULL)
> p_offset: 0 (0)
> p_vaddr: 0
> p_paddr: 0
> p_filesz: 0 (0)
> p_memsz: 0 (0)
> p_flags: 0 ()
> p_align: 0
> Elf64_Phdr:
> p_type: 0 (PT_NULL)
> p_offset: 4 (4)
> p_vaddr: 160
> p_paddr: 0
> p_filesz: 0 (0)
> p_memsz: 536 (218)
> p_flags: 0 ()
> p_align: 0
> Elf64_Phdr:
> p_type: 0 (PT_NULL)
> p_offset: 30064771073 (700000001)
> p_vaddr: 4000001000
> p_paddr: fff80000
> p_filesz: 4294443008 (fff80000)
> p_memsz: 524288 (80000)
> p_flags: 0 ()
> p_align: 524288
> Elf64_Phdr:
> p_type: 1000 (?)
> p_offset: 30064771073 (700000001)
> p_vaddr: 1000
> p_paddr: 0
> p_filesz: 0 (0)
> p_memsz: 3221225472 (c0000000)
> p_flags: 0 ()
> p_align: 3221225472
> readmem: read_kdump()
> crash: pv_init_ops exists: ARCH_PVOPS
> gdb vmlinux
> GNU gdb (GDB) 7.6
> Copyright (C) 2013 Free Software Foundation, Inc.
> License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
> This is free software: you are free to change and redistribute it.
> There is NO WARRANTY, to the extent permitted by law. Type "show copying"
> and "show warranty" for details.
> This GDB was configured as "x86_64-unknown-linux-gnu"...
>
> <readmem: ffffffff8165e250, KVADDR, "cpu_possible_mask", 8, (FOE),
> 7fffd7e36838>
> <read_kdump: addr: ffffffff8165e250 paddr: 165e250 cnt: 8>
> <readmem: ffffffff8165e240, KVADDR, "cpu_present_mask", 8, (FOE),
> 7fffd7e36838>
> <read_kdump: addr: ffffffff8165e240 paddr: 165e240 cnt: 8>
> <readmem: ffffffff8165e248, KVADDR, "cpu_online_mask", 8, (FOE),
> 7fffd7e36838>
> <read_kdump: addr: ffffffff8165e248 paddr: 165e248 cnt: 8>
> <readmem: ffffffff8165e238, KVADDR, "cpu_active_mask", 8, (FOE),
> 7fffd7e36838>
> <read_kdump: addr: ffffffff8165e238 paddr: 165e238 cnt: 8>
> <readmem: ffffffff819658d8, KVADDR, "pv_init_ops", 8, (ROE), 7fffd7e47a38>
> <read_kdump: addr: ffffffff819658d8 paddr: 19658d8 cnt: 8>
> <readmem: ffffffff81db6e18, KVADDR, "timekeeper xtime_sec", 8, (ROE),
> 7fffd7e36838>
> <read_kdump: addr: ffffffff81db6e18 paddr: 1db6e18 cnt: 8>
> xtime timespec.tv_sec: 0: Wed Dec 31 18:00:00 1969
> <readmem: ffffffff81951284, KVADDR, "init_uts_ns", 390, (ROE), cfe2dc>
> <read_kdump: addr: ffffffff81951284 paddr: 1951284 cnt: 390>
> utsname:
> sysname:
> nodename:
> release:
> version:
> machine:
> domainname:
> base kernel version: 0.1.9
> <readmem: ffffffff8164d100, KVADDR, "accessible check", 8, (ROE|Q),
> 7fffd7e33ba8>
> <read_kdump: addr: ffffffff8164d100 paddr: 164d100 cnt: 8>
> <readmem: ffffffff8164d100, KVADDR, "read_string characters", 1499, (ROE|Q),
> 7fffd7e35f10>
> <read_kdump: addr: ffffffff8164d100 paddr: 164d100 cnt: 1499>
> WARNING: cannot read linux_banner string
> linux_banner:
>
> crash: vmlinux and vmss.core do not match!
>
> Usage:
>
> crash [OPTION]... NAMELIST MEMORY-IMAGE[@ADDRESS] (dumpfile form)
> crash [OPTION]... [NAMELIST] (live system form)
>
> Enter "crash -h" for details.
>
> Using something like readelf:
>
> $ readelf --file-header vmss.core
> ELF Header:
> Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
> Class: ELF64
> Data: 2's complement, little endian
> Version: 1 (current)
> OS/ABI: UNIX - System V
> ABI Version: 0
> Type: CORE (Core file)
> Machine: Advanced Micro Devices X86-64
> Version: 0x1
> Entry point address: 0x0
> Start of program headers: 128 (bytes into file)
> Start of section headers: 64 (bytes into file)
> Flags: 0x0
> Size of this header: 64 (bytes)
> Size of program headers: 56 (bytes)
> Number of program headers: 4
> Size of section headers: 64 (bytes)
> Number of section headers: 1
> Section header string table index: 0
>
> Shows that the file has one section header (resize_elf_header didn't seem to
> do anything with section headers) and 4 program headers:
>
> Program Headers:
> Type Offset VirtAddr PhysAddr
> FileSiz MemSiz Flags Align
> NOTE 0x0000000000000160 0x0000000000000000 0x0000000000000000
> 0x0000000000000218 0x0000000000000000 0
> LOAD 0x0000004000001000 0x00000000fff80000 0x00000000fff80000
> 0x0000000000080000 0x0000000000080000 RWE 1000
> LOAD 0x0000000000001000 0x0000000000000000 0x0000000000000000
> 0x00000000c0000000 0x00000000c0000000 RWE 1000
> LOAD 0x00000000c0001000 0x0000000100000000 0x0000000100000000
> 0x0000003f40000000 0x0000003f40000000 RWE 1000
>
> The changes I made (note that I only tested the ELF64 code). In the first
> calculation of the header_size below it seems to assume that there will
> always be an ELF header followed by program headers (one for the note
> program header and then e_phnum-1 load program headers) but doesn't take
> into account the potential existence of a section header:
>
> --- crash-7.1.9/netdump.c 2017-04-20 13:53:16.000000000 -0500
> +++ crash-7.1.9.new/netdump.c 2017-06-22 23:48:23.282639936 -0500
> @@ -455,16 +455,20 @@ resize_elf_header(int fd, char *file, ch
> {
> case NETDUMP_ELF32:
> case KDUMP_ELF32:
> - num_pt_load_segments = elf32->e_phnum - 1;
> - header_size = sizeof(Elf32_Ehdr) + sizeof(Elf32_Phdr) +
> - (sizeof(Elf32_Phdr) * num_pt_load_segments);
> + header_size = elf32->e_shoff +
> + elf32->e_shnum * elf32->e_shentsize;
> + if (elf32->e_phoff > elf32->e_shoff)
> + header_size = elf32->e_phoff +
> + elf32->e_phnum * elf32->e_phentsize;
> break;
>
> case NETDUMP_ELF64:
> case KDUMP_ELF64:
> - num_pt_load_segments = elf64->e_phnum - 1;
> - header_size = sizeof(Elf64_Ehdr) + sizeof(Elf64_Phdr) +
> - (sizeof(Elf64_Phdr) * num_pt_load_segments);
> + header_size = elf64->e_shoff +
> + elf64->e_shnum * elf64->e_shentsize;
> + if (elf64->e_phoff > elf64->e_shoff)
> + header_size = elf64->e_phoff +
> + elf64->e_phnum * elf64->e_phentsize;
> break;
> }
>
> @@ -494,26 +498,34 @@ resize_elf_header(int fd, char *file, ch
> {
> case NETDUMP_ELF32:
> case KDUMP_ELF32:
> - load32 = (Elf32_Phdr
> *)&eheader[sizeof(Elf32_Ehdr)+sizeof(Elf32_Phdr)];
> + load32 = (Elf32_Phdr *)&eheader[elf32->e_phoff];
> p_offset32 = load32->p_offset;
> - for (i = 0; i < num_pt_load_segments; i++, load32 += 1) {
> + for (i = 0; i < elf32->e_phnum; i++, load32 += 1) {
> if (load32->p_offset &&
> (p_offset32 > load32->p_offset))
> p_offset32 = load32->p_offset;
> }
> - header_size = (size_t)p_offset32;
> + if (header_size == p_offset32) {
> + return header_size;
> + } else {
> + header_size = (size_t)p_offset32;
> + }
> break;
>
> case NETDUMP_ELF64:
> case KDUMP_ELF64:
> - load64 = (Elf64_Phdr
> *)&eheader[sizeof(Elf64_Ehdr)+sizeof(Elf64_Phdr)];
> + load64 = (Elf64_Phdr *)&eheader[elf64->e_phoff];
> p_offset64 = load64->p_offset;
> - for (i = 0; i < num_pt_load_segments; i++, load64 += 1) {
> + for (i = 0; i < elf64->e_phnum; i++, load64 += 1) {
> if (load64->p_offset &&
> (p_offset64 > load64->p_offset))
> p_offset64 = load64->p_offset;
> }
> - header_size = (size_t)p_offset64;
> + if (header_size == p_offset64) {
> + return header_size;
> + } else {
> + header_size = (size_t)p_offset64;
> + }
> break;
> }
>
> This shouldn't be considered a patch (yet) but more of a request for comments
> so about if section headers are expected in an ELF64 vmcore and if I can
> more generally trust values like e_shoff, e_shnum, e_shentsize, e_phoff,
> e_phnum, and e_phentsize from the ELF header in all vmcores so I can change
> the code to cope with the potential presense of a section header? I'm sort
> of hoping that you will tell me I can rely on them being valid.
>
> Thanks
> Shane
I can't confidently tell you that those fields can be trusted.
I just did a quick test of your patch against a sampling of ~50
64-bit ELF dumpfiles that I've got hanging around, and all of them
generate the message:
WARNING: Elf64_Nhdr pointer: 130d2b8 ELF header end: 130d2b8
i.e., where the offsets are equal. And then the session either fails,
or there are other odd messages.
These ELF vmcores consisted of netdumps, kdumps (both regular and
flattened format), virsh dumps, simple ELF vmcores from the snap.so
extension module, and even the single vmss.core file I have on hand.
In fact the vmss.core file I have does not have any section headers:
$ readelf -a vmss.core
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: CORE (Core file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x0
Start of program headers: 64 (bytes into file)
Start of section headers: 0 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 56 (bytes)
Number of program headers: 4
Size of section headers: 0 (bytes)
Number of section headers: 0
Section header string table index: 0
There are no sections in this file.
There are no sections to group in this file.
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
NOTE 0x0000000000000120 0x0000000000000000 0x0000000000000000
0x0000000000000218 0x0000000000000000 0
LOAD 0x0000000100001000 0x00000000fff80000 0x00000000fff80000
0x0000000000080000 0x0000000000080000 RWE 1000
LOAD 0x0000000000001000 0x0000000000000000 0x0000000000000000
0x00000000c0000000 0x00000000c0000000 RWE 1000
LOAD 0x00000000c0001000 0x0000000100000000 0x0000000100000000
0x0000000040000000 0x0000000040000000 RWE 1000
There is no dynamic section in this file.
There are no relocations in this file.
The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.
Dynamic symbol information is not available for displaying symbols.
No version information found in this file.
Displaying notes found at file offset 0x00000120 with length 0x00000218:
Owner Data size Description
CORE 0x00000150 NT_PRSTATUS (prstatus structure)
CORE 0x00000088 NT_PRPSINFO (prpsinfo structure)
CORE 0x00000010 NT_TASKSTRUCT (task structure)
What's in the new section anyway?
In any case, in order to absolutely prevent any backwards-compatibility
issues, I would be hesitant to make changes unless you can segregate your
code changes such that they only execute if the elf->e_shnum member is non-zero.
Thanks,
Dave
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