[Crash-utility] [PATCH] filesys: Skip 'vmlinux.o' while searching for vmlinux in 'find_booted_kernel()'
Bhupesh Sharma
bhsharma at redhat.com
Wed Feb 28 19:34:07 UTC 2018
Hi Dave,
On Wed, Feb 28, 2018 at 8:48 PM, Dave Anderson <anderson at redhat.com> wrote:
> Hi Bhupesh,
>
> Thanks -- mystery solved!
>
> I kind of hate to simply use the file name string as the qualifier, though.
> Looking at the ELF headers of the vmlinux.o and vmlinux file, there are
> several notable differences:
>
> $ readelf -a vmlinux
> 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: EXEC (Executable file)
> Machine: AArch64
> Version: 0x1
> Entry point address: 0xffff000008080000
> Start of program headers: 64 (bytes into file)
> Start of section headers: 186677936 (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: 42
> Section header string table index: 41
> ...
>
>
> $ readelf -a vmlinux.o
> 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: REL (Relocatable file)
> Machine: AArch64
> Version: 0x1
> Entry point address: 0x0
> Start of program headers: 0 (bytes into file)
> Start of section headers: 478638376 (bytes into file)
> Flags: 0x0
> Size of this header: 64 (bytes)
> Size of program headers: 0 (bytes)
> Number of program headers: 0
> Size of section headers: 64 (bytes)
> Number of section headers: 34816
> Section header string table index: 34815
> ...
>
> There is the "Type", the "Entry point address", and perhaps most notably,
> the vmlinux.o's "Number of program headers:" count of 0, which obviously
> would preclude the file from being usable.
Indeed. I was loath to use the string 'vmlinux.o' directly also, but I
was worried if modifying the checks in ''find_booted_kernel()'' would
break existing systems ...
>
> I would prefer to beef up the currently-existing is_kernel() function,
> and use it instead of is_elf_file(). Let me take a look at what works.
... sure, let me know if I can help in any way with the same.
Regards,
Bhupesh
>
> ----- Original Message -----
>> I found this particular issue while compiling and installing
>> a kernel from source on a RHEL arm64 machine. I used the following
>> commands to build and install the kernel from source:
>>
>> # make olddefconfig
>> # make
>> # make modules_install INSTALL_MOD_STRIP=1 && make install
>>
>> This compiles and installs both 'vmlinux' and 'vmlinux.o' at
>> the following standard location - '/lib/modules/<kernel>/build/
>>
>> Now the problem can be reproduced using the following steps:
>> 1. Update grub to boot the freshly compiled kernel.
>> 2. Reboot machine.
>> 3. Now, run crash on live system, it errors out with the following
>> messages:
>>
>> crash: invalid kernel virtual address: 8470 type: "possible"
>> WARNING: cannot read cpu_possible_map
>> crash: invalid kernel virtual address: 8270 type: "present"
>> WARNING: cannot read cpu_present_map
>> crash: invalid kernel virtual address: 8070 type: "online"
>> WARNING: cannot read cpu_online_map
>> crash: invalid kernel virtual address: 8670 type: "active"
>> WARNING: cannot read cpu_active_map
>>
>> crash: cannot resolve "_stext"
>>
>> 4. Enabling some debug messages in 'find_booted_kernel()' function
>> tells us that it finds 'vmlinux.o' earlier than 'vmlinux' and accepts
>> that as the kernel being boot'ed:
>>
>> find_booted_kernel: check: /lib/modules/4.14.0/build/vmlinux.o
>> find_booted_kernel: found: /lib/modules/4.14.0/build/vmlinux.o
>>
>> 5. Now the problem happens due to following check inside
>> 'find_booted_kernel()' function:
>>
>> if (mount_point(kernel) ||
>> !file_readable(kernel) ||
>> !is_elf_file(kernel))
>> continue;
>>
>> 6. Since 'vmlinux.o' is a elf file, is readable and is not
>> mount'able, so the check in point 5 fails and we incorrectly
>> accept this as the kernel being boot'ed, even though
>> there was a 'vmlinux' present inside
>> '/lib/modules/<kernel>/build'.
>>
>> 7. Now, later when crash tries to access symbols (like _stext)
>> using this kernel symbol file, we see errors.
>>
>> Fix this by skipping 'vmlinux.o' from the check in point 5,
>> so that we can select 'vmlinux' correctly as the kernel file.
>>
>> After this fix, crash no longer errors out and we can use
>> full functionality on the crash prompt.
>>
>> Signed-off-by: Bhupesh Sharma <bhsharma at redhat.com>
>> ---
>> filesys.c | 14 +++++++++++++-
>> 1 file changed, 13 insertions(+), 1 deletion(-)
>>
>> diff --git a/filesys.c b/filesys.c
>> index 1b44ad5cefa8..9f240d6115a1 100644
>> --- a/filesys.c
>> +++ b/filesys.c
>> @@ -623,9 +623,21 @@ find_booted_kernel(void)
>>
>> sprintf(kernel, "%s%s", searchdirs[i], dp->d_name);
>>
>> + /* We may run into a special case with
>> + * 'vmlinux.o' been present inside
>> + * '/lib/modules/<kernel>/build/',
>> + * which has some special characteristics -
>> + * this is a elf file, is readable and is not
>> + * mount'able.
>> + *
>> + * So we need to handle this properly here to
>> + * make sure that we get to the real 'vmlinux'
>> + * rather than the 'vmlinux.o'
>> + */
>> if (mount_point(kernel) ||
>> !file_readable(kernel) ||
>> - !is_elf_file(kernel))
>> + !is_elf_file(kernel) ||
>> + !(strcmp(dp->d_name, "vmlinux.o")))
>> continue;
>>
>> if (CRASHDEBUG(1))
>> --
>> 2.7.4
>>
>>
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