Hi Vivek,
Thanks for the quick reply!
On 10/26/06, Vivek Goyal <[email protected]> wrote:
On Thu, Oct 26, 2006 at 06:49:57PM +0900, Magnus Damm wrote:
> Kdump: Align 64-bit ELF crash notes correctly (x86_64, powerpc)
>
> The current ELF code aligns data to 32-bit addresses, regardless if ELFCLASS32
> or ELFCLASS64 is used. This works well for the 32-bit case, but for 64-bit
> notes we should (of course) align to 64-bit addresses. At least if we intend
> to follow the "ELF-64 Object File Format, Version 1.5 Draft 2, May 27, 1998".
>
> Unfortunately this change affects 3 pieces of code:
> - The regular Linux kernel: See x86_64 and powerpc changes below.
> - The "crash" kernel: Needs to align properly when merging notes, see below.
> - The utilities that read the vmcore files: Crash, GDB and so on.
>
Hi Magnus,
Interesting observation. Going through the ELF-64 Object File format,
version 1.5, it does look like that note data should be aligned to
8byte boundary for 64bit and not 4 byte boundary.
But given the fact that as of today, gdb, readelf parse the notes correctly,
then they are broken too and needs to be changed?
Well, yes, other tools needs to be updated as well.
I just quickly went through the code for readelf from binutils-2.16.1
and in readelf.c, process_corefile_note_segment() always treats
alignment as 4. Broken. The bfd code in the same package seems to be
correct in most 64-bit cases though, struct elf_size_info ->
log_file_align seems to be the value we are looking for. It is set to
3 for at least elf64-{alpha, hppa, mips, s390, sparc}. I've not been
able to figure out the status for ppc and x86_64.
I do however believe that no-one has been troubled by this so far
because we happen to have well formed data. It seems like the only
string that is passed to append_elf_note() in the kdump case is "CORE"
and it is 5 bytes including terminating zero.
The old code adds 3 bytes and divides by 4 to get 2 32-bit values.
With my fix we would add 7 bytes and divide by 8 to get 1 64-bit value.
In both cases we pad to a total of 8 bytes. So we seem to be lucky with "CORE".
This also means that my fix doesn't affect the format of the data in
the current case.
I just looked at process core dumper (binfmt_elf.c) and that too also
seems to be creating notes aligned at 4byte boundary (alignfile()).
Same seems to be the case of /proc/kcore ((storenote()). Notes seem to
be 4byte aligned.
So looks like, everywhere in kernel and tool chain we are still following
the assumption of notes being 4byte aligned even for 64bit.
I think if you are fixing it, then please fix it for /proc/kcore and
process core dumps too so that kernel exports a consistent image and then
tool chain folks can do the modifications.
Yes, that makes sense. I will have a look at it. I assume that
following the spec is what we want to do, but if someone disagrees
please shout _now_!
Thanks,
/ magnus
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
