Re: [patch] ia64: Order of operations bug in PT_LOAD segment reader

Simon Horman
2008-10-23T00:01:11Z

On Wed, Oct 22, 2008 at 04:47:19PM -0700, Jay Lan wrote:
> Simon Horman wrote:
> > On Tue, Oct 21, 2008 at 09:52:27AM -0700, Jay Lan wrote:
> >> Simon Horman wrote:
> >>
> >> Hi Simon,
> >>
> >> Just got back from vacation. Sorry for late response.
> >>
> >>> This bug was discovered by Jay Lan and he also proposed this fix, however
> >>> thee is some discussion about what if any related changes should be made at
> >>> the same time.
> >>>
> >>> The bug comes about because the break statment was never executed because
> >>> the if clause would bever be true because the if clause will never be true
> >>> because & has higher precedence than !=.
> >>>
> >>> My position on this is that with the if logic fixed, as per this patch, the
> >>> break statment and the rest of the while() loop makes sense and should work
> >>> as intended.
> >>>
> >>> As I understand it, Jay's position is that the code should be simplified,
> >>> after all it never worked as intended.
> >>>
> >>> There is a related kernel bug that lead Jay to discover this problem.
> >>> The kernel bug has been resolved by Tony Luck and was
> >>> included in Linus's tree between 2.6.27-rc8 and 2.6.27-rc9 as
> >>> "[IA64] Put the space for cpu0 per-cpu area into .data section".
> >>>
> >>> Now that the kernel bug is out of the way, I am providing this patch to
> >>> continue discussion on what to do on the kexec-tools side of things. I do
> >>> not intend to apply this patch until there is some conclusion in the
> >>> discussion between Jay and myself.
> >> I think this patch is not right for two reasons:
> >> 1) The if-statement below has never proved the correctness of
> >> its intent because the 'break' statement never got executed
> >> due to a logic error.
> >> if (loaded_segments[loaded_segments_num].end !> >> (phdr->p_paddr & ~(ELF_PAGE_SIZE-1)))
> >> break;
> >> 2) With your patch in my testing, the kdump kernel boot hung
> >> even earlier in a PAL_CALL that was not returned to the kernel.
> >> I understand that my test case was based on a kernel without
> >> Tony's latest fix, but that was the only situation we can
> >> see the if-statement becomes true. I do not know any other
> >> way to make a memory gap happen. However, when it happens,
> >> your patch only makes kdump kenrel boot hang earlier.
> >>
> >> I still root for my patch because the kdump kernel would boot
> >> correctly even if a memory gap indeed happened. ;) However,
> >> if you do not feel comfortable with my patch, i think the best
> >> alternative is to take out the if-statement above completely.
> >
> > Point taken, just to clarify, this is the patch you would like merged?
>
> Hi Simon,
>
> Yes. This is the patch that i would like merged.

Thanks. I've applied the change.

> > From: Jay Lan <jlan@sgi.com>
> >
> > IA64: better calculate PT_LOAD segment size
> >
> > This patch combines consecutive PL_LOAD segments into one.
> > The end address of the last PL_LOAD segment, calculated by
> > adding p_memsz to p_paddr & rounded up to ELF_PAGE_SIZE,
> > will be the end address of this loaded_segments[] entry.
> >
> > This patch fixes the kdump kernel MCA problem caused by under-
> > allocation of memory and a "kdump broken on ALtix 350" problem
> > reported by Bernhard Walle.
> >
> > Simon, this patch replaces my previous patch I submitted on the
> > underallocation issue.

--
Simon Horman
VA Linux Systems Japan K.K., Sydney, Australia Satellite Office
H: www.vergenet.net/~horms/ W: www.valinux.co.jp/en