linux-kernel - Re: PATCH: Add --size-check=[error|warning]

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Message-ID: <20110314122539.GA27205@elte.hu>
Date:	Mon, 14 Mar 2011 13:25:39 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Alan Modra <amodra@...il.com>
Cc:	Jan Beulich <JBeulich@...ell.com>, "H.J. Lu" <hjl.tools@...il.com>,
	binutils@...rceware.org, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: PATCH: Add --size-check=[error|warning]

(resend, fixed the To line)

* Alan Modra <amodra@...il.com> wrote:

> On Mon, Mar 14, 2011 at 10:55:34AM +0100, Ingo Molnar wrote:
> > The thing is, it is absolutely, breath-takingy incompetent for
> 
> kernel developers to write such poor asm!  And not notice the error
> for 4 years! [...]

It is not 'poor asm'.

The 'bug' is just a slight assymetry in ENTRY()/END() debug-symbols sequences, with 
lots of assembly code between the ENTRY() and the END(). Here's an example:

         ENTRY(xen_do_hypervisor_callback)   # do_hypervisor_callback(struct *pt_regs)
           ...
         END(do_hypervisor_callback)

Human reviewers almost never catch such small mismatches, and binutils never even 
warned about it either - for over a decade.

Now kernel bisections are insta-broken on latest binutils, and there's nothing to do 
about it on the kernel side as during bisection all later fixes are unfolded. The 
fix itself i already applied - but my argument was not about that:

> [...] Oh, and the binutils developers to write such a poor assembler in the first 
> place.  ;-)
> 
> Seriously, you are complaining because something is fixed??

No, i reported this bug because the kernel build gets broken going back 130,000 
commits, breaking bisection and causing other damage - while issuing a warning 
message would achieve the same effect of warning the developer about the mismatch.

> > The correct solution is to turn it into a warning as me and others have suggested.
> 
> I disagree.  The whole world is not the linux kernel.  I think HJ is
> bending over backwards to even offer a switch that turns the error
> into a warning.

It's not about a switch at all - it's to not break builds by default. I.e. the 
default behavior should be to issue a warning and ignore the directive.

This is a very simple concept of compatibility: the build environment should always 
be very permissive - stuff that build fine before should be allowed to build.

Also, i hope you are not suggesting to break projects just because they are not 
important to you personally? The fix is exceedingly simple to do for the binutils 
project - and impossible to do for the kernel project (because during bisection - 
which is a very powerful debugging tool - older versions of the source get checked 
out).

Thanks,

	Ingo
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