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Re: as bug (was: Re: smp/up alternatives crash when CONFIG_HOTPLUG_CPU)
- From: Denis Vlasenko <vda at ilport dot com dot ua>
- To: Alan Modra <amodra at bigpond dot net dot au>
- Cc: binutils at sourceware dot org, Ingo Molnar <mingo at elte dot hu>, Linus Torvalds <torvalds at osdl dot org>, Gerd Hoffmann <kraxel at suse dot de>, Andrew Morton <akpm at osdl dot org>, linux-kernel at vger dot kernel dot org, Chuck Ebbert <76306 dot 1226 at compuserve dot com>, binutils at sources dot redhat dot com
- Date: Fri, 5 May 2006 16:13:24 +0300
- Subject: Re: as bug (was: Re: smp/up alternatives crash when CONFIG_HOTPLUG_CPU)
- References: <20060419094630.GA14800@elte.hu> <200605051145.54643.vda@ilport.com.ua> <20060505122029.GB11597@bubble.grove.modra.org>
On Friday 05 May 2006 15:20, Alan Modra wrote:
> On Fri, May 05, 2006 at 11:45:54AM +0300, Denis Vlasenko wrote:
> >
> > .section .smp_altinstr_replacement,"awx"
> > .section .sched.text,"ax",@progbits
> > call _spin_unlock #
> > 661:
> > 2: jle 2b #
> > 662:
> > .section .smp_altinstr_replacement,"awx"
> > .fill 662b-661b,1,0x42
>
> gas should give a better error message here, but really, gas shouldn't
> be expected to assemble this. In essence, you have forward references
> in that expression for the .fill length..
Yes, this should be an error, not warning. It produces miscompiled
object modules.
> Some background: Gas is a single pass assembler. It emits code and
> data into "frags", buffers containing some fixed number of bytes and
> possibly a variable length tail. The variable length part allows
> various features, notably that of variable length instructions. Symbols
> are defined relative to their frags. Until the frag addresses are
> finalized, an expresion involving subtraction of two symbols in
> different frags cannot be evaluated correctly. With the testcase above
> you have exactly that situation. The x86 "jle" instruction can be two
> sizes, either 6 bytes or 2 bytes depending on the offset needed, and gas
> doesn't have the smarts to recognize that the "jle" above is just 2
> bytes. Instead, it assumes a variable size, putting the "jle" in its
> own frag. This means that label "661" and "662" are in separate frags
> with "661" at offset 5 in its frag, and "662" at offset 0.
>
> Since you define the ".smp_altinstr_replacement" section before the
> ".sched.text section", gas tries to finalize ".smp_altinstr_replacement"
> first. When it tries to calculate the fill size using
> (<base addr "662" frag>+<offset "662">)
> - (<base addr "661" frag>+<offset "661">)
> the frag base addresses have not yet been set, and zero is used. ie.
> gas tries to assemble ".fill -5,1,0x42".
Thanks for the explanation.
--
vda