linux-kernel - Re: [uClinux-dev] [PATCH] Valid relocation symbol for FLAT format on ARM, v2

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Message-ID: <20100104185834.GC19197@Pogoplug>
Date:	Mon, 4 Jan 2010 10:58:34 -0800
From:	Jun Sun <jsun@...sun.net>
To:	uClinux development list <uclinux-dev@...inux.org>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [uClinux-dev] [PATCH] Valid relocation symbol for FLAT format
	on ARM, v2

On Sat, Jan 02, 2010 at 11:09:07PM +0000, Jamie Lokier wrote:
> Jun Sun wrote:
> > +/* [jsun] new gcc 4.x generates ANCHOR symbols in order to reduce the size
> > + * of GOT table for PIC code. It is possible the ANCHOR is placed beyond
> > + * the end of data/bss segment up to 4K bytes(12 bits), because ARM allows
> > + * negative 12-bit offset. Thus we allow 0x1000 extra in reloc address range.
> > + */
> 
> Can you provide a small test program and *exact* GCC version and
> Binutils version which triggers this?  People have reported using GCC
> 4.x on ARM uClinux for some time, and I haven't seen complaints
> about this problem before.
>

gcc is derived from code saucery's 2008Q2 release, I believe. It is
gcc 4.3.3. So there is minor chance that this ANCHOR feature
is a code sourcery feature rather than a generic gcc 4.3.3. Please help
verify.

I attached the sample code and the assembly output. The command line used
to generate this is:

arm-uclinuxeabi-gcc  -c -Os                     -g -fomit-frame-pointer -pipe -msoft-float -fno-common -fno-builtin -Wall   -DEMBED -D__PIC__ -fpic -msingle-pic-base -Dlinux -D__linux__ -Dunix -D__uClinux__ -S helloworld.c

You can see from the assembly output that .LANCHOR1 is set to 8184, well
beyond the real end of bss/data segment. And it is easy to see why, because
variable x2 is then referred as a negative offset to LANCHOR1.

Cheers.

Jun

View attachment "helloworld.c" of type "text/plain" (279 bytes)

View attachment "helloworld.s" of type "text/plain" (7046 bytes)