Re: [Xen-devel] [PATCH v3 07/16] x86/boot: create *.lnk files with linker script

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.06.16 at 11:06, <daniel.kiper@xxxxxxxxxx> wrote:
> On Tue, May 24, 2016 at 06:52:39AM -0600, Jan Beulich wrote:
>> >>> On 24.05.16 at 14:28, <daniel.kiper@xxxxxxxxxx> wrote:
>> > On Tue, May 24, 2016 at 03:05:06AM -0600, Jan Beulich wrote:
>> >> >>> On 15.04.16 at 14:33, <daniel.kiper@xxxxxxxxxx> wrote:
>> >> > +  /DISCARD/ : {
>> >> > +        /*
>> >> > +         * .got.plt section is used only by dynamic linker
>> >> > +         * and our output is not supposed to be loaded by
>> >> > +         * dynamic linker. Additionally, it just contains
>> >> > +         * .PLT0 which is referenced from nowhere. So, we
>> >> > +         * can safely drop .got.plt here.
>> >> > +         *
>> >> > +         * Ha! This should be really discarded here. However,
>> >> > +         * .got.plt section contains _GLOBAL_OFFSET_TABLE_
>> >> > +         * symbol too and it is used as a reference for relative
>> >> > +         * addressing (and only for that thing). Hence, ld
>> >> > +         * complains if we remove that section because it
>> >> > +         * cannot find _GLOBAL_OFFSET_TABLE_. So, drop .got.plt
>> >> > +         * section during conversion to plain binary format.
>> >> > +         * Please check build32.mk for more details.
>> >> > +         */
>> >> > +        /* *(.got.plt) */
>> >> > +  }
>> >>
>> >> I'm afraid this needs more investigation: Afaik there should be no
>> >
>> > I am not sure what else we should look for.
>>
>> The reason why such an empty .got.plt gets created in the first place.
>> If e.g. that turns out to be a bug in (some versions of) binutils, then
>> that bug should be named here as the reason.
> 
> If PIC/PIE code is build then .got.plt exists in executable even if it
> is not linked with dynamic libraries.

Well - then just don't force -fPIC or -fPIE for the compilation of this
code?

>> >> reason for the linker to create an otherwise empty .got.plt in the
>> >
>> > As I wrote above. It contains _GLOBAL_OFFSET_TABLE_ which is used
>> > as a reference for relative addressing.
>>
>> But we don't use any such, so without being needed I don't think
>> the symbol needs to be created.
> 
> R_386_GOTPC and R_386_GOTOFF relocations use address of 
> _GLOBAL_OFFSET_TABLE_
> as a reference. So, it is needed during linking phase. However, later it is
> not needed.  Hence, .got.plt with _GLOBAL_OFFSET_TABLE_ and .PLT0 can safely
> be dropped.

These two relocation types should not appear for non-PIC/PIE code.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel