llvm dev - Jan 2021 - (LLD / lto ) How to avoid GOT code generation

On Sat, Jan 9, 2021 at 12:03 PM Moshtaghi, Alireza via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hi
>
> I’m generating a special shared object in x86_64 large memory model
(-mcmodel=large).
>
> For this object, I pass -z notext to lld to leave the text relocations
alone and normally it builds and works with no problem.
>
> But when I add link time optimization, lld generates R_X86_64_GOTOFF64
which stops itself from linking. (Note that when I objdump -r on my elf objects
without lto, this relocation is not generated; but for some reason lld decides
to treat the non pic lto object as a pic object and generate got/pic code)
>
> How can I tell lld to not generate GOT / PIC code ?
>
>
>
> Here is the example:
>
> When I compile as follows, my shared lib is generated and works fine
>
> clang -mcmodel=large -o sample.o -c sample.c.  # -flto=thin fails
>
> ld.lld -Bshareable -z notext -o out.so sample.o
>
>
>
> but when I compile with -flto=thin, lld errors that R_X86_64_GOTOFF64 can’t
be used against foo and foo_call and wants me to compile with -fPIC but I don’t
want to use PIC and GOT
>
>
>
> sample.c :
>
> extern int foo;
>
> int* bar = &foo;
>
>
>
> int foo_call (int, int *);
>
>
>
> int foo_call (int a, int *b) {
>
>   return a+ *b;
>
> }
>
>
>
> int dummy (void) {
>
>   int *fooptr = &foo;
>
>   return foo_call (1, fooptr);
>
> }
clang -fpic -flto -mcmodel=large -o sample.o -c sample.c will work.

Your compile mode is -fno-pic (Clang default for Linux), which can
only be linked in -no-pie mode.
-fpie is compatible with -no-pie and -pie.
-fpic is compatible with -no-pie, -pie and -shared (-Bsharable).

-fno-pic + -shared can sometimes work, work in more cases with -z
notext, but still not always.

For this case: the formula for R_X86_64_GOTOFF64 is S+A-GOT where S
represents the symbol value.
Since foo is undefined, the relocation cannot be resolved at link
time. LLD does not produce a dynamic relocation
because R_X86_64_GOTOFF64 is not a generally acceptable dynamic
relocation type by ld.so implementations.

On Sat, Jan 9, 2021 at 10:45 PM Fāng-ruì Sòng <maskray at google.com>
wrote:
>
> On Sat, Jan 9, 2021 at 12:03 PM Moshtaghi, Alireza via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Hi
> >
> > I’m generating a special shared object in x86_64 large memory model
(-mcmodel=large).
> >
> > For this object, I pass -z notext to lld to leave the text relocations
alone and normally it builds and works with no problem.
> >
> > But when I add link time optimization, lld generates R_X86_64_GOTOFF64
which stops itself from linking. (Note that when I objdump -r on my elf objects
without lto, this relocation is not generated; but for some reason lld decides
to treat the non pic lto object as a pic object and generate got/pic code)
> >
> > How can I tell lld to not generate GOT / PIC code ?
> >
> >
> >
> > Here is the example:
> >
> > When I compile as follows, my shared lib is generated and works fine
> >
> > clang -mcmodel=large -o sample.o -c sample.c.  # -flto=thin fails
> >
> > ld.lld -Bshareable -z notext -o out.so sample.o
> >
> >
> >
> > but when I compile with -flto=thin, lld errors that R_X86_64_GOTOFF64
can’t be used against foo and foo_call and wants me to compile with -fPIC but I
don’t want to use PIC and GOT
> >
> >
> >
> > sample.c :
> >
> > extern int foo;
> >
> > int* bar = &foo;
> >
> >
> >
> > int foo_call (int, int *);
> >
> >
> >
> > int foo_call (int a, int *b) {
> >
> >   return a+ *b;
> >
> > }
> >
> >
> >
> > int dummy (void) {
> >
> >   int *fooptr = &foo;
> >
> >   return foo_call (1, fooptr);
> >
> > }
>
> clang -fpic -flto -mcmodel=large -o sample.o -c sample.c will work.
>
> Your compile mode is -fno-pic (Clang default for Linux), which can
> only be linked in -no-pie mode.
> -fpie is compatible with -no-pie and -pie.
> -fpic is compatible with -no-pie, -pie and -shared (-Bsharable).
>
> -fno-pic + -shared can sometimes work, work in more cases with -z
> notext, but still not always.
>
> For this case: the formula for R_X86_64_GOTOFF64 is S+A-GOT where S
> represents the symbol value.
> Since foo is undefined, the relocation cannot be resolved at link
> time. LLD does not produce a dynamic relocation
> because R_X86_64_GOTOFF64 is not a generally acceptable dynamic
> relocation type by ld.so implementations.
I'd say R_X86_64_GOTOFF64 is a direct access relocation type, not a
GOT indirection relocation type.

A GOT indirection relocation type should compute to a place in GOT
(e.g. G+A, G+GOT+A-P), but R_X86_64_GOTOFF64 (S+A-GOT) is different.
"GOT" refers to the address of _GLOBAL_OFFSET_TABLE_.

The large code model needs the GOT base ("GOT" in x86-64 psABI:
"Represents the address of the global offset table") as a fixed place
in the text segment to
compute the addresses of symbols. The GOT base is not used in the GOT
indirection manner.