[PATCH] i386: Issue an error for baseless @GOT relocation with relaxation

[H.J. Lu]

On Wed, Nov 5, 2025 at 11:31 PM Rainer Orth <ro@cebitec.uni-bielefeld.de> wrote:
>
> Hi Jan,
>
> > On 27.10.2025 11:09, H.J. Lu wrote:
> >> On Mon, Oct 27, 2025 at 5:52 PM Jan Beulich <jbeulich@suse.com> wrote:
> >>>
> >>> On 27.10.2025 10:31, H.J. Lu wrote:
> >>>> On Mon, Oct 27, 2025 at 4:41 PM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>>
> >>>>> On 26.10.2025 00:28, H.J. Lu wrote:
> >>>>>> When -mrelax-relocations=no is used, don't generate baseless R_386_GOT32X
> >>>>>> relocation.  Issue an error instead.
> >>>>>>
> >>>>>> PR gas/33552
> >>>>>> * config/tc-i386.c (output_disp): Issue an error for baseless @GOT
> >>>>>> relocation without relaxation in 32-bit.
> >>>>>> * testsuite/gas/i386/got-no-relax-error.l: New file.
> >>>>>> * testsuite/gas/i386/got-no-relax-error.s: Likewise.
> >>>>>> * testsuite/gas/i386/got-no-relax.d: Don't use got.s.  Remove
> >>>>>> R_386_GOT32X entries.
> >>>>>> * testsuite/gas/i386/i386.exp: Run got-no-relax-error.
> >>>>>>
> >>>>>
> >>>>> Please can you add a reference to the part of the spec that puts in place
> >>>>> this restriction?
> >>>>
> >>>> It is more of an ISA restriction.  In i386 psABI, there are 2 kinds of PLTs,
> >>>> one with EBX for PIC and the other without EBX for non-PIC.  The baseless
> >>>> @GOT reloc can only be used with non-PIC PLT.
> >>>
> >>> But the two PLT kinds must be specified somewhere. As must be the relocation
> >>
> >> It is in i386 psABI.   They are implemented in elf32-i386.c.
> >>
> >>> types. Furthermore, how does the PLT come into play here when we're talking
> >>> of a GOT reloc? R_386-GOT32X is G + A - GOT / G + A, i.e. nothing PLT-ish in
> >>
> >> jmp *foo@GOT
> >>
> >> was added for -fno-plt.  For the same reason as non-PIC PLT, it only works
> >> without PIC.
> >>
> >>> there. The description in the psABI also clearly mentions forms with and
> >>> without base register.
> >>
> >> It is covered in:
> >>
> >> https://gitlab.com/x86-psABIs/i386-ABI
> >
> > Hmm, yes, I see the text there was extended at some point. Having a single
> > reloc with two different calculations is, however, a no-go. Even more so with
> > an arch like x86, where you can't reliably move backwards in the instruction
> > byte stream.
>
> apparently this patch has never been tested either natively on Solaris
> or with --enable-x86-relax-relocations=no, the only configurations where
> it makes a noticeable difference.  When doing one finds that it causes
> quite a number of testsuite regressions:

I withdrew my patch.  Please see

https://sourceware.org/bugzilla/show_bug.cgi?id=33552

for why R_386_GOT32X was generated for

jmp *_start@GOT

> * On both amd64-pc-solaris2.11 and i386-pc-solaris2.11, there's
>
>   FAIL: i386 general
>   FAIL: i386 intel (AT&T disassembly)
>   FAIL: i386 intel
>   FAIL: i386 intel-ok (directive)
>   FAIL: i386 intel-ok (cmdline option)
>   FAIL: i386 jump
>   FAIL: i386 quoted symbols
>   FAIL: gas/i386/align-branch-2a
>   FAIL: gas/i386/align-branch-2b
>   FAIL: gas/i386/align-branch-2c
>   FAIL: -mlfence-before-indirect-branch=all
>   FAIL: -mlfence-before-indirect-branch=register
>   FAIL: -mlfence-before-indirect-branch=memory
>   FAIL: i386 gotpc
>   FAIL: i386 non-pic tls
>   FAIL: Check tls relocation 32 bit-mode
>   FAIL: i386 inval-tls
>   FAIL: i386 intel-got
>
>   All fail with the new error.
>
> * On i386-pc-solaris2.11, there are also
>
>   FAIL: Run pr2404
>   FAIL: Run pr2404n
>   FAIL: Run pr33409
>   FAIL: --gc-sections with KEEP
>
>   all in generic code.
>
> Even worse, I ran a gcc trunk bootstraps on i386-pc-solaris2.11 with the
> patched gas and both Solaris ld and GNU ld.  This breaks bootstrap,
> unfortunately: compiling the PIC version of libgomp/barrier.c causes gas
> to abort:
>
> /var/tmp//cc.rNzya.s: Assembler messages:
> /var/tmp//cc.rNzya.s:33: Internal error in output_disp at /vol/src/gnu/binutils/hg/master/local/gas/config/tc-i386.c:13092.
> Please report this bug.
>
> I believe there's a far easier way to address this issue: when one
> simply doesn't emit R_386_GOT32X in tc-i386.c (output_disp) without
> generate_relax_relocations, gas behaves the same way as before gas 2.26
> when R_386_GOT32X support was introduced, namely only ever emitting
> R_386_GOT32 instead.  The only other change to gas is to change
> gas/testsuite/gas/i386/got-no-relax.d not to expect R_386_GOT32X any
> longer.  If one removes the parts that rely on -march=+mpx (which isn't
> supported either before gas 2.26 and with Solaris as) and the part
> requiring Intel syntax support (which Solaris as lackes), the source for
> that test (i386/got.s) can easily be assembled with both gas 2.25
> (before R_386_GOT32X was introduced) and Solaris as (which never knew
> about R_386_GOT32X).
>
> With this patch I get clean testsuite results on both Solaris/x86 and
> Linux/x86_64.  I've also bootstrapped gcc trunk successfully with the
> patched gas and both Solaris ld and GNU ld.  Those builds didn't show
> any testsuite regressions.
>
> I believe such a change restores gas behaviour to what it was before gas
> 2.26.
>
> WDYT?
>
>         Rainer
>
>
> gas: Never emit R_386_GOT32X with -mrelax-relocations=no [PR33552]
>
> To avoid gas generating R_386_GOT32X in some cases even with
> -mrelax-relocations=no, this patch only emits that reloc if
> generate_relax_relocations is true, changing the gas/i386/got-no-relax.d
> testcase to match.
>
> Tested on {i386,amd64}-pc-solaris2.11 and {i686,x86_64}-pc-linux-gnu.
> Also with gcc trunk bootstraps on i386-pc-solaris2.11 with both Solaris
> ld and GNU ld.
>
> --
> -----------------------------------------------------------------------------
> Rainer Orth, Center for Biotechnology, Bielefeld University
>
>
> 2025-11-05  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>
>
>         gas:
>         PR gas/33552
>         * config/tc-i386.c (output_disp) [OBJ_ELF]: Don't emit
>         R_386_GOT32X unless generate_relax_relocations.
>         * testsuite/gas/i386/got-no-relax.d: Only expect R_386_GOT32.
>


-- 
H.J.