Re: [PATCH] i386: Avoid avoid rely on linker optimization to avoid relocation

["H.J. Lu via Libc-alpha" <libc-alpha@sourceware.org>]

On Thu, Nov 17, 2022 at 9:28 AM Adhemerval Zanella Netto
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 17/11/22 13:52, H.J. Lu wrote:
> > On Thu, Nov 17, 2022 at 8:42 AM Adhemerval Zanella Netto
> > <adhemerval.zanella@linaro.org> wrote:
> >>
> >>
> >>
> >> On 17/11/22 13:07, H.J. Lu wrote:
> >>> On Thu, Nov 17, 2022 at 4:43 AM Adhemerval Zanella
> >>> <adhemerval.zanella@linaro.org> wrote:
> >>>>
> >>>> lld does not implement all the linker optimization to avoid the GOT
> >>>> relocation as done by binutils (bfd/elf32-i386.c:elf_i386_convert_load_reloc).
> >>>> The current 'movl main@GOT(%ebx), %eax' will then create a GOT
> >>>> relocation when building with lld, which make static-pie status to
> >>>> not being able to start the provided main function.
> >>>>
> >>>> The change uses a __wrap_main local symbol, which in turn calls main
> >>>> (similar as used by aarch64 and s390x).
> >>>>
> >>>> Checked on i686-linux-gnu with binutils and lld.
> >>>> ---
> >>>>  sysdeps/i386/start.S | 13 +++++++++----
> >>>>  1 file changed, 9 insertions(+), 4 deletions(-)
> >>>>
> >>>> diff --git a/sysdeps/i386/start.S b/sysdeps/i386/start.S
> >>>> index 4ec04bdfd7..d593c4de00 100644
> >>>> --- a/sysdeps/i386/start.S
> >>>> +++ b/sysdeps/i386/start.S
> >>>> @@ -98,11 +98,10 @@ ENTRY (_start)
> >>>>         pushl main@GOT(%ebx)
> >>>>  # else
> >>>>         /* Avoid relocation in static PIE since _start is called before
> >>>> -          it is relocated.  Don't use "leal main@GOTOFF(%ebx), %eax"
> >>>> -          since main may be in a shared object.  Linker will convert
> >>>> -          "movl main@GOT(%ebx), %eax" to "leal main@GOTOFF(%ebx), %eax"
> >>>> +          it is relocated.  This also avoid rely on linker optimization to
> >>>> +          transform 'movl main@GOT(%ebx), %eax' to 'leal main@GOTOFF(%ebx)'
> >>>>            if main is defined locally.  */
> >>>> -       movl main@GOT(%ebx), %eax
> >>>> +       955774751b71c4bc94029dd541ad9d34634ec995 __wrap_main@GOTOFF(%ebx), %eax
> >>>>         pushl %eax
> >>>>  # endif
> >>>>
> >>>> @@ -130,6 +129,12 @@ ENTRY (_start)
> >>>>  1:     movl    (%esp), %ebx
> >>>>         ret
> >>>>  #endif
> >>>> +
> >>>> +#if defined PIC && !defined SHARED
> >>>> +__wrap_main:
> >>>> +       _CET_ENDBR
> >>>> +       jmp     main
> >>>
> >>> Shouldn't it be "jmp main@PLT"?
> >>
> >> My understanding is for static build we can handle main as hidden, so there
> >> is no need use a PLT relocation here.
> >
> > The original comments have
> >
> >   it is relocated.  Don't use "leal main@GOTOFF(%ebx), %eax"
> >   since main may be in a shared object.  Linker will convert
> >
> > If this is only used for static build, we can use "leal
> > main@GOTOFF(%ebx), %eax".
> >
>
> It does not work with binutils (and that's why you installed 955774751b71c4):
>
> $ make test t=elf/tst-main1
> [...]
> csu/crt1.o(.text+0x1e): unresolvable R_386_GOTOFF relocation against symbol `main'
> collect2: error: ld returned 1 exit status
>
> The start.o (PIC and !SHARED) is used for crt1.o.  With 'main' requirement, leal
> does work for building tst-main1, so maybe one option could to add a .global main
> to pull the symbol.

That is because it is also used for dynamic executables.  I think PLT
should be used.


-- 
H.J.