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 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
> >> +       leal __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".

-- 
H.J.