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

[Adhemerval Zanella Netto via Libc-alpha <libc-alpha@sourceware.org>]

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.