* [PATCH v2] i386: Avoid avoid rely on linker optimization to avoid relocation
@ 2022-11-17 18:13 Adhemerval Zanella
2022-11-17 19:54 ` H.J. Lu
0 siblings, 1 reply; 3+ messages in thread
From: Adhemerval Zanella @ 2022-11-17 18:13 UTC (permalink / raw)
To: libc-alpha, Fangrui Song, H . J . Lu
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..23e4f2b012 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@PLT
+#endif
END (_start)
/* To fulfill the System V/i386 ABI we need this symbol. Yuck, it's so
--
2.34.1
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH v2] i386: Avoid avoid rely on linker optimization to avoid relocation
2022-11-17 18:13 [PATCH v2] i386: Avoid avoid rely on linker optimization to avoid relocation Adhemerval Zanella
@ 2022-11-17 19:54 ` H.J. Lu
2022-11-17 20:53 ` Fangrui Song
0 siblings, 1 reply; 3+ messages in thread
From: H.J. Lu @ 2022-11-17 19:54 UTC (permalink / raw)
To: Adhemerval Zanella; +Cc: libc-alpha, Fangrui Song
On Thu, Nov 17, 2022 at 10:13 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..23e4f2b012 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@PLT
> +#endif
> END (_start)
>
> /* To fulfill the System V/i386 ABI we need this symbol. Yuck, it's so
> --
> 2.34.1
>
LGTM.
Thanks.
--
H.J.
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH v2] i386: Avoid avoid rely on linker optimization to avoid relocation
2022-11-17 19:54 ` H.J. Lu
@ 2022-11-17 20:53 ` Fangrui Song
0 siblings, 0 replies; 3+ messages in thread
From: Fangrui Song @ 2022-11-17 20:53 UTC (permalink / raw)
To: Adhemerval Zanella; +Cc: H.J. Lu, libc-alpha
On 2022-11-17, H.J. Lu wrote:
>On Thu, Nov 17, 2022 at 10:13 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..23e4f2b012 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@PLT
>> +#endif
>> END (_start)
>>
>> /* To fulfill the System V/i386 ABI we need this symbol. Yuck, it's so
>> --
>> 2.34.1
>>
>
>LGTM.
>
>Thanks.
LGTM, too.
Reviewed-by: Fangrui Song <maskray@google.com>
BTW: do you know when start.o/start.os is assembled without -DPIC?
I find that in both --enable-static-pie and --disable-static-pie modes,
both start.o and start.os use -DPIC. start.os uses an additional -DSHARED.
Is the non-`#ifdef __PIC__` path dead?
^ permalink raw reply [flat|nested] 3+ messages in thread
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2022-11-17 18:13 [PATCH v2] i386: Avoid avoid rely on linker optimization to avoid relocation Adhemerval Zanella
2022-11-17 19:54 ` H.J. Lu
2022-11-17 20:53 ` Fangrui Song
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