llvm dev - Jan 2019 - Why -pie option force LLD to output shared obj file type, not executable?

Hello Rui,
I'm enabling the LLD in the Uefi firmware edk2 build. I meet a problem about
the -pie option and cannot output the executable type obj file correctly. I need
your advice.
The Uefi firmware executable binary is the position independent + small code
mode in 64bits. So we always add the options "-Wl,-pie -mcmodel=small"
in our clang build toolchain. These options work well with binutils LD, but
cannot work with LLD. I see the LLD uses the below code to force the output obj
file as shared obj type if link with -pie. Is there any way to let LLD output
executable type obj with -pie option? I'm OK if I have to link twice, first
output DYN and then covert to EXEC in some way. Could you give some link command
examples on it?

lld\ELF\Writer.cpp
static uint16_t getELFType() {
  if (Config->Pic)
    return ET_DYN;
  if (Config->Relocatable)
    return ET_REL;
  return ET_EXEC;
}


Thanks
Steven

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Hello Steven,

I'm not Rui, but I may be able to help. We recently ran into something
like this on AArch64 with pr39810 lld would not accept --pie and
--shared simultaneously.

In general even on ld.bfd use of the --pie linker option sets the file
type to ET_DYN, ld.bfd has a special case when the start of the text
section is non 0 (I think). This was described in
https://sourceware.org/ml/binutils/2013-12/msg00081.html . As far as I
know this was for non-position independent executables that had been
compiled -fPIE just to get the small code model.

In the AArch64 case the problem was that --shared was being used to
work around ld.bfd using ET_EXEC, unfortunately it seems like you need
the ET_EXEC behaviour? If so I don't think that there are any flags
that you can give to lld to change this.

My understanding is that the only difference in the file ld.bfd
generates is the e_type field in the ELF header. It would be trivial
to write a quick tool to change the field from ET_DYN to ET_EXEC.

Hope this is of some use.

Peter




On Wed, 23 Jan 2019 at 15:47, Shi, Steven via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hello Rui,
>
> I’m enabling the LLD in the Uefi firmware edk2 build. I meet a problem
about the -pie option and cannot output the executable type obj file correctly.
I need your advice.
>
> The Uefi firmware executable binary is the position independent + small
code mode in 64bits. So we always add the options “-Wl,-pie -mcmodel=small” in
our clang build toolchain. These options work well with binutils LD, but cannot
work with LLD. I see the LLD uses the below code to force the output obj file as
shared obj type if link with -pie. Is there any way to let LLD output executable
type obj with -pie option? I’m OK if I have to link twice, first output DYN and
then covert to EXEC in some way. Could you give some link command examples on
it?
>
>
>
> lld\ELF\Writer.cpp
>
> static uint16_t getELFType() {
>
>   if (Config->Pic)
>
>     return ET_DYN;
>
>   if (Config->Relocatable)
>
>     return ET_REL;
>
>   return ET_EXEC;
>
> }
>
>
>
>
>
> Thanks
>
> Steven
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

lld sets ET_DYN instead of ET_EXEC if `-pic` is specified, which is
basically the same behavior except the case that Peter explained.
Unfortunately, there's no way to set ET_EXEC using lld, but the
"type"
field is 2 byte long and at offset 16, so you can edit it using a binary
editor to change it from ET_DYN to ET_EXEC (although it's super hacky).

If you have GNU sed, you can do with the following command to change the
field value:

sed -E -i -e '1s/^(.{16})../\1\x2\x0/' your-executable-file

Does that work for you?

On Wed, Jan 23, 2019 at 7:47 AM Shi, Steven <steven.shi at intel.com>
wrote:
> Hello Rui,
>
> I’m enabling the LLD in the Uefi firmware edk2 build. I meet a problem
> about the -pie option and cannot output the executable type obj file
> correctly. I need your advice.
>
> The Uefi firmware executable binary is the position independent + small
> code mode in 64bits. So we always add the options “-Wl,-pie -mcmodel=small”
> in our clang build toolchain. These options work well with binutils LD, but
> cannot work with LLD. I see the LLD uses the below code to force the output
> obj file as shared obj type if link with -pie. Is there any way to let LLD
> output executable type obj with -pie option? I’m OK if I have to link
> twice, first output DYN and then covert to EXEC in some way. Could you give
> some link command examples on it?
>
>
>
> lld\ELF\Writer.cpp
>
> static uint16_t getELFType() {
>
>   if (Config->Pic)
>
>     return ET_DYN;
>
>   if (Config->Relocatable)
>
>     return ET_REL;
>
>   return ET_EXEC;
>
> }
>
>
>
>
>
> Thanks
>
> Steven
>
>
>
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Hi Peter, Rui
Thank you for the info.


Thanks
Steven

From: Rui Ueyama [mailto:ruiu at google.com]
Sent: Thursday, January 24, 2019 3:02 AM
To: Shi, Steven <steven.shi at intel.com>
Cc: llvm-dev at lists.llvm.org
Subject: Re: Why -pie option force LLD to output shared obj file type, not
executable?

lld sets ET_DYN instead of ET_EXEC if `-pic` is specified, which is basically
the same behavior except the case that Peter explained. Unfortunately,
there's no way to set ET_EXEC using lld, but the "type" field is 2
byte long and at offset 16, so you can edit it using a binary editor to change
it from ET_DYN to ET_EXEC (although it's super hacky).

If you have GNU sed, you can do with the following command to change the field
value:

sed -E -i -e '1s/^(.{16})../\1\x2\x0/' your-executable-file

Does that work for you?

On Wed, Jan 23, 2019 at 7:47 AM Shi, Steven <steven.shi at
intel.com<mailto:steven.shi at intel.com>> wrote:
Hello Rui,
I’m enabling the LLD in the Uefi firmware edk2 build. I meet a problem about the
-pie option and cannot output the executable type obj file correctly. I need
your advice.
The Uefi firmware executable binary is the position independent + small code
mode in 64bits. So we always add the options “-Wl,-pie -mcmodel=small” in our
clang build toolchain. These options work well with binutils LD, but cannot work
with LLD. I see the LLD uses the below code to force the output obj file as
shared obj type if link with -pie. Is there any way to let LLD output executable
type obj with -pie option? I’m OK if I have to link twice, first output DYN and
then covert to EXEC in some way. Could you give some link command examples on
it?

lld\ELF\Writer.cpp
static uint16_t getELFType() {
  if (Config->Pic)
    return ET_DYN;
  if (Config->Relocatable)
    return ET_REL;
  return ET_EXEC;
}


Thanks
Steven

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