llvm dev - Sep 2020 - [lld] [arm] Linker Cannot Set Custom Section Type to NOBITS

I am linking a program to be loaded in an ARM Cortex-M0+ based
microcontroller. In the linker script, I have a section allocated for the
stack which roughly looks like the following.
.stack : { . += __stack_size__; } > ram

Using the linker in the gcc arm toolchain, arm-none-eabi-ld, this section
is automatically set to type NOBITS, however, when linking with version
10.0.0 of ld.lld, the section type is PROGBITS.
Here is the output when using readelf.

*ld.lld*
Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk
Inf Al
  ...
  [ 5] .stack            PROGBITS        1ffff000 0014f0 000400 00   A  0
0  1

*arm-none-eabi-ld*
Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk
Inf Al
  ...
  [ 5] .stack            NOBITS          1ffff000 01f000 000400 00  WA  0
0  1

Reading online, I found that I should add the (NOLOAD) directive to my
linker script, so I tried the following.
.stack (NOLOAD) :  { . += __stack_size__; } > ram

But this change to the linker script does not seem to affect the output ELF
file. Interestingly, if I add the (NOLOAD) directive to the script's
.data section,
the .data section will change to type NOBITS.
I'd like to know if I'm making any errors, either in the section
definition
itself or when I call the linker.

Thank you,
Daniel Way
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Hello Daniel,
>From looking at the code I would expect LLD to give the .stack section type
SHT_NOBITS if (NOLOAD) is used. If (NOLOAD) is not used then it is difficult to
tell what the type of the section should be as it is likely to depend on the
type of the previous Output Section. It is possible that ld.bfd is inheriting
the type and LLD is not. I recommend being as explicit as possible in the linker
script and using (NOLOAD) if you can.
A change since LLD 10.0 https://reviews.llvm.org/D64930 may have made LLD closer
to ld.bfd in this area. If you are able to try out LLD 11.0 when it releases (or
build it from source) that would be great.

If all else fails it would be great if you could raise a PR with an example so
we can look into it. The lld --reproduce=<filename> can capture the
dependencies.

Peter

________________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Daniel
Way via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 01 September 2020 08:26
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] [lld] [arm] Linker Cannot Set Custom Section Type to       
NOBITS

I am linking a program to be loaded in an ARM Cortex-M0+ based microcontroller.
In the linker script, I have a section allocated for the stack which roughly
looks like the following.
.stack : { . += __stack_size__; } > ram

Using the linker in the gcc arm toolchain, arm-none-eabi-ld, this section is
automatically set to type NOBITS, however, when linking with version 10.0.0 of
ld.lld, the section type is PROGBITS.
Here is the output when using readelf.

ld.lld
Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  ...
  [ 5] .stack            PROGBITS        1ffff000 0014f0 000400 00   A  0   0  1

arm-none-eabi-ld
Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  ...
  [ 5] .stack            NOBITS          1ffff000 01f000 000400 00  WA  0   0  1

Reading online, I found that I should add the (NOLOAD) directive to my linker
script, so I tried the following.
.stack (NOLOAD) :  { . += __stack_size__; } > ram

But this change to the linker script does not seem to affect the output ELF
file. Interestingly, if I add the (NOLOAD) directive to the script's .data
section, the .data section will change to type NOBITS.
I'd like to know if I'm making any errors, either in the section
definition itself or when I call the linker.

Thank you,
Daniel Way

Hello Peter,

Thank you for your help. I finally had some time to build LLD from source
and confirmed that the (NOLOAD) directive does work properly in LLD 12.0.

I wanted to find out where this was fixed so I tried some different
versions of LLD. I pulled versions 10.1 and 11.0 from llvm-project/releases
and found that the issue is solved in both: the .stack section is shown as
NOBITS in the elf file.

Best regards,
Daniel Way


On Tue, Sep 1, 2020 at 7:32 PM Peter Smith <Peter.Smith at arm.com> wrote:
> Hello Daniel,
>
> From looking at the code I would expect LLD to give the .stack section
> type SHT_NOBITS if (NOLOAD) is used. If (NOLOAD) is not used then it is
> difficult to tell what the type of the section should be as it is likely to
> depend on the type of the previous Output Section. It is possible that
> ld.bfd is inheriting the type and LLD is not. I recommend being as explicit
> as possible in the linker script and using (NOLOAD) if you can.
>
> A change since LLD 10.0 https://reviews.llvm.org/D64930 may have made LLD
> closer to ld.bfd in this area. If you are able to try out LLD 11.0 when it
> releases (or build it from source) that would be great.
>
> If all else fails it would be great if you could raise a PR with an
> example so we can look into it. The lld --reproduce=<filename> can
capture
> the dependencies.
>
> Peter
>
> ________________________________________
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of
Daniel Way
> via llvm-dev <llvm-dev at lists.llvm.org>
> Sent: 01 September 2020 08:26
> To: llvm-dev at lists.llvm.org
> Subject: [llvm-dev] [lld] [arm] Linker Cannot Set Custom Section Type to
>       NOBITS
>
> I am linking a program to be loaded in an ARM Cortex-M0+ based
> microcontroller. In the linker script, I have a section allocated for the
> stack which roughly looks like the following.
> .stack : { . += __stack_size__; } > ram
>
> Using the linker in the gcc arm toolchain, arm-none-eabi-ld, this section
> is automatically set to type NOBITS, however, when linking with version
> 10.0.0 of ld.lld, the section type is PROGBITS.
> Here is the output when using readelf.
>
> ld.lld
> Section Headers:
>   [Nr] Name              Type            Addr     Off    Size   ES Flg Lk
> Inf Al
>   ...
>   [ 5] .stack            PROGBITS        1ffff000 0014f0 000400 00   A  0
>  0  1
>
> arm-none-eabi-ld
> Section Headers:
>   [Nr] Name              Type            Addr     Off    Size   ES Flg Lk
> Inf Al
>   ...
>   [ 5] .stack            NOBITS          1ffff000 01f000 000400 00  WA  0
>  0  1
>
> Reading online, I found that I should add the (NOLOAD) directive to my
> linker script, so I tried the following.
> .stack (NOLOAD) :  { . += __stack_size__; } > ram
>
> But this change to the linker script does not seem to affect the output
> ELF file. Interestingly, if I add the (NOLOAD) directive to the
script's
> .data section, the .data section will change to type NOBITS.
> I'd like to know if I'm making any errors, either in the section
> definition itself or when I call the linker.
>
> Thank you,
> Daniel Way
>
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