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

[Daniel Way via llvm-dev]

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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