[lld] How do I prevent .note sections from being eliminated?

Project Infrastructure LLVM Dev List Archives
#1

Hi,

I'm working on an AMDGPU target for lld, and I'm trying to link a single
.o file with a .note section, but the .note section is missing from the
resulting a.out file. How can I control which sections get copied over
to the executable? Is there a target hook to implement?

Thanks,
Tom

#2

Hi,

If you need to control content of output .note section, take a look at
ARMExidxSection, MipsReginfoSection, MipsOptionsSection classes. If
you need to copy sections from input to output, use SDataSection (from
Hexagon) as a reference point.

#3

Hi,

If you need to control content of output .note section, take a look at
ARMExidxSection, MipsReginfoSection, MipsOptionsSection classes. If
you need to copy sections from input to output, use SDataSection (from
Hexagon) as a reference point.

Hi Simon,

Thanks for the reply. I have a few more follow up questions.

ELFFile::handleSectionWithNoSymbols() returns false for SHT_NOTE. This
means that ELFFile::createAtoms() never creates an atom for .note sections.
It seems like even if I use something like SDataSection, then I will still
need to modify ELFFIle::handleSectionWithNoSymbols(). What is the best
way to do this? Should I make the function virtual and override it in
AMDGPUELFFile.cpp?

In ELFDefinedAtom::doContentType(), SHT_NOTE sections are only assigned
the typeR[OW]Note ContentType if they have the SHF_ALLOC flag. This is
because any section without SHF_ALLOC is assigned the typeNoAlloc
ContentType before the section type is ever considered. Is this a bug
or are SHT_NOTE sections required to have the SHF_ALLOC flag?

Thanks,
Tom

#4

ELFFile::handleSectionWithNoSymbols() returns false for SHT_NOTE. This
means that ELFFile::createAtoms() never creates an atom for .note sections.
It seems like even if I use something like SDataSection, then I will still
need to modify ELFFIle::handleSectionWithNoSymbols(). What is the best
way to do this? Should I make the function virtual and override it in
AMDGPUELFFile.cpp?

Yes, you can make handleSectionWithNoSymbols() virtual and override it
in ELFFile class descendant.

In ELFDefinedAtom::doContentType(), SHT_NOTE sections are only assigned
the typeR[OW]Note ContentType if they have the SHF_ALLOC flag. This is
because any section without SHF_ALLOC is assigned the typeNoAlloc
ContentType before the section type is ever considered. Is this a bug
or are SHT_NOTE sections required to have the SHF_ALLOC flag?

It depends on your ABI requirement. If a .note section does not need
to occupy memory during a process execution it should not have
the SHF_ALLOC flag.

#5

> ELFFile::handleSectionWithNoSymbols() returns false for SHT_NOTE. This
> means that ELFFile::createAtoms() never creates an atom for .note sections.
> It seems like even if I use something like SDataSection, then I will still
> need to modify ELFFIle::handleSectionWithNoSymbols(). What is the best
> way to do this? Should I make the function virtual and override it in
> AMDGPUELFFile.cpp?

Yes, you can make handleSectionWithNoSymbols() virtual and override it
in ELFFile class descendant.

> In ELFDefinedAtom::doContentType(), SHT_NOTE sections are only assigned
> the typeR[OW]Note ContentType if they have the SHF_ALLOC flag. This is
> because any section without SHF_ALLOC is assigned the typeNoAlloc
> ContentType before the section type is ever considered. Is this a bug
> or are SHT_NOTE sections required to have the SHF_ALLOC flag?

It depends on your ABI requirement. If a .note section does not need
to occupy memory during a process execution it should not have
the SHF_ALLOC flag.

What I want to do is emit a .note section *without* SHF_ALLOC. However,
the current implementation won't let me do this, because it assigns the
section a content type of typeNoAlloc, rather than typeR[OW]Note.
I'm just wondering the best way to fix this.

-Tom

#6

Oh, now I see the point. The simplest way to fix the problem is to teach
the ELFDefinedAtom::doContentType() method to understand that there is
SHT_NOTE sections without SHF_ALLOC flag. It is enough to return typeRWNote
for such sections. But maybe a more accurate solution is to introduce
new ContentType like typeNoAllocNote.