Re: [PATCH v3 2/9] vmlinux.lds.h: Add .symtab, .strtab, and .shstrtab to STABS_DEBUG

[Fangrui Song]

On 2020-06-24, Arvind Sankar wrote:
> On Wed, Jun 24, 2020 at 09:16:43AM -0700, Fangrui Song wrote:
> > On 2020-06-24, Arvind Sankar wrote:
> > >On Tue, Jun 23, 2020 at 06:49:33PM -0700, Kees Cook wrote:
> > >> When linking vmlinux with LLD, the synthetic sections .symtab, .strtab,
> > >> and .shstrtab are listed as orphaned. Add them to the STABS_DEBUG section
> > >> so there will be no warnings when --orphan-handling=warn is used more
> > >> widely. (They are added above comment as it is the more common
> > >
> > >Nit 1: is "after .comment" better than "above comment"? It's above in the
> > >sense of higher file offset, but it's below in readelf output.
> >
> > I mean this order:)
> >
> >    .comment
> >    .symtab
> >    .shstrtab
> >    .strtab
> >
> > This is the case in the absence of a linker script if at least one object file has .comment (mostly for GCC/clang version information) or the linker is LLD which adds a .comment
> >
> > >Nit 2: These aren't actually debugging sections, no? Is it better to add
> > >a new macro for it, and is there any plan to stop LLD from warning about
> > >them?
> >
> > https://reviews.llvm.org/D75149 "[ELF] --orphan-handling=: don't warn/error for unused synthesized sections"
> > described that .symtab .shstrtab .strtab are different in GNU ld.
> > Since many other GNU ld synthesized sections (.rela.dyn .plt ...) can be renamed or dropped
> > via output section descriptions, I don't understand why the 3 sections
> > can't be customized.
>
> So IIUC, lld will now warn about .rela.dyn etc only if they're non-empty?

HEAD and future 11.0.0 will not warn about unused synthesized sections
like .rela.dyn

For most synthesized sections, empty = unused.

> > I created a feature request: https://sourceware.org/bugzilla/show_bug.cgi?id=26168
> > (If this is supported, it is a consistent behavior to warn for orphan
> > .symtab/.strtab/.shstrtab
> >
> > There may be 50% chance that the maintainer decides that "LLD diverges"
> > I would disagree: there is no fundamental problems with .symtab/.strtab/.shstrtab which make them special in output section descriptions or orphan handling.)
>
> .shstrtab is a little special in that it can't be discarded if the ELF
> file contains any sections at all. But yeah, there's no reason they
> can't be renamed or placed in a custom location in the file.

https://sourceware.org/pipermail/binutils/2020-March/000179.html
proposes -z nosectionheader. With this option, I believe .shstrtab is
not needed. /DISCARD/ : { *(.shstrtab) }  should achieve a similar effect.