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[PATCH 0/7] ELF: Don't require section header on ELF objects

H.J. Lu hjl.tools@gmail.com
Thu Mar 19 01:32:40 GMT 2020 
On Wed, Mar 18, 2020 at 5:48 PM Fangrui Song <i@maskray.me> wrote:
>
> >Section header isn't mandatory on ELF executable nor shared library.
> >This patch set adds a new linker option, -z nosectionheader, to omit
> >ELF section header when building an executable or shared library, an
> >objcopy and strip option, --remove-section-header, to remove ELF
> >section header from an executable or shared library.
> >
> >The PT_DYNAMIC segment contains DT_HASH/DT_GNU_HASH/DT_MIPS_XHASH,
> >DT_STRTAB, DT_SYMTAB, DT_STRSZ and DT_SYMENT entries, which can be
> >used to reconstruct dynamic symbol table when section header isn't
> >available.  For DT_HASH, the number of dynamic symbol table entries
> >equals the number of chains.  For DT_GNU_HASH/DT_MIPS_XHASH, only
> >defined symbols with non-STB_LOCAL indings are in hash table.  Since
> >in dynamic symbol table, all symbols with STB_LOCAL binding are placed
> >before symbols with other bindings and all undefined symbols are placed
> >before defined ones, the highest symbol index in DT_GNU_HASH and
> >DT_MIPS_XHASH is the highest dynamic symbol table index.
> >
> >
> >H.J. Lu (5):
> >  bfd: Reconstruct dynamic symbol table from PT_DYNAMIC segment
> >  readelf: Compute dynamic symbol table size from hash table
> >  binutils: Add --remove-section-header tests
> >  ld: Add tests for -z nosectionheader and --remove-section-header
> >  ld: Add -z nosectionheader test to bootstrap.exp
> >
> >Kaylee Blake (2):
> >  ELF: Don't require section header on ELF objects
> >  ld: Add a simple test for -z nosectionheader

> For objcopy --remove-section-header, we can probably implement an
> enhanced feature --strip-sections first. eu-strip and llvm-objcopy
> (since https://reviews.llvm.org/D38335) have --strip-sections.

I have renamed --remove-section-header to --strip-sections on
users/hjl/pr25617/master branch:

https://gitlab.com/x86-binutils/binutils-gdb/-/tree/users/hjl/pr25617/master

Kaylee, is your paper work with FSF in order? I will submit the updated
patch set after your paper is on file with FSF.

> --strip-sections strips the section header table and sections not
> covered by a segment. In llvm-objcopy, a non-SHF_ALLOC section can
> covered by a segment.

I think objcopy does it by default.

>
> For -z nosectionheader, I am questioning about the potential use case.
> As others have commented on the thread "ELF: Don't require section header on ELF objects",
> such shared objects should not be used for link. For runtime only use
> cases, perhaps we can do that with a post-processing tool.

-z nosectionheader works on both executable and shared library.   The
use case for shared library can be dlopen-only shared library.

>
> For readelf dumping of PT_DYNAMIC and DT_GNU_HASH, I think they are useful features.

I submitted  a separate readelf patch:

https://sourceware.org/pipermail/binutils/2020-March/110211.html

Thanks.

-- 
H.J.
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