Re: [PATCH] x86-64: Add -z mark-plt and -z nomark-plt

[Jan Beulich <jbeulich@suse.com>]

On 26.09.2023 18:13, H.J. Lu via Binutils wrote:
> The PLT entry in executables and shared libraries contains an indirect
> branch, like
> 
>  	jmp *foo@GOTPCREL(%rip)
> 	push $index_foo
> 	jmp .PLT0
> 
> or
> 
> 	endbr64
>  	jmp *foo@GOTPCREL(%rip)
>  	NOP padding
> 
> which is used to branch to the function, foo, defined in another object.
> Each R_X86_64_JUMP_SLOT relocation has a corresponding PLT entry.
> 
> The dynamic tags have been added to the x86-64 psABI to mark such PLT
> entries:
> 
> https://gitlab.com/x86-psABIs/x86-64-ABI/-/commit/6d824a52a42d173eb838b879616c1be5870b593e
> 
> Add an x86-64 linker option, -z mark-plt, to mark PLT entries with
> 
>  #define DT_X86_64_PLT     (DT_LOPROC + 0)
>  #define DT_X86_64_PLTSZ   (DT_LOPROC + 1)
>  #define DT_X86_64_PLTENT  (DT_LOPROC + 3)
> 
> 1. DT_X86_64_PLT: The address of the procedure linkage table.
> 2. DT_X86_64_PLTSZ: The total size, in bytes, of the procedure linkage
> table.
> 3. DT_X86_64_PLTENT: The size, in bytes, of a procedure linkage table
> entry.
> 
> and set the r_addend field of the R_X86_64_JUMP_SLOT relocation to the
> memory offset of the indirect branch instruction.  The dynamic linker
> can use these tags to update the PLT section to direct branch.
> 
> bfd/
> 
> 	* elf-linker-x86.h (elf_linker_x86_params): Add mark_plt.
> 	* elf64-x86-64.c (elf_x86_64_finish_dynamic_symbol): Set the
> 	r_addend of R_X86_64_JUMP_SLOT to the indirect branch offset
> 	in PLT entry for -z mark-plt.
> 	* elfxx-x86.c (_bfd_x86_elf_size_dynamic_sections): Add
> 	DT_X86_64_PLT, DT_X86_64_PLTSZ and DT_X86_64_PLTENT for
> 	-z mark-plt.
> 	(_bfd_x86_elf_finish_dynamic_sections): Set DT_X86_64_PLT,
> 	DT_X86_64_PLTSZ and DT_X86_64_PLTENT.
> 	(_bfd_x86_elf_get_synthetic_symtab): Ignore addend for
> 	JUMP_SLOT relocation.
> 	(_bfd_x86_elf_link_setup_gnu_properties): Set
> 	plt_indirect_branch_offset.
> 	* elfxx-x86.h (elf_x86_plt_layout): Add plt_indirect_branch_offset.
> 
> binutils/
> 
> 	* readelf.c (get_x86_64_dynamic_type): New function.
> 	(get_dynamic_type): Call get_x86_64_dynamic_type.
> 
> include/
> 
> 	* elf/x86-64.h (DT_X86_64_PLT): New.
> 	(DT_X86_64_PLTSZ): Likewise.
> 	(DT_X86_64_PLTENT): Likewise.
> 
> ld/
> 	* emulparams/elf32_x86_64.sh: Source x86-64-plt.sh.
> 	* emulparams/elf_x86_64.sh: Likewise.
> 	* emulparams/x86-64-plt.sh: New file.
> 	* testsuite/ld-x86-64/mark-plt-1.s: Likewise.
> 	* testsuite/ld-x86-64/mark-plt-1a-x32.d: Likewise.
> 	* testsuite/ld-x86-64/mark-plt-1a.d: Likewise.
> 	* testsuite/ld-x86-64/mark-plt-1b-x32.d: Likewise.
> 	* testsuite/ld-x86-64/mark-plt-1b.d: Likewise.
> 	* testsuite/ld-x86-64/mark-plt-1c-x32.d: Likewise.
> 	* testsuite/ld-x86-64/mark-plt-1c.d: Likewise.
> 	* testsuite/ld-x86-64/mark-plt-1d-x32.d: Likewise.
> 	* testsuite/ld-x86-64/mark-plt-1d.d: Likewise.
> 	* testsuite/ld-x86-64/x86-64.exp: Run -z mark-plt tests.

Don't you also want to add text to ld.texi? There it would also be made
clear what the default setting is ("off" aiui). A related question would
be whether it would make sense to allow configure time selection of the
default.

Jan