elf: Add ELF_DYNAMIC_AFTER_RELOC to rewrite PLT

Add ELF_DYNAMIC_AFTER_RELOC to allow target specific processing after
relocation.

For x86-64, add

 #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.

With the r_addend field of the R_X86_64_JUMP_SLOT relocation set to the
memory offset of the indirect branch instruction.

Define ELF_DYNAMIC_AFTER_RELOC for x86-64 to rewrite the PLT section
with direct branch after relocation when the lazy binding is disabled.

PLT rewrite is disabled by default since SELinux may disallow modifying
code pages and ld.so can't detect it in all cases.  Use

$ export GLIBC_TUNABLES=glibc.cpu.plt_rewrite=1

to enable PLT rewrite with 32-bit direct jump at run-time or

$ export GLIBC_TUNABLES=glibc.cpu.plt_rewrite=2

to enable PLT rewrite with 32-bit direct jump and on APX processors with
64-bit absolute jump at run-time.

Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/scripts/glibcelf.py b/scripts/glibcelf.py
index c5e5dda48e..5f3813f326 100644
--- a/scripts/glibcelf.py
+++ b/scripts/glibcelf.py
@@ -439,6 +439,8 @@ class DtRISCV(Dt):
     """Supplemental DT_* constants for EM_RISCV."""
 class DtSPARC(Dt):
     """Supplemental DT_* constants for EM_SPARC."""
+class DtX86_64(Dt):
+    """Supplemental DT_* constants for EM_X86_64."""
 _dt_skip = '''
 DT_ENCODING DT_PROCNUM
 DT_ADDRRNGLO DT_ADDRRNGHI DT_ADDRNUM
@@ -451,6 +453,7 @@ DT_MIPS_NUM
 DT_PPC_NUM
 DT_PPC64_NUM
 DT_SPARC_NUM
+DT_X86_64_NUM
 '''.strip().split()
 _register_elf_h(DtAARCH64, prefix='DT_AARCH64_', skip=_dt_skip, parent=Dt)
 _register_elf_h(DtALPHA, prefix='DT_ALPHA_', skip=_dt_skip, parent=Dt)
@@ -461,6 +464,7 @@ _register_elf_h(DtPPC, prefix='DT_PPC_', skip=_dt_skip, parent=Dt)
 _register_elf_h(DtPPC64, prefix='DT_PPC64_', skip=_dt_skip, parent=Dt)
 _register_elf_h(DtRISCV, prefix='DT_RISCV_', skip=_dt_skip, parent=Dt)
 _register_elf_h(DtSPARC, prefix='DT_SPARC_', skip=_dt_skip, parent=Dt)
+_register_elf_h(DtX86_64, prefix='DT_X86_64_', skip=_dt_skip, parent=Dt)
 _register_elf_h(Dt, skip=_dt_skip, ranges=True)
 del _dt_skip