On Thu, Mar 15, 2018 at 04:36:23PM -0700, Matthias Kaehlcke wrote:

Hi Greg,

The below patch recently landed in Linus' tree:

commit b21ebf2fb4cde1618915a97cc773e287ff49173e Author: H.J. Lu hjl.tools@gmail.com Date: Wed Feb 7 14:20:09 2018 -0800

x86: Treat R_X86_64_PLT32 as R_X86_64_PC32

On i386, there are 2 types of PLTs, PIC and non-PIC.  PIE and shared
objects must use PIC PLT.  To use PIC PLT, you need to load
_GLOBAL_OFFSET_TABLE_ into EBX first.  There is no need for that on
x86-64 since x86-64 uses PC-relative PLT.

On x86-64, for 32-bit PC-relative branches, we can generate PLT32
relocation, instead of PC32 relocation, which can also be used as
a marker for 32-bit PC-relative branches.  Linker can always reduce
PLT32 relocation to PC32 if function is defined locally.   Local
functions should use PC32 relocation.  As far as Linux kernel is
concerned, R_X86_64_PLT32 can be treated the same as R_X86_64_PC32
since Linux kernel doesn't use PLT.

R_X86_64_PLT32 for 32-bit PC-relative branches has been enabled in
binutils master branch which will become binutils 2.31.

[ hjl is working on having better documentation on this all, but a few
  more notes from him:

   "PLT32 relocation is used as marker for PC-relative branches. Because
    of EBX, it looks odd to generate PLT32 relocation on i386 when EBX
    doesn't have GOT.

    As for symbol resolution, PLT32 and PC32 relocations are almost
    interchangeable. But when linker sees PLT32 relocation against a
    protected symbol, it can resolved locally at link-time since it is
    used on a branch instruction. Linker can't do that for PC32
    relocation"

  but for the kernel use, the two are basically the same, and this
  commit gets things building and working with the current binutils
  master   - Linus ]

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

The commit message mentions that "R_X86_64_PLT32 for 32-bit PC-relative branches has been enabled in binutils master branch which will become binutils 2.31."

This probably means that builds of older kernels could fail with binutils >= 2.31 and that the patch should be included in stable.

FYI: for pre-v4.14 kernel you might also want to include the below patch. It isn't strictly needed, but is trivial and including it avoids a conflict.

commit eda9cec4c9a12208a6f69fbe68f72a6311d50032 Author: Josh Poimboeuf jpoimboe@redhat.com Date: Fri Nov 3 07:58:54 2017 -0500

x86/module: Detect and skip invalid relocations

Thanks for the info, both patches are now queued up.

greg k-h