Add patch "x86: Treat R_X86_64_PLT32 as R_X86_64_PC32" to stable?

Add patch "x86: Treat R_X86_64_PLT32 as R_X86_64_PC32" to stable?

[Matthias Kaehlcke]

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

Matthias

Re: Add patch "x86: Treat R_X86_64_PLT32 as R_X86_64_PC32" to stable?

[Greg Kroah-Hartman]

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