Re: [PATCH] gold: Remove BND from 64-bit x86-64 IBT PLT

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Dec 6, 2022 at 1:44 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> Since MPX support has been removed from x86-64 psABI, remove BND from
> 64-bit IBT PLT by using 32-bit IBT PLT.
>
>         PR gold/29851
>         * x86_64.cc (Output_data_plt_x86_64_ibt<32>::first_plt_entry):
>         Renamed to ...
>         (Output_data_plt_x86_64_ibt<size>::first_plt_entry): This.
>         (Output_data_plt_x86_64_ibt<64>::first_plt_entry): Removed.
>         (Output_data_plt_x86_64_ibt<size>::do_fill_first_plt_entry):
>         Drop the size == 32 check.
>         (Output_data_plt_x86_64_ibt<32>::plt_entry): Renamed to ...
>         (Output_data_plt_x86_64_ibt<size>::plt_entry): This.
>         (Output_data_plt_x86_64_ibt<64>::plt_entry): Removed.
>         (Output_data_plt_x86_64_ibt<32>::aplt_entry): Renamed to ...
>         (Output_data_plt_x86_64_ibt<size>::aplt_entry): This.
>         (Output_data_plt_x86_64_ibt<64>::aplt_entry): Removed.
>         (Output_data_plt_x86_64_ibt<size>::do_fill_plt_entry): Drop the
>         size == 32 check.
>         (Output_data_plt_x86_64_ibt<size>::fill_aplt_entry): Likewise.
> ---
>  gold/x86_64.cc | 57 ++++++++------------------------------------------
>  1 file changed, 9 insertions(+), 48 deletions(-)
>
> diff --git a/gold/x86_64.cc b/gold/x86_64.cc
> index 86b232a5e2d..83684b90fcf 100644
> --- a/gold/x86_64.cc
> +++ b/gold/x86_64.cc
> @@ -2328,12 +2328,10 @@ Output_data_plt_x86_64_ibt<size>::set_final_data_size()
>
>  // The first entry in the IBT PLT.
>
> -template<>
> +template<int size>
>  const unsigned char
> -Output_data_plt_x86_64_ibt<32>::first_plt_entry[plt_entry_size] =
> +Output_data_plt_x86_64_ibt<size>::first_plt_entry[plt_entry_size] =
>  {
> -  // MPX isn't supported for x32, so we don't need the BND prefix.
> -  // From AMD64 ABI Draft 0.98, page 76
>    0xff, 0x35,           // pushq contents of memory address
>    0, 0, 0, 0,           // replaced with address of .got + 8
>    0xff, 0x25,           // jmp indirect
> @@ -2341,18 +2339,6 @@ Output_data_plt_x86_64_ibt<32>::first_plt_entry[plt_entry_size] =
>    0x90, 0x90, 0x90, 0x90 // noop (x4)
>  };
>
> -template<>
> -const unsigned char
> -Output_data_plt_x86_64_ibt<64>::first_plt_entry[plt_entry_size] =
> -{
> -  // Use the BND prefix so that IBT is compatible with MPX.
> -  0xff, 0x35,          // pushq contents of memory address
> -  0, 0, 0, 0,          // replaced with address of .got + 8
> -  0xf2, 0xff, 0x25,    // bnd jmp indirect
> -  0, 0, 0, 0,          // replaced with address of .got + 16
> -  0x0f, 0x1f, 0x00     // nop
> -};
> -
>  template<int size>
>  void
>  Output_data_plt_x86_64_ibt<size>::do_fill_first_plt_entry(
> @@ -2362,7 +2348,7 @@ Output_data_plt_x86_64_ibt<size>::do_fill_first_plt_entry(
>  {
>    // Offsets to the addresses needing relocation.
>    const unsigned int roff1 = 2;
> -  const unsigned int roff2 = (size == 32) ? 8 : 9;
> +  const unsigned int roff2 = 8;
>
>    memcpy(pov, first_plt_entry, plt_entry_size);
>    // We do a jmp relative to the PC at the end of this instruction.
> @@ -2376,9 +2362,9 @@ Output_data_plt_x86_64_ibt<size>::do_fill_first_plt_entry(
>
>  // Subsequent entries in the IBT PLT.
>
> -template<>
> +template<int size>
>  const unsigned char
> -Output_data_plt_x86_64_ibt<32>::plt_entry[plt_entry_size] =
> +Output_data_plt_x86_64_ibt<size>::plt_entry[plt_entry_size] =
>  {
>    // From AMD64 ABI Draft 1.0-rc1, Chapter 13.
>    0xf3, 0x0f, 0x1e, 0xfa,      // endbr64
> @@ -2389,24 +2375,11 @@ Output_data_plt_x86_64_ibt<32>::plt_entry[plt_entry_size] =
>    0x90, 0x90                   // nop
>  };
>
> -template<>
> -const unsigned char
> -Output_data_plt_x86_64_ibt<64>::plt_entry[plt_entry_size] =
> -{
> -  // From AMD64 ABI Draft 1.0-rc1, Chapter 13.
> -  0xf3, 0x0f, 0x1e, 0xfa,      // endbr64
> -  0x68,                                // pushq immediate
> -  0, 0, 0, 0,                  // replaced with offset into relocation table
> -  0xf2, 0xe9,                  // bnd jmpq relative
> -  0, 0, 0, 0,                  // replaced with offset to start of .plt
> -  0x90                         // nop
> -};
> -
>  // Entries in the IBT Additional PLT.
>
> -template<>
> +template<int size>
>  const unsigned char
> -Output_data_plt_x86_64_ibt<32>::aplt_entry[aplt_entry_size] =
> +Output_data_plt_x86_64_ibt<size>::aplt_entry[aplt_entry_size] =
>  {
>    // From AMD64 ABI Draft 1.0-rc1, Chapter 13.
>    0xf3, 0x0f, 0x1e, 0xfa,      // endbr64
> @@ -2416,18 +2389,6 @@ Output_data_plt_x86_64_ibt<32>::aplt_entry[aplt_entry_size] =
>    0x90, 0x90                   // nop
>  };
>
> -template<>
> -const unsigned char
> -Output_data_plt_x86_64_ibt<64>::aplt_entry[aplt_entry_size] =
> -{
> -  // From AMD64 ABI Draft 1.0-rc1, Chapter 13.
> -  0xf3, 0x0f, 0x1e, 0xfa,      // endbr64
> -  0xf2, 0xff, 0x25,            // bnd jmpq indirect
> -  0, 0, 0, 0,                  // replaced with address of symbol in .got
> -  0x0f, 0x1f, 0x04, 0x00,      // nop
> -  0x90,                                // nop
> -};
> -
>  template<int size>
>  unsigned int
>  Output_data_plt_x86_64_ibt<size>::do_fill_plt_entry(
> @@ -2440,7 +2401,7 @@ Output_data_plt_x86_64_ibt<size>::do_fill_plt_entry(
>  {
>    // Offsets to the addresses needing relocation.
>    const unsigned int roff1 = 5;
> -  const unsigned int roff2 = (size == 32) ? 10 : 11;
> +  const unsigned int roff2 = 10;
>
>    memcpy(pov, plt_entry, plt_entry_size);
>    elfcpp::Swap_unaligned<32, false>::writeval(pov + roff1, plt_index);
> @@ -2459,7 +2420,7 @@ Output_data_plt_x86_64_ibt<size>::fill_aplt_entry(
>      unsigned int plt_index)
>  {
>    // Offset to the address needing relocation.
> -  const unsigned int roff = (size == 32) ? 6 : 7;
> +  const unsigned int roff = 6;
>
>    // Check PC-relative offset overflow in PLT entry.
>    uint64_t plt_got_pcrel_offset = (got_address + got_offset
> --
> 2.38.1
>

Hi Cary,

Is it OK for gold?

Thanks.

-- 
H.J.