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Re: Update x86-64 PLT for MPX
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: GCC Development <gcc at gcc dot gnu dot org>, GNU C Library <libc-alpha at sourceware dot org>, Binutils <binutils at sourceware dot org>, GDB <gdb at sourceware dot org>, "x86-64-abi at googlegroups dot com" <x86-64-abi at googlegroups dot com>
- Cc: Igor Zamyatin <izamyatin at gmail dot com>
- Date: Wed, 19 Feb 2014 11:51:16 -0800
- Subject: Re: Update x86-64 PLT for MPX
- Authentication-results: sourceware.org; auth=none
- References: <CAMe9rOqfBoicMbVBoX3BkuQhFVD_nFt_BWatbAEL4ZkmgfPEXg at mail dot gmail dot com> <CAMe9rOrUZB8ifn8oe07Q8aqcfZZesBWtoEid3ajfs2nzspMcdw at mail dot gmail dot com> <CAMe9rOqOHTBo72s0b+++WfzfTHVWSO3MWOPN2M1QQ2+gTdCURQ at mail dot gmail dot com>
On Mon, Jan 27, 2014 at 1:50 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Mon, Jan 27, 2014 at 1:42 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Sat, Jan 18, 2014 at 8:11 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> Hi,
>>>
>>> Here is the proposal to update x86-64 PLT for MPX. The linker change
>>> is implemented on hjl/mpx/pltext8 branch. Any comments/feedbacks?
>>>
>>> Thanks.
>>>
>>> --
>>> H.J.
>>> ---
>>> Intel MPX:
>>>
>>> http://software.intel.com/en-us/file/319433-017pdf
>>>
>>> introduces 4 bound registers, which will be used for parameter passing
>>> in x86-64. Bound registers are cleared by branch instructions. Branch
>>> instructions with BND prefix will keep bound register contents. This leads
>>> to 2 requirements to 64-bit MPX run-time:
>>>
>>> 1. Dynamic linker (ld.so) should save and restore bound registers during
>>> symbol lookup.
>>> 2. Change the current 16-byte PLT0:
>>>
>>> ff 35 08 00 00 00 pushq GOT+8(%rip)
>>> ff 25 00 10 00 jmpq *GOT+16(%rip)
>>> 0f 1f 40 00 nopl 0x0(%rax)
>>>
>>> and 16-byte PLT1:
>>>
>>> ff 25 00 00 00 00 jmpq *name@GOTPCREL(%rip)
>>> 68 00 00 00 00 pushq $index
>>> e9 00 00 00 00 jmpq PLT0
>>>
>>> which clear bound registers, to preserve bound registers.
>>>
>>> We use 2 new relocations:
>>>
>>> #define R_X86_64_PC32_BND 39 /* PC relative 32 bit signed with BND prefix */
>>> #define R_X86_64_PLT32_BND 40 /* 32 bit PLT address with BND prefix */
>>>
>>> to mark branch instructions with BND prefix.
>>>
>>> When linker sees any R_X86_64_PC32_BND or R_X86_64_PLT32_BND relocations,
>>> it switches to a different PLT0:
>>>
>>> ff 35 08 00 00 00 pushq GOT+8(%rip)
>>> f2 ff 25 00 10 00 bnd jmpq *GOT+16(%rip)
>>> 0f 1f 00 nopl (%rax)
>>>
>>> to preserve bound registers for symbol lookup and it also creates an
>>> external PLT section, .pl.bnd. Linker will create a BND PLT1 entry
>>> in .plt:
>>>
>>> 68 00 00 00 00 pushq $index
>>> f2 e9 00 00 00 00 bnd jmpq PLT0
>>> 0f 1f 44 00 00 nopl 0(%rax,%rax,1)
>>>
>>> and a 8-byte BND PLT entry in .plt.bnd:
>>>
>>> f2 ff 25 00 00 00 00 bnd jmpq *name@GOTPCREL(%rip)
>>> 90 nop
>>>
>>> Otherwise, linker will create a legacy PLT entry in .plt:
>>>
>>> 68 00 00 00 00 pushq $index
>>> e9 00 00 00 00 jmpq PLT0
>>> 66 0f 1f 44 00 00 nopw 0(%rax,%rax,1)
>>>
>>> and a 8-byte legacy PLT in .plt.bnd:
>>>
>>> ff 25 00 00 00 00 jmpq *name@GOTPCREL(%rip)
>>> 66 90 xchg %ax,%ax
>>>
>>> The initial value of the GOT entry for "name" will be set to the the
>>> "pushq" instruction in the corresponding entry in .plt. Linker will
>>> resolve reference of symbol "name" to the entry in the second PLT,
>>> .plt.bnd.
>>>
>>> Prelink stores the offset of pushq of PLT1 (plt_base + 0x10) in GOT[1]
>>> and GOT[1] is stored in GOT[3]. We can undo prelink in GOT by computing
>>> the corresponding the pushq offset with
>>>
>>> GOT[1] + (GOT offset - &GOT[3]) * 2
>>>
>>> Since for each entry in .plt except for PLT0 we create a 8-byte entry in
>>> .plt.bnd, there is extra 8-byte per PLT symbol.
>>>
>>> We also investigated the 16-byte entry for .plt.bnd. We compared the
>>> 8-byte entry vs the the 16-byte entry for .plt.bnd on Sandy Bridge.
>>> There are no performance differences in SPEC CPU 2000/2006 as well as
>>> micro benchmarks.
>>>
>>> Pros:
>>> No change to undo prelink in dynamic linker.
>>> Only 8-byte memory overhead for each PLT symbol.
>>> Cons:
>>> Extra .plt.bnd section is needed.
>>> Extra 8 byte for legacy branches to PLT.
>>> GDB is unware of the new layout of .plt and .plt.bnd.
>>
>> Hi,
>>
>> I am enclosing the updated x86-64 psABI with PLT change.
>> I checkeMy email is rejected due to PDF attachment. I am resubmitting it with
> out PDF file.
> d it onto hjl/mpx/master branch at
>>
>> https://github.com/hjl-tools/x86-64-psABI
>>
>> I will check in the binutils changes if there are no disagreements
>> in 2 weeks.
>>
>> Thanks.
>
>
> My email is rejected due to PDF attachment. I am resubmitting it with
> out PDF file.
I pushed the MPX binutils change into master:
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=0ff2b86e7c14177ec7f9e1257f8e697814794017
--
H.J.