On Mon, Jan 27, 2014 at 1:50 PM, H.J. Lu <hjl.to...@gmail.com> wrote: > On Mon, Jan 27, 2014 at 1:42 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >> On Sat, Jan 18, 2014 at 8:11 AM, H.J. Lu <hjl.to...@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.
