The intention of this post is to highlight some of the similarities and differences between ELF and Mach-O dynamic linking that I encountered while building memprof.

I hope to write more posts about similarities and differences in other aspects of Mach-O and ELF that I stumbled across to shed some light on what goes on down there and provide (in some cases) the only documentation.

Procedure Linkage Table

The procedure linkage table (PLT) is used to determine the absolute address of a function at runtime. Both Mach-O and ELF objects have PLTs that are generated at compile time. The initial table simply invokes the dynamic linker which finds the symbol you want. The way this works is very similar at a high level in ELF and Mach-O, but there are some implementation differences that I thought were worth mentioning.

Mach-O PLT arrangement

Mach-O objects have several different sections across different segments that are all involved to create a PLT entry for a specific symbol.

Consider the following assembly stub which calls out to the PLT entry for malloc:

# MACH-O calling a PLT entry (ELF is nearly identical)
0x000000010008c504 [str_new+52]:	callq  0x10009ebbc [dyld_stub_malloc]

The dyld_stub prefix is added by GDB to let the user know that the callq instruction is calling a PLT entry and not malloc itself. The address 0x10009ebbc is the first instruction of malloc‘s PLT entry in this Mach-O object. In Mach-O terminology, the instruction at 0x10009ebbc is called a symbol stub. Symbol stubs in Mach-O objects are found in the __TEXT segment in the __symbol_stub1 section.

Let’s examine some instructions at the symbol stub address above:

# MACH-O "symbol stubs" for malloc and other functions
0x10009ebbc [dyld_stub_malloc]:	  jmpq   *0x3ae46(%rip)        # 0x1000d9a08
0x10009ebc2 [dyld_stub_realloc]:  jmpq   *0x3ae48(%rip)        # 0x1000d9a10
0x10009ebc8 [dyld_stub_seekdir$INODE64]:	jmpq   *0x3ae4c(%rip)  # 0x1000d9a20
. . . .

Each Mach-O symbol stub is just a single jmpq instruction. That jmpq instruction either:

• Invokes the dynamic linker to find the symbol and transfer execution there

OR

• Transfers execution directly to the function.

via an entry in a table.

In the example above, GDB is telling us that the address of the table entry for malloc is 0x1000d9a08. This table entry is stored in a section called the __la_symbol_ptr within the __DATA segment.

Before malloc has been resolved, the address in that table entry points to a helper function which (eventually) invokes the dynamic linker to find malloc and fill in its address in the table entry.

Let’s take a look at what a few entries of the helper functions look like:

# MACH-O stub helpers
0x1000a08d4 [stub helpers+6986]:	pushq  $0x3b73
0x1000a08d9 [stub helpers+6991]:	jmpq   0x10009ed8a [stub helpers]
0x1000a08de [stub helpers+6996]:	pushq  $0x3b88
0x1000a08e3 [stub helpers+7001]:	jmpq   0x10009ed8a [stub helpers]
0x1000a08e8 [stub helpers+7006]:	pushq  $0x3b9e
0x1000a08ed [stub helpers+7011]:	jmpq   0x10009ed8a [stub helpers]
. . . .

Each symbol that has a PLT entry has 2 instructions above; a pair of pushq and jmpq. This instruction sequence sets an ID for the desired function and then invokes the dynamic linker. The dynamic linker looks up this ID so it knows which function it should be looking for.

ELF PLT arrangement

ELF objects have the same mechanism, but organize each PLT entry into chunks instead of splicing them out across different sections. Let’s take a look at a PLT entry for malloc in an ELF object:

# ELF complete PLT entry for malloc
0x40f3d0 [malloc@plt]:	jmpq   *0x2c91fa(%rip)        # 0x6d85d0
0x40f3d6 [malloc@plt+6]:	pushq  $0x2f
0x40f3db [malloc@plt+11]:	jmpq   0x40f0d0
. . . .

Much like a Mach-O object, an ELF object uses a table entry to direct the flow of execution to either invoke the dynamic linker or transfer directly to the desired function if it has already been resolved.

Two differences to point out here:

1. ELF puts the entire PLT entry together in nicely named section called plt instead of splicing it out across multiple sections.
2. The table entries indirected through with the initial jmpq instruction are stored in a section named: .got.plt.

Both invoke an assembly trampoline…

Both Mach-O and ELF objects are set up to invoke the runtime dynamic linker. Both need an assembly trampoline to bridge the gap between the application and the linker. On 64bit Intel based systems, linkers in both systems must comply to the same Application Binary Interace (ABI).

Strangely enough, the two linkers have slightly different assembly trampolines even though they share the same calling convention^{1 2}.

Both trampolines ensure that the program stack is 16-byte aligned to comply with the amd64 ABI’s calling convention. Both trampolines also take care to save the “general purpose” caller-saved registers prior to invoking the dynamic link, but it turns out that the trampoline in Linux does not save or restore the SSE registers. It turns out that this “shouldn’t” matter, so long as glibc takes care not to use any of those registers in the dynamic linker. OSX takes a more conservative approach and saves and restores the SSE registers before and after calling out the dynamic linker.

I’ve included a snippet from the two trampolines below and some comments so you can see the differences up close.

Different trampolines for the same ABI

The OSX trampoline:

dyld_stub_binder:
  pushq   %rbp
  movq    %rsp,%rbp
  subq    $STACK_SIZE,%rsp  # at this point stack is 16-byte aligned because two meta-parameters where pushed
  movq    %rdi,RDI_SAVE(%rsp) # save registers that might be used as parameters
  movq    %rsi,RSI_SAVE(%rsp)
  movq    %rdx,RDX_SAVE(%rsp)
  movq    %rcx,RCX_SAVE(%rsp)
  movq    %r8,R8_SAVE(%rsp)
  movq    %r9,R9_SAVE(%rsp)
  movq    %rax,RAX_SAVE(%rsp)
  movdqa    %xmm0,XMMM0_SAVE(%rsp)
  movdqa    %xmm1,XMMM1_SAVE(%rsp)
  movdqa    %xmm2,XMMM2_SAVE(%rsp)
  movdqa    %xmm3,XMMM3_SAVE(%rsp)
  movdqa    %xmm4,XMMM4_SAVE(%rsp)
  movdqa    %xmm5,XMMM5_SAVE(%rsp)
  movdqa    %xmm6,XMMM6_SAVE(%rsp)
  movdqa    %xmm7,XMMM7_SAVE(%rsp)
  movq    MH_PARAM_BP(%rbp),%rdi  # call fastBindLazySymbol(loadercache, lazyinfo)
  movq    LP_PARAM_BP(%rbp),%rsi
  call    __Z21_dyld_fast_stub_entryPvl

The OSX trampoline saves all the caller saved registers as well as the the %xmm0 - %xmm7 registers prior to invoking the dynamic linker with that last call instruction. These registers are all restored after the call instruction, but I left that out for the sake of brevity.

The Linux trampoline:

  subq $56,%rsp
  cfi_adjust_cfa_offset(72) # Incorporate PLT
  movq %rax,(%rsp)  # Preserve registers otherwise clobbered.
  movq %rcx, 8(%rsp)
  movq %rdx, 16(%rsp)
  movq %rsi, 24(%rsp)
  movq %rdi, 32(%rsp)
  movq %r8, 40(%rsp)
  movq %r9, 48(%rsp)
  movq 64(%rsp), %rsi # Copy args pushed by PLT in register.
  movq %rsi, %r11   # Multiply by 24
  addq %r11, %rsi
  addq %r11, %rsi
  shlq $3, %rsi
  movq 56(%rsp), %rdi # %rdi: link_map, %rsi: reloc_offset
  call _dl_fixup    # Call resolver.

The Linux trampoline doesn’t touch the SSE registers because it assumes that the dynamic linker will not modify them thus avoiding a save and restore.

Conclusion

• Tracing program execution from call site to the dynamic linker is pretty interesting and there is a lot to learn along the way.
• glibc not saving and restoring %xmm0-%xmm7 kind of scares me, but there is a unit test included that disassembles the built ld.so searching it to make sure that those registers are never touched. It is still a bit frightening.
• Stay tuned for more posts explaining other interesting similarities and differences between Mach-O and ELF coming soon.

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References

1. http://developer.apple.com/mac/library/documentation/DeveloperTools/Conceptual/LowLevelABI/140-x86-64_Function_Calling_Conventions/x86_64.html#//apple_ref/doc/uid/TP40005035-SW1 [↩]
2. http://www.x86-64.org/documentation/abi.pdf [↩]