This is the mail archive of the mailing list for the binutils project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Allow pie links to create PLT entries

On Thu, Jan 29, 2015 at 11:48 AM, H.J. Lu <> wrote:
> On Thu, Jan 29, 2015 at 11:00 AM, Sriraman Tallam <> wrote:
>> Hi,
>>     Here is a simple example that fails to link with -pie but which
>> should work just fine without having to use -fPIE.
>> ======
>> int extern_func();
>> int main()
>> {
>>   extern_func();
>>   return 0;
>> }
>> =====
>> int extern_func()
>> {
>>   return 1;
>> }
>> $ g++ -fPIC -shared -o
>> $ g++ -lbar -pie
>> ld: error: foo.o: requires dynamic R_X86_64_PC32 reloc against
>> '_Z11extern_funcv' which may overflow at runtime; recompile with -fPIC
>> It fails because the linker disallows creating a PLT for
>> R_X86_64_PC32 reloc when it is perfectly fine to do so.  Note that I
>> could have recompiled with -fPIE or -fPIC but I still think
>> this can be allowed.  With support for copy relocations in pie in gold
>> and with this support, the cases where we would need to use -fPIE to
>> get working pie links is smaller.  This would help us link non-PIE
>> objects into pie executables.
> You can't do it for x86 since EBX isn't setup for calling via PLT.
> For x86-64, there should be little difference between PIE
> and non-PIE code.

True but that little difference is sometimes causing non-trivial
performance penalties. With copyrelocations support for PIE added
recently, one big difference causing non-trivial performance penalty
went away.  However, there are still differences in the way global
arrays are accessed.  For instance,

uint32 a[] = {1, 2, 3, 4}

a[i] can be accessed with one insn without -fPIE, whereas with -fPIE,
we need two. One extra to get the 64-bit address of a.

Without -fPIE:

movslq   0x1655(%rip),%rax  # 401b80 <i>
mov    0x401b30(,%rax,4),%esi # a[i]

With -fPIE:

movslq 0x16c5(%rip),%rdx        # <i>
lea    0x166e(%rip),%rax      # <&a>
mov    (%rax,%rdx,4),%esi   # a[i]

I wish we could use just one insn to do the last two in the -fPIE
case, using PC-relative addressing like:
mov  0x166e(%rip, %rdx, 4), %esi

What do you gain by building PIE without
> -fPIE on x86-64?

We can avoid re-compiling the same file twice for pie and non-pie
links.  If we could just link non-PIE objects into pie, we get
position independence and ASLR without any performance penalty.

Is it alright to make it X86_64 specific?


>> Attached patch to fix this. I have only tested this patch on X86_64.
>> Please review.
>> Thanks
>> Sri
> --
> H.J.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]