This is the mail archive of the binutils@sourceware.org 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: [PATCH] PR gold/18609


2015-07-22 4:06 GMT+03:00 Cary Coutant <ccoutant@gmail.com>:
>>>>>> If you remove those changes, won't it generate an unused GOT slot
>>>>>> when GOTPCREL relocation is converted to PC-relative relocation?
>>>>>
>>>>> Yes, it can generate unused GOT slots.
>>>>
>>>> I think we should add a testcase to check for the unused GOT slot.
>>>> Please check if you can implement similar heuristic in gold:
>>>>
>>>> https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commitdiff;h=59cab532835904f368b0aa99267afba5fda5ded2
>>>
>>> No addresses available at the time of Target_x86_64<size>::Scan::local
>>> and *::global work, so not clear how to use some heuristics here...
>>
>> There are no addresses available in ld.bfd neither.  An estimate was
>> used in ld.bfd.
>
> In gold, I think the best you can do is check to see if the branch and
> its target are in the same output section, then use
> output_section_offset() plus offset within the input section to check
> distance between the two. At the time you're scanning relocations, we
> haven't even done a tentative layout of output sections, so you'll
> have to be pessimistic if the two aren't in the same output section.

Do you mean uint64_t Object::output_section_offset(unsigned int shndx)?
Why we should use it if checked what branch and target are in the same
section so distance depends on offsets in that section only?
I propose the following patch, comments are welcome.

2015-08-27  Andrew Senkevich  <andrew.senkevich@intel.com>

gold/
        PR gold/18609
        * x86_64.cc (Target_x86_64::Relocate::relocate): Added overflow check
        for convert mov foo@GOTPCREL(%rip), %reg to lea foo(%rip), %reg.
        (Target_x86_64::Scan::global, Target_x86_64::Scan::local): Added check
        to avoid unused PLT slots.

diff --git a/gold/x86_64.cc b/gold/x86_64.cc
index 007af1d..db17443 100644
--- a/gold/x86_64.cc
+++ b/gold/x86_64.cc
@@ -2344,6 +2344,32 @@ Target_x86_64<size>::Scan::reloc_needs_plt_for_ifunc(
   return flags != 0;
 }

+template<int size, int valsize>
+class x86_64_overflow_check
+{
+public:
+  typedef typename elfcpp::Elf_types<size>::Elf_Addr Address;
+
+  static inline bool
+  has_overflow_signed(Address value)
+  {
+    // limit = 1 << (valsize - 1) without shift count exceeding size of type
+    Address limit = static_cast<Address>(1) << ((valsize - 1) >> 1);
+    limit <<= ((valsize - 1) >> 1);
+    limit <<= ((valsize - 1) - 2 * ((valsize - 1) >> 1));
+    return value + limit > (limit << 1) - 1;
+  }
+
+  static inline bool
+  has_overflow_unsigned(Address value)
+  {
+    Address limit = static_cast<Address>(1) << ((valsize - 1) >> 1);
+    limit <<= ((valsize - 1) >> 1);
+    limit <<= ((valsize - 1) - 2 * ((valsize - 1) >> 1));
+    return value > (limit << 1) - 1;
+  }
+};
+
 // Scan a relocation for a local symbol.

 template<int size>
@@ -2480,14 +2506,21 @@ Target_x86_64<size>::Scan::local(Symbol_table* symtab,
        // The symbol requires a GOT section.
        Output_data_got<64, false>* got = target->got_section(symtab, layout);

-       // If the relocation symbol isn't IFUNC,
-       // and is local, then we will convert
+       typename elfcpp::Elf_types<size>::Elf_Addr value
+           = reloc.get_r_offset() + reloc.get_r_addend() - lsym.get_st_value();
+
+       // If the relocation symbol isn't IFUNC, and is local,
+       // and branch and its target are in the same output section,
+       // and distance isn't overflow, then we will convert
        // mov foo@GOTPCREL(%rip), %reg
        // to lea foo(%rip), %reg.
        // in Relocate::relocate.
        if (r_type == elfcpp::R_X86_64_GOTPCREL
            && reloc.get_r_offset() >= 2
-           && !is_ifunc)
+           && !is_ifunc
+           && strcmp(object->section_name(lsym.get_st_shndx()).c_str(),
+                     ".text") == 0
+           && !x86_64_overflow_check<size, 32>::has_overflow_signed(value))
          {
            section_size_type stype;
            const unsigned char* view = object->section_contents(data_shndx,
@@ -2496,7 +2529,6 @@ Target_x86_64<size>::Scan::local(Symbol_table* symtab,
              break;
          }

-
        // The symbol requires a GOT entry.
        unsigned int r_sym = elfcpp::elf_r_sym<size>(reloc.get_r_info());

@@ -2905,14 +2937,21 @@ Target_x86_64<size>::Scan::global(Symbol_table* symtab,
       {
        // The symbol requires a GOT entry.
        Output_data_got<64, false>* got = target->got_section(symtab, layout);
+       Sized_symbol<size>* ssym = static_cast<Sized_symbol<size>*>(gsym);
+       typename elfcpp::Elf_types<size>::Elf_Addr value
+           = reloc.get_r_offset() + reloc.get_r_addend() - ssym->value();

        // If we convert this from
        // mov foo@GOTPCREL(%rip), %reg
        // to lea foo(%rip), %reg.
-       // in Relocate::relocate, then there is nothing to do here.
+       // in Relocate::relocate and
+       // branch and its target are in the same output section and
+       // distance isn't overflow, then there is nothing to do here.
        if (r_type == elfcpp::R_X86_64_GOTPCREL
            && reloc.get_r_offset() >= 2
-           && Target_x86_64<size>::can_convert_mov_to_lea(gsym))
+           && Target_x86_64<size>::can_convert_mov_to_lea(gsym)
+           && gsym->output_section() == output_section
+           && !x86_64_overflow_check<size, 32>::has_overflow_signed(value))
          {
            section_size_type stype;
            const unsigned char* view = object->section_contents(data_shndx,
@@ -3543,7 +3582,10 @@ Target_x86_64<size>::Relocate::relocate(
       // mov foo@GOTPCREL(%rip), %reg
       // to lea foo(%rip), %reg.
       // if possible.
-      if (rela.get_r_offset() >= 2
+      typename elfcpp::Elf_types<size>::Elf_Addr value;
+      value = psymval->value(object, addend) - address;
+      if (!x86_64_overflow_check<size, 32>::has_overflow_signed(value)
+         && rela.get_r_offset() >= 2
          && view[-2] == 0x8b
          && ((gsym == NULL && !psymval->is_ifunc_symbol())
              || (gsym != NULL


--
WBR,
Andrew


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