elf: lazy relocate the PLT GOT

Instead of looking up the actual symbols, adjust the existing entry to point
back at the PLT.  This will later case the stub at the first three entries
of the PLT to get called with the symbol index on the stack, for run-time
resolution.

elf.cc

@@ -348,12 +348,31 @@ namespace elf {
         }
     }
 
+    void elf_object::relocate_pltgot()
+    {
+        auto rel = dynamic_ptr<Elf64_Rela>(DT_JMPREL);
+        auto nrel = dynamic_val(DT_PLTRELSZ) / sizeof(*rel);
+        for (auto p = rel; p < rel + nrel; ++p) {
+            auto info = p->r_info;
+              u32 sym = info >> 32;
+              u32 type = info & 0xffffffff;
+              assert(type = R_X86_64_JUMP_SLOT);
+              void *addr = _base + p->r_offset;
+              // The JUMP_SLOT entry already points back to the PLT, just
+              // make sure it is relocated relative to the object base.
+              *static_cast<u64*>(addr) += reinterpret_cast<u64>(_base);
+        }
+    }
+
     void elf_object::relocate()
     {
         assert(!dynamic_exists(DT_REL));
         if (dynamic_exists(DT_RELA)) {
             relocate_rela();
         }
+        if (dynamic_exists(DT_JMPREL)) {
+            relocate_pltgot();
+        }
     }
 
     unsigned long

elf.hh

@@ -272,6 +272,7 @@ namespace elf {
         symbol_module symbol(unsigned idx);
         Elf64_Xword symbol_tls_module(unsigned idx);
         void relocate_rela();
+        void relocate_pltgot();
     protected:
         program& _prog;
 	Elf64_Ehdr _ehdr;