Guess where unknown functions are in .rel.plt

1 files changed, 90 insertions, 6 deletions

diff --git a/src/model/relocate.c b/src/model/relocate.c
index 1b4f9ff..972bda3 100644
--- a/src/model/relocate.c
+++ b/src/model/relocate.c
@@ -1,8 +1,86 @@
 #include <assert.h>
 #include <stdlib.h>
+#include <string.h>
 #include <libelfu/libelfu.h>
 
 
+static void* subFindByName(ElfuElf *me, ElfuScn *ms, void *aux1, void *aux2)
+{
+  char *name = (char*)aux1;
+  (void)aux2;
+
+  if (elfu_mScnName(me, ms)) {
+    if (!strcmp(elfu_mScnName(me, ms), name)) {
+      return ms;
+    }
+  }
+
+  /* Continue */
+  return NULL;
+}
+
+/* Hazard a guess where a function may be found in the PLT */
+static GElf_Word pltLookupVal(ElfuElf *metarget, char *name)
+{
+  ElfuScn *relplt;
+  ElfuScn *plt;
+  ElfuRel *rel;
+  GElf_Word j;
+
+  relplt = elfu_mScnForall(metarget, subFindByName, ".rel.plt", NULL);
+  if (!relplt) {
+    ELFU_WARN("dynsymLookupVal: Could not find .rel.plt section in destination ELF.\n");
+    return 0;
+  }
+
+  plt = elfu_mScnForall(metarget, subFindByName, ".plt", NULL);
+  if (!plt) {
+    ELFU_WARN("dynsymLookupVal: Could not find .plt section in destination ELF.\n");
+    return 0;
+  }
+
+
+  /* Look up name */
+  assert(relplt->reltab);
+  assert(relplt->linkptr);
+  assert(relplt->linkptr->symtab);
+  j = 0;
+  CIRCLEQ_FOREACH(rel, &relplt->reltab->rels, elem) {
+    GElf_Word i;
+    ElfuSym *sym;
+
+    j++;
+
+    if (rel->type != R_386_JMP_SLOT) {
+      continue;
+    }
+
+    sym = CIRCLEQ_FIRST(&relplt->linkptr->symtab->syms);
+    for (i = 1; i < rel->sym; i++) {
+      sym = CIRCLEQ_NEXT(sym, elem);
+    }
+
+    if (!sym->name) {
+      continue;
+    }
+
+    if (!strcmp(sym->name, name)) {
+      /* If this is the symbol we are looking for, then in an x86 binary
+       * the jump to the dynamic symbol is probably at offset (j * 16)
+       * from the start of the PLT, where j is the PLT entry and 16 is
+       * the number of bytes the machine code in a PLT entry take. */
+      GElf_Addr addr = plt->shdr.sh_addr + (16 * j);
+      ELFU_DEBUG("dynsymLookupVal: Guessing symbol '%s' is in destination memory at %jx (PLT entry #%d).\n", name, addr, j);
+      return addr;
+    }
+  }
+
+  ELFU_WARN("dynsymLookupVal: Could not find symbol '%s' in destination ELF.\n", name);
+
+  return 0;
+}
+
+
 static GElf_Word symtabLookupVal(ElfuElf *metarget, ElfuScn *msst, GElf_Word entry)
 {
   GElf_Word i;
@@ -26,9 +104,16 @@ static GElf_Word symtabLookupVal(ElfuElf *metarget, ElfuScn *msst, GElf_Word ent
       if (sym->scnptr) {
         assert(elfu_mScnByOldscn(metarget, sym->scnptr));
         return elfu_mScnByOldscn(metarget, sym->scnptr)->shdr.sh_addr + sym->value;
+      } else if (sym->shndx == SHN_UNDEF) {
+        /* Look the symbol up in .dyn.plt. If it cannot be found there then
+         * .rel.dyn may need to be expanded with a COPY relocation so the
+         * dynamic linker fixes up the (TODO). */
+        return pltLookupVal(metarget, sym->name);
+      } else if (sym->shndx == SHN_ABS) {
+        return sym->value;
       } else {
-        // TODO: UNDEF, ABS, or COMMON
-        ELFU_WARN("symtabLookupVal: Returning 0 for UNDEF, ABS, or COMMON symbol.\n");
+        ELFU_WARN("symtabLookupVal: Symbol binding COMMON is not supported, using 0.\n");
+        return 0;
       }
       break;
     case STT_SECTION:
@@ -36,11 +121,10 @@ static GElf_Word symtabLookupVal(ElfuElf *metarget, ElfuScn *msst, GElf_Word ent
       assert(elfu_mScnByOldscn(metarget, sym->scnptr));
       return elfu_mScnByOldscn(metarget, sym->scnptr)->shdr.sh_addr;
     case STT_FILE:
-      // TODO
-      ELFU_WARN("symtabLookupVal: Returning 0 for FILE symbol.\n");
-      break;
+      ELFU_WARN("symtabLookupVal: Symbol type FILE is not supported, using 0.\n");
+      return 0;
     default:
-      ELFU_WARN("symtabLookupVal: Unknown symbol type %d.\n", sym->type);
+      ELFU_WARN("symtabLookupVal: Unknown symbol type %d for %s.\n", sym->type, sym->name);
       return 0;
   }
 }