Merge "Make the linker relocatable."

linker/Android.mk

@@ -10,27 +10,9 @@ LOCAL_SRC_FILES:= \
 	dlfcn.c \
 	debugger.c
 
-ifeq ($(TARGET_ARCH),sh)
-# SH-4A series virtual address range from 0x00000000 to 0x7FFFFFFF.
-LINKER_TEXT_BASE := 0x70000100
-else
-# This is aligned to 4K page boundary so that both GNU ld and gold work.  Gold
-# actually produces a correct binary with starting address 0xB0000100 but the
-# extra objcopy step to rename symbols causes the resulting binary to be misaligned
-# and unloadable.  Increasing the alignment adds an extra 3840 bytes in padding
-# but switching to gold saves about 1M of space.
-LINKER_TEXT_BASE := 0xB0001000
-endif
+LOCAL_LDFLAGS := -shared
 
-# The maximum size set aside for the linker, from
-# LINKER_TEXT_BASE rounded down to a megabyte.
-LINKER_AREA_SIZE := 0x01000000
-
-LOCAL_LDFLAGS := -Wl,-Ttext,$(LINKER_TEXT_BASE)
-
-LOCAL_CFLAGS += -DPRELINK
-LOCAL_CFLAGS += -DLINKER_TEXT_BASE=$(LINKER_TEXT_BASE)
-LOCAL_CFLAGS += -DLINKER_AREA_SIZE=$(LINKER_AREA_SIZE)
+LOCAL_CFLAGS += -fno-stack-protector
 
 # Set LINKER_DEBUG to either 1 or 0
 #

linker/linker.c

@@ -313,15 +313,6 @@ static void free_info(soinfo *si)
     freelist = si;
 }
 
-#ifndef LINKER_TEXT_BASE
-#error "linker's makefile must define LINKER_TEXT_BASE"
-#endif
-#ifndef LINKER_AREA_SIZE
-#error "linker's makefile must define LINKER_AREA_SIZE"
-#endif
-#define LINKER_BASE ((LINKER_TEXT_BASE) & 0xfff00000)
-#define LINKER_TOP  (LINKER_BASE + (LINKER_AREA_SIZE))
-
 const char *addr_to_name(unsigned addr)
 {
     soinfo *si;
@@ -332,10 +323,6 @@ const char *addr_to_name(unsigned addr)
         }
     }
 
-    if((addr >= LINKER_BASE) && (addr < LINKER_TOP)){
-        return "linker";
-    }
-
     return "";
 }
 
@@ -354,14 +341,12 @@ _Unwind_Ptr dl_unwind_find_exidx(_Unwind_Ptr pc, int *pcount)
     soinfo *si;
     unsigned addr = (unsigned)pc;
 
-    if ((addr < LINKER_BASE) || (addr >= LINKER_TOP)) {
     for (si = solist; si != 0; si = si->next){
         if ((addr >= si->base) && (addr < (si->base + si->size))) {
             *pcount = si->ARM_exidx_count;
             return (_Unwind_Ptr)(si->base + (unsigned long)si->ARM_exidx);
         }
     }
-    }
    *pcount = 0;
     return NULL;
 }
@@ -420,6 +405,33 @@ static Elf32_Sym *_elf_lookup(soinfo *si, unsigned hash, const char *name)
     return NULL;
 }
 
+/*
+ * Essentially the same method as _elf_lookup() above, but only
+ * searches for LOCAL symbols
+ */
+static Elf32_Sym *_elf_lookup_local(soinfo *si, unsigned hash, const char *name)
+{
+    Elf32_Sym *symtab = si->symtab;
+    const char *strtab = si->strtab;
+    unsigned n = hash % si->nbucket;;
+
+    TRACE_TYPE(LOOKUP, "%5d LOCAL SEARCH %s in %s@0x%08x %08x %d\n", pid,
+               name, si->name, si->base, hash, hash % si->nbucket);
+    for(n = si->bucket[hash % si->nbucket]; n != 0; n = si->chain[n]){
+        Elf32_Sym *s = symtab + n;
+        if (strcmp(strtab + s->st_name, name)) continue;
+        if (ELF32_ST_BIND(s->st_info) != STB_LOCAL) continue;
+        /* no section == undefined */
+        if(s->st_shndx == 0) continue;
+
+        TRACE_TYPE(LOOKUP, "%5d FOUND LOCAL %s in %s (%08x) %d\n", pid,
+                   name, si->name, s->st_value, s->st_size);
+        return s;
+    }
+
+    return NULL;
+}
+
 static unsigned elfhash(const char *_name)
 {
     const unsigned char *name = (const unsigned char *) _name;
@@ -443,7 +455,17 @@ _do_lookup(soinfo *si, const char *name, unsigned *base)
     soinfo *lsi = si;
     int i;
 
-    /* Look for symbols in the local scope first (the object who is
+    /* If we are trying to find a symbol for the linker itself, look
+     * for LOCAL symbols first. Avoid using LOCAL symbols for other
+     * shared libraries until we have a better understanding of what
+     * might break by doing so. */
+    if (si->flags & FLAG_LINKER) {
+        s = _elf_lookup_local(si, elf_hash, name);
+        if(s != NULL)
+            goto done;
+    }
+
+    /* Look for symbols in the local scope (the object who is
      * searching). This happens with C++ templates on i386 for some
      * reason.
      *
@@ -452,6 +474,7 @@ _do_lookup(soinfo *si, const char *name, unsigned *base)
      * dynamic linking.  Some systems return the first definition found
      * and some the first non-weak definition.   This is system dependent.
      * Here we return the first definition found for simplicity.  */
+
     s = _elf_lookup(si, elf_hash, name);
     if(s != NULL)
         goto done;
@@ -1628,10 +1651,10 @@ static int link_image(soinfo *si, unsigned wr_offset)
     DEBUG("%5d si->base = 0x%08x si->flags = 0x%08x\n", pid,
           si->base, si->flags);
 
-    if (si->flags & FLAG_EXE) {
+    if (si->flags & (FLAG_EXE | FLAG_LINKER)) {
         /* Locate the needed program segments (DYNAMIC/ARM_EXIDX) for
-         * linkage info if this is the executable. If this was a
-         * dynamic lib, that would have been done at load time.
+         * linkage info if this is the executable or the linker itself. 
+         * If this was a dynamic lib, that would have been done at load time.
          *
          * TODO: It's unfortunate that small pieces of this are
          * repeated from the load_library routine. Refactor this just
@@ -1958,7 +1981,12 @@ int main(int argc, char **argv)
 
 static void * __tls_area[ANDROID_TLS_SLOTS];
 
-unsigned __linker_init(unsigned **elfdata)
+/*
+ * This code is called after the linker has linked itself and
+ * fixed it's own GOT. It is safe to make references to externs
+ * and other non-local data at this point.
+ */
+static unsigned __linker_init_post_relocation(unsigned **elfdata)
 {
     static soinfo linker_soinfo;
 
@@ -2158,3 +2186,69 @@ sanitize:
           si->entry);
     return si->entry;
 }
+
+/*
+ * Find the value of AT_BASE passed to us by the kernel. This is the load
+ * location of the linker.
+ */
+static unsigned find_linker_base(unsigned **elfdata) {
+    int argc = (int) *elfdata;
+    char **argv = (char**) (elfdata + 1);
+    unsigned *vecs = (unsigned*) (argv + argc + 1);
+    while (vecs[0] != 0) {
+        vecs++;
+    }
+
+    /* The end of the environment block is marked by two NULL pointers */
+    vecs++;
+
+    while(vecs[0]) {
+        if (vecs[0] == AT_BASE) {
+            return vecs[1];
+        }
+        vecs += 2;
+    }
+
+    return 0; // should never happen
+}
+
+/*
+ * This is the entry point for the linker, called from begin.S. This
+ * method is responsible for fixing the linker's own relocations, and
+ * then calling __linker_init_post_relocation().
+ *
+ * Because this method is called before the linker has fixed it's own
+ * relocations, any attempt to reference an extern variable, extern
+ * function, or other GOT reference will generate a segfault.
+ */
+unsigned __linker_init(unsigned **elfdata) {
+    unsigned linker_addr = find_linker_base(elfdata);
+    Elf32_Ehdr *elf_hdr = (Elf32_Ehdr *) linker_addr;
+    Elf32_Phdr *phdr =
+        (Elf32_Phdr *)((unsigned char *) linker_addr + elf_hdr->e_phoff);
+
+    soinfo linker_so;
+    memset(&linker_so, 0, sizeof(soinfo));
+
+    linker_so.base = linker_addr;
+    linker_so.dynamic = (unsigned *) -1;
+    linker_so.phdr = phdr;
+    linker_so.phnum = elf_hdr->e_phnum;
+    linker_so.flags |= FLAG_LINKER;
+    linker_so.wrprotect_start = 0xffffffff;
+    linker_so.wrprotect_end = 0;
+
+    if (link_image(&linker_so, 0)) {
+        // It would be nice to print an error message, but if the linker
+        // can't link itself, there's no guarantee that we'll be able to
+        // call write() (because it involves a GOT reference).
+        //
+        // This situation should never occur unless the linker itself
+        // is corrupt.
+        exit(-1);
+    }
+
+    // We have successfully fixed our own relocations. It's safe to run
+    // the main part of the linker now.
+    return __linker_init_post_relocation(elfdata);
+}

linker/linker.h

@@ -83,6 +83,7 @@ typedef struct soinfo soinfo;
 #define FLAG_LINKED     0x00000001
 #define FLAG_ERROR      0x00000002
 #define FLAG_EXE        0x00000004 // The main executable
+#define FLAG_LINKER     0x00000010 // The linker itself
 
 #define SOINFO_NAME_LEN 128