Re: [gold] Error out when there is an access beyond the end of the merged section

[Alexander Ivchenko <aivchenk at gmail dot com>]

Hi Cary,

I think you are right that it would be better to give an error from
Merged_symbol_value::value_from_output_section, but I think that just
replacing gold_assert is wrong:

Here is how "found" is initialized:
bool found = object->merge_map()->get_output_offset(NULL, input_shndx,
                                                    input_offset,
                                                    &output_offset,
                                                    object);

And Object_merge_map::get_output_offset can return false in three cases:

1)
  if (map == NULL
      || (merge_map != NULL && map->merge_map != merge_map))
    return false;

2)
  std::vector<Input_merge_entry>::const_iterator p =
    std::lower_bound(map->entries.begin(), map->entries.end(),
                     entry, Input_merge_compare());
  if (p == map->entries.end() || p->input_offset > input_offset)
    {
      if (p == map->entries.begin())
        return false;
      --p;
      gold_assert(p->input_offset <= input_offset);
    }

and

3)
  if (input_offset - p->input_offset
      >= static_cast<section_offset_type>(p->length))
       return false;

AFAIU, for cases 1) and 2) the gold_assert in
Merged_symbol_value::value_from_output_section should really be hit,
and only 3) should give us an error with "access beyond end of merged
section".

So, I think, we should move gold_assert to 1), 2) in order to catch
that situation. Something like that:


diff --git a/gold/ChangeLog b/gold/ChangeLog
index 264f127..8608aaa 100644
--- a/gold/ChangeLog
+++ b/gold/ChangeLog
@@ -1,3 +1,12 @@
+2014-07-02  Alexander Ivchenko  <alexander.ivchenko@intel.com>
+
+ * reloc.cc (Merged_symbol_value<size>::value_from_output_section):
+ error out (instead of gold_assert) when we see that there is an
+ access beyond the end of the merged section.
+ * merge.cc (Object_merge_map::get_output_offset): Add two additional
+ asserts.
+ * merge.h (Object_merge_map::get_output_offset): Adjust the comment.
+
 2014-06-27  Alan Modra  <amodra@gmail.com>

  * symtab.cc (Symbol::should_add_dynsym_entry): Don't make inline.
diff --git a/gold/merge.cc b/gold/merge.cc
index 6d444e6..8c91b20 100644
--- a/gold/merge.cc
+++ b/gold/merge.cc
@@ -148,6 +148,14 @@ Object_merge_map::get_output_offset(const
Merge_map* merge_map,
     section_offset_type* output_offset)
 {
   Input_merge_map* map = this->get_input_merge_map(shndx);
+
+  // If this assertion fails, then it means that we are called from
+  // Merged_symbol_value<size>::value_from_output_section and some
+  // relocation was against a portion of an input merge section which
+  // we didn't map to the output file and we didn't explicitly discard.
+  // We should always map all portions of input merge sections.
+  gold_assert(merge_map != NULL || map != NULL);
+
   if (map == NULL
       || (merge_map != NULL && map->merge_map != merge_map))
     return false;
@@ -166,6 +174,11 @@ Object_merge_map::get_output_offset(const
Merge_map* merge_map,
      entry, Input_merge_compare());
   if (p == map->entries.end() || p->input_offset > input_offset)
     {
+      // Just like in the assert in the beginning of this method: we are
+      // called from Merged_symbol_value<size>::value_from_output_section
+      // and failed to map some portion of some input merge section.
+      gold_assert(merge_map != NULL || p != map->entries.begin());
+
       if (p == map->entries.begin())
  return false;
       --p;
diff --git a/gold/merge.h b/gold/merge.h
index b4fd8e1..ff4f3af 100644
--- a/gold/merge.h
+++ b/gold/merge.h
@@ -61,8 +61,9 @@ class Object_merge_map
       section_size_type length, section_offset_type output_offset);

   // Get the output offset for an input address.  MERGE_MAP is the map
-  // we are looking for, or NULL if we don't care.  The input address
-  // is at offset OFFSET in section SHNDX.  This sets *OUTPUT_OFFSET
+  // we are looking for, or NULL if we don't care - in this case, we
+  // assume that failing to find the output offset is fatal.  The input
+  // address is at offset OFFSET in section SHNDX.  This sets *OUTPUT_OFFSET
   // to the offset in the output section; this will be -1 if the bytes
   // are not being copied to the output.  This returns true if the
   // mapping is known, false otherwise.  *OUTPUT_OFFSET is relative to
diff --git a/gold/reloc.cc b/gold/reloc.cc
index 115ab37..dd86a08 100644
--- a/gold/reloc.cc
+++ b/gold/reloc.cc
@@ -1478,11 +1478,14 @@ Merged_symbol_value<size>::value_from_output_section(
       input_offset,
       &output_offset);

-  // If this assertion fails, it means that some relocation was
-  // against a portion of an input merge section which we didn't map
-  // to the output file and we didn't explicitly discard.  We should
-  // always map all portions of input merge sections.
-  gold_assert(found);
+  if (!found)
+    {
+      // FIXME: const_cast is ugly..
+      object->error(_("access beyond end of merged section (%s+0x%lx)"),
+    const_cast<Relobj*>(object)->section_name(input_shndx).c_str(),
+    static_cast<long>(input_offset));
+      return 0;
+    }

   if (output_offset == -1)
     return 0;



There is an unfortunate necessity of const_cast, if we want to print
out the name of the section.
What do you think?


--Alexander

2014-07-02 0:18 GMT+04:00 Cary Coutant <ccoutant@google.com>:
>> +2014-07-01  Alexander Ivchenko  <alexander.ivchenko@intel.com>
>> +
>> + * merge.cc (Object_merge_map::get_output_offset): error out when we see
>> + that there is an access beyond the end of the merged section.
>> + * merge.h (Object_merge_map::get_output_offset): Add new argument.
>> + (Merge_map::get_output_offset): Adjust
>> + Object_merge_map::get_output_offset call with additional argument.
>> + * reloc.cc (Merged_symbol_value<size>::value_from_output_section):
>> + Ditto.
>
> I don't think Object_merge_map or Merge_map should be printing this
> error. Instead, I'd prefer to issue the error from
> Merged_symbol_value::value_from_output_section in place of the
> gold_assert. Everything needed to print the error message is readily
> available there.
>
> It would be nice to print the section name as well as the offset. You
> should just be able to replace gold_assert with this:
>
>   if (!found)
>     {
>       object->error(_("access beyond end of merged section (%s+%ld)"),
>                     object->section_name(input_shndx),
>                     static_cast<long>(input_offset));
>       return 0;
>     }
>
> Thanks for looking at this!
>
> -cary