Find .plt section in object files generated by recent ld

[uweigand]

Summary

Code in ObjectFileELF::ParseTrampolineSymbols assumes that the sh_info
field of the .rel(a).plt section identifies the .plt section.

However, with recent GNU ld this is no longer true. As a result of this:
https://sourceware.org/bugzilla/show_bug.cgi?id=18169
in object files generated with current linkers the sh_info field of
.rel(a).plt now points to the .got.plt section (or .got on some targets).

This causes LLDB to fail to identify any PLT stubs, causing a number of
test case failures.

This patch changes LLDB to simply always look for the .plt section by
name. This should be safe across all linkers and targets.

[clayborg]

Other ELF experts should OK this one as well, but looks good to me though.

[tberghammer]

Looks good as I am not aware of any system where the plt sections has different section name

[clayborg]

We can create a new SectionType enumeration in lldb-enumerations.h. Then in ObjectFileELF::CreateSections() we would classify the ".plt" section as eSectionTypeProcedureLinkageTable:

static ConstString g_sect_name_plt (".plt");
if (name == g_sect_name_plt)
    sect_type = eSectionTypeProcedureLinkageTable;

Then you can find sections by type no matter what the file format is. This isn't needed for this fix, but if sections contain common information in different file formats, this is handy to avoid having to know to look for sections by name. For instance, DWARF debug info is contained in ".debug_info" and ".debug_abbrev" in ELF, but in MachO the sections are "debug_info" and "debug_abbrev". So we made section types for them in the SectionType enumeration and then the DWARF parser just asks the lldb_private::ObjectFile for the sections by type (eSectionTypeDWARFDebugInfo and eSectionTypeDWARFDebugAbbrev).

lldb/trunk/source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp

 unsigned
 ObjectFileELF::ParseTrampolineSymbols(Symtab *symbol_table,
                                       user_id_t start_id,
                                       const ELFSectionHeaderInfo *rel_hdr,
                                       user_id_t rel_id)
 {
     assert(rel_hdr->sh_type == SHT_RELA || rel_hdr->sh_type == SHT_REL);
 
-    // The link field points to the associated symbol table. The info field
-    // points to the section holding the plt.
+    // The link field points to the associated symbol table.
     user_id_t symtab_id = rel_hdr->sh_link;
-    user_id_t plt_id = rel_hdr->sh_info;
 
     // If the link field doesn't point to the appropriate symbol name table then
     // try to find it by name as some compiler don't fill in the link fields.
     if (!symtab_id)
         symtab_id = GetSectionIndexByName(".dynsym");
-    if (!plt_id)
-        plt_id = GetSectionIndexByName(".plt");
+
+    // Get PLT section.  We cannot use rel_hdr->sh_info, since current linkers
+    // point that to the .got.plt or .got section instead of .plt.
+    user_id_t plt_id = GetSectionIndexByName(".plt");
 
     if (!symtab_id || !plt_id)
         return 0;
 
     // Section ID's are ones based;
     symtab_id++;
     plt_id++;