[PATCH v3 2/2] Fix: ppc64le support

[Hemant Kumar <hemant at linux dot vnet dot ibm dot com>]

PPC64 ELF ABI v2 has a Global entry point and a local entry point
for the functions. We need the Local entry point in order to probe
these functions. However, the DIE for these functions in debuginfo
return the function.entrypc which is same as the global entry point.
The local entry point is not encoded in the debuginfo of the ELFs. The
offset to local entry point is however encoded in the st_other field
of these symbols in the symbol table.
We need to use this field to adjust the sym.st_value to actually point
to the local entry point instead of the global entry point.

This patch is in relation to this bug :
https://sourceware.org/bugzilla/show_bug.cgi?id=17638

So, while adding symbols to the sym_table, we add an offset of
PPC64_LOCAL_ENTRY_OFFSET(sym.st_other) to st_value.
And when the function address is queried in query_dwarf_func(), we give
priority to the cached sym_table, where we can retrieve the adjusted
entry address of the function. If we don't get any address from the
symbol table, then we proceed to get from the debuginfo.

Macro definition PPC64_LOCAL_ENTRY_OFFSET has been picked up from glibc.
It won't be defined if we are building systemtap on a machine having
older elf.h and hence, won't recognize PPC64_LOCAL_ENTRY_OFFSET.

Signed-off-by: Hemant Kumar <hemant@linux.vnet.ibm.com>
---
 tapsets.cxx | 67 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 65 insertions(+), 2 deletions(-)

diff --git a/tapsets.cxx b/tapsets.cxx
index 326ba6a..d8d4222 100644
--- a/tapsets.cxx
+++ b/tapsets.cxx
@@ -66,7 +66,13 @@ extern "C" {
 using namespace std;
 using namespace __gnu_cxx;
 
-
+// for elf.h where PPC64_LOCAL_ENTRY_OFFSET isn't defined
+#ifndef PPC64_LOCAL_ENTRY_OFFSET
+#define STO_PPC64_LOCAL_BIT    5
+#define STO_PPC64_LOCAL_MASK   (7 << STO_PPC64_LOCAL_BIT)
+#define PPC64_LOCAL_ENTRY_OFFSET(other)                                        \
+ (((1 << (((other) & STO_PPC64_LOCAL_MASK) >> STO_PPC64_LOCAL_BIT)) >> 2) << 2)
+#endif
 
 // ------------------------------------------------------------------------
 
@@ -2051,6 +2057,18 @@ query_dwarf_inline_instance (Dwarf_Die * die, dwarf_query * q)
     }
 }
 
+static bool
+is_filtered_func_exists (func_info_map_t filtered, func_info *fi)
+{
+  for (unsigned i = 0; i < filtered.size(); i++)
+    {
+      if ((filtered[i].entrypc == fi->entrypc) && (filtered[i].name == fi->name))
+        return true;
+      }
+
+  return false;
+}
+
 static int
 query_dwarf_func (Dwarf_Die * func, dwarf_query * q)
 {
@@ -2101,7 +2119,34 @@ query_dwarf_func (Dwarf_Die * func, dwarf_query * q)
           q->dw.function_line (&func.decl_line);
 
           Dwarf_Addr entrypc;
-          if (q->dw.function_entrypc (&entrypc))
+
+          func.entrypc = 0;
+          Dwarf_Addr bias;
+          Dwfl_Module *mod = q->dw.module;
+          Elf* elf = (dwarf_getelf (dwfl_module_getdwarf (mod, &bias))
+                      ?: dwfl_module_getelf (mod, &bias));
+
+           GElf_Ehdr ehdr_mem;
+           GElf_Ehdr* em = gelf_getehdr (elf, &ehdr_mem);
+           if (em == NULL) throw SEMANTIC_ERROR (_("Couldn't get elf header"));
+
+           /* Giving priority to sym_table for ppc64*/
+           if ((em->e_machine == EM_PPC64) && (q->dw.mod_info->sym_table))
+             {
+               list<func_info *> *fis;
+               fis = q->dw.mod_info->sym_table->lookup_symbol(func.name);
+               if (fis && !fis->empty())
+                 {
+                   for (list<func_info*>::iterator it=fis->begin(); it!=fis->end() ; ++it)
+                     {
+                       func.entrypc = (*it)->addr;
+                       if (is_filtered_func_exists(q->filtered_functions, &func))
+                         continue;
+                       q->filtered_functions.push_back(func);
+                     }
+                 }
+              }
+            else if (!func.entrypc && q->dw.function_entrypc (&entrypc))
             {
               func.entrypc = entrypc;
               q->filtered_functions.push_back (func);
@@ -8156,6 +8201,14 @@ symbol_table::get_from_elf()
   int syments = dwfl_module_getsymtab(mod);
   assert(syments);
   prepare_section_rejection(mod);
+  Dwarf_Addr bias;
+  Elf* elf = (dwarf_getelf (dwfl_module_getdwarf (mod, &bias))
+              ?: dwfl_module_getelf (mod, &bias));
+
+  GElf_Ehdr ehdr_mem;
+  GElf_Ehdr* em = gelf_getehdr (elf, &ehdr_mem);
+  if (em == NULL) throw SEMANTIC_ERROR (_("Couldn't get elf header"));
+
   for (int i = 1; i < syments; ++i)
     {
       GElf_Sym sym;
@@ -8184,6 +8237,16 @@ symbol_table::get_from_elf()
       addr = sym.st_value;
       reject = reject_section(section);
 #endif
+     /*
+      * For ELF ABI v2 on PPC64 LE, we need to adjust sym.st_value corresponding
+      * to the bits of sym.st_other. These bits will tell us what's the offset
+      * of the local entry point from the global entry point.
+      *
+      * st_other field is currently only used with ABIv2 on ppc64
+      */
+      if ((em->e_machine == EM_PPC64) && (GELF_ST_TYPE(sym.st_info) == STT_FUNC)
+           && sym.st_other)
+        addr += PPC64_LOCAL_ENTRY_OFFSET(sym.st_other);
 
       if (name && GELF_ST_TYPE(sym.st_info) == STT_FUNC)
         add_symbol(name, (GELF_ST_BIND(sym.st_info) == STB_WEAK),
-- 
1.9.3