Date:	Thu, 17 Jul 2008 11:58:34 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	systemtap@...rceware.org
Subject: Re: [RFC] systemtap: begin the process of using proper kernel APIs
	(part1: use kprobe symbol_name/offset instead of address)

On Thu, 2008-07-17 at 09:18 -0500, James Bottomley wrote:
> OK, thought about it.  There seem to be two possible solutions
> 
>      1. Get systemtap always to offset from non-static functions.  This
>         will use the standard linker to ensure uniqueness (a module
>         qualifier will still need to be added to the struct kprobe for
>         lookup, since modules can duplicate unexported kernel symbols).
>      2. Add the filename as a discriminator for duplicate symbols in the
>         kallsyms program (would still need module qualifier).  This is
>         appealing because the path name would be printed in the kernel
>         trace to help with oops tracking
> 
> This is where negotiations come in.  To me 2. looks to be better because
> it will help us with oops tracking.  On the other hand, it's usually
> pretty obvious from the stack trace context which files the duplicate
> symbols are actually in; what do other people think?

Just by way of illustration, this is systemtap fixed up according to
suggestion number 1.  You can see now using your test case that we get:

# probes
kernel.function("do_open@...block_dev.c:929") /* pc=<lookup_bdev+0x90> */ /* <- kernel.function("do_open") */
kernel.function("do_open@...nfsctl.c:24") /* pc=<sys_nfsservctl+0x6a> */ /* <- kernel.function("do_open") */
kernel.function("do_open@.../mqueue.c:642") /* pc=<sys_mq_unlink+0x130> */ /* <- kernel.function("do_open") */

James

---

>>From 0203b75a9ca97fc7463e6372baee897d1029b799 Mon Sep 17 00:00:00 2001
From: James Bottomley <James.Bottomley@...senPartnership.com>
Date: Tue, 15 Jul 2008 13:25:00 -0500
Subject: Part1 use symbol_name/offset to locate dwarf probes

---
 tapsets.cxx |  119 +++++++++++++++++++++++++++++++++++++++++------------------
 1 files changed, 83 insertions(+), 36 deletions(-)

diff --git a/tapsets.cxx b/tapsets.cxx
index 4d1e469..da198c9 100644
--- a/tapsets.cxx
+++ b/tapsets.cxx
@@ -491,6 +491,7 @@ func_info
   Dwarf_Addr entrypc;
   Dwarf_Addr prologue_end;
   bool weak;
+  bool global;
   // Comparison functor for list of functions sorted by address. The
   // two versions that take a Dwarf_Addr let us use the STL algorithms
   // upper_bound, equal_range et al., but we don't know whether the
@@ -591,6 +592,7 @@ symbol_table
   module_info *mod_info;	// associated module
   map<string, func_info*> map_by_name;
   vector<func_info*> list_by_addr;
+  vector<func_info*> global_list_by_addr;
   typedef vector<func_info*>::iterator iterator_t;
   typedef pair<iterator_t, iterator_t> range_t;
 #ifdef __powerpc__
@@ -598,7 +600,7 @@ symbol_table
 #endif
   // add_symbol doesn't leave symbol table in order; call
   // symbol_table::sort() when done adding symbols.
-  void add_symbol(const char *name, bool weak, Dwarf_Addr addr,
+  void add_symbol(const char *name, bool weak, bool global, Dwarf_Addr addr,
                                                Dwarf_Addr *high_addr);
   void sort();
   enum info_status read_symbols(FILE *f, const string& path);
@@ -611,7 +613,7 @@ symbol_table
   void purge_syscall_stubs();
   func_info *lookup_symbol(const string& name);
   Dwarf_Addr lookup_symbol_address(const string& name);
-  func_info *get_func_containing_address(Dwarf_Addr addr);
+  func_info *get_func_containing_address(Dwarf_Addr addr, bool global);
 
   symbol_table(module_info *mi) : mod_info(mi) {}
   ~symbol_table();
@@ -2306,13 +2308,15 @@ struct dwarf_derived_probe: public derived_probe
                        const string& module,
                        const string& section,
 		       Dwarf_Addr dwfl_addr,
-		       Dwarf_Addr addr,
+		       string symbol,
+		       unsigned int offset,
 		       dwarf_query & q,
                        Dwarf_Die* scope_die);
 
   string module;
   string section;
-  Dwarf_Addr addr;
+  string kprobe_symbol;
+  unsigned int kprobe_offset;
   bool has_return;
   bool has_maxactive;
   long maxactive_val;
@@ -2835,7 +2839,7 @@ dwarf_query::query_module_symtab()
       // Find the "function" in which the indicated address resides.
       Dwarf_Addr addr =
       		(has_function_num ? function_num_val : statement_num_val);
-      fi = sym_table->get_func_containing_address(addr);
+      fi = sym_table->get_func_containing_address(addr, false);
       if (!fi)
         {
           cerr << "Warning: address "
@@ -3260,9 +3264,18 @@ dwarf_query::add_probe_point(const string& funcname,
 
   if (! bad)
     {
+      struct module_info *mi = dw.mod_info;
+      if (!mi->sym_table)
+	mi->get_symtab(this);
+      struct symbol_table *sym_tab = mi->sym_table;
+      func_info *symbol = sym_tab->get_func_containing_address(addr, true);
+
       sess.unwindsym_modules.insert (module);
       probe = new dwarf_derived_probe(funcname, filename, line,
-                                      module, reloc_section, addr, reloc_addr, *this, scope_die);
+                                      module, reloc_section, reloc_addr,
+				      symbol->name,
+				      (unsigned int)(addr - symbol->addr),
+				      *this, scope_die);
       results.push_back(probe);
     }
 }
@@ -4380,7 +4393,8 @@ dwarf_derived_probe::printsig (ostream& o) const
   // function instances.  This is distinct from the verbose/clog
   // output, since this part goes into the cache hash calculations.
   sole_location()->print (o);
-  o << " /* pc=0x" << hex << addr << dec << " */";
+  o << " /* pc=<" << kprobe_symbol << "+0x" << hex
+    << kprobe_offset << dec << "> */";
   printsig_nested (o);
 }
 
@@ -4406,22 +4420,26 @@ dwarf_derived_probe::dwarf_derived_probe(const string& funcname,
                                          // NB: dwfl_addr is the virtualized
                                          // address for this symbol.
                                          Dwarf_Addr dwfl_addr,
-                                         // addr is the section-offset for
-                                         // actual relocation.
-                                         Dwarf_Addr addr,
+                                         // symbol is the closest known symbol
+                                         // and offset is the offset from the symbol
+					 string symbol,
+					 unsigned int offset,
                                          dwarf_query& q,
                                          Dwarf_Die* scope_die /* may be null */)
   : derived_probe (q.base_probe, new probe_point(*q.base_loc) /* .components soon rewritten */ ),
-    module (module), section (section), addr (addr),
+    module (module), section (section), kprobe_symbol(symbol),
+    kprobe_offset(offset),
     has_return (q.has_return),
     has_maxactive (q.has_maxactive),
     maxactive_val (q.maxactive_val)
 {
   // Assert relocation invariants
+#if 0
   if (section == "" && dwfl_addr != addr) // addr should be absolute
     throw semantic_error ("missing relocation base against", q.base_loc->tok);
   if (section != "" && dwfl_addr == addr) // addr should be an offset
     throw semantic_error ("inconsistent relocation address", q.base_loc->tok);
+#endif
 
   this->tok = q.base_probe->tok;
 
@@ -4620,8 +4638,8 @@ dwarf_derived_probe_group::emit_module_decls (systemtap_session& s)
   // Let's find some stats for the three embedded strings.  Maybe they
   // are small and uniform enough to justify putting char[MAX]'s into
   // the array instead of relocated char*'s.
-  size_t module_name_max = 0, section_name_max = 0, pp_name_max = 0;
-  size_t module_name_tot = 0, section_name_tot = 0, pp_name_tot = 0;
+  size_t pp_name_max = 0, pp_name_tot = 0;
+  size_t symbol_name_name_max  = 0, symbol_name_name_tot = 0;
   size_t all_name_cnt = probes_by_module.size(); // for average
   for (p_b_m_iterator it = probes_by_module.begin(); it != probes_by_module.end(); it++)
     {
@@ -4630,9 +4648,8 @@ dwarf_derived_probe_group::emit_module_decls (systemtap_session& s)
         size_t var##_size = (expr) + 1;                 \
         var##_max = max (var##_max, var##_size);        \
         var##_tot += var##_size; } while (0)
-      DOIT(module_name, p->module.size());
-      DOIT(section_name, p->section.size());
       DOIT(pp_name, lex_cast_qstring(*p->sole_location()).size());
+      DOIT(symbol_name_name, p->kprobe_symbol.size());
 #undef DOIT
     }
 
@@ -4652,11 +4669,10 @@ dwarf_derived_probe_group::emit_module_decls (systemtap_session& s)
       if (s.verbose > 2) clog << "stap_dwarf_probe *" << #var << endl;  \
     }
 
-  CALCIT(module);
-  CALCIT(section);
   CALCIT(pp);
+  CALCIT(symbol_name);
 
-  s.op->newline() << "const unsigned long address;";
+  s.op->newline() << "unsigned int offset;";
   s.op->newline() << "void (* const ph) (struct context*);";
   s.op->newline(-1) << "} stap_dwarf_probes[] = {";
   s.op->indent(1);
@@ -4673,9 +4689,8 @@ dwarf_derived_probe_group::emit_module_decls (systemtap_session& s)
           assert (p->maxactive_val >= 0 && p->maxactive_val <= USHRT_MAX);
           s.op->line() << " .maxactive_val=" << p->maxactive_val << ",";
         }
-      s.op->line() << " .address=0x" << hex << p->addr << dec << "UL,";
-      s.op->line() << " .module=\"" << p->module << "\",";
-      s.op->line() << " .section=\"" << p->section << "\",";
+      s.op->line() << " .symbol_name=\"" << p->kprobe_symbol << "\",";
+      s.op->line() << " .offset=0x" << hex << p->kprobe_offset << dec << ",";
       s.op->line() << " .pp=" << lex_cast_qstring (*p->sole_location()) << ",";
       s.op->line() << " .ph=&" << p->name;
       s.op->line() << " },";
@@ -4735,11 +4750,10 @@ dwarf_derived_probe_group::emit_module_init (systemtap_session& s)
   s.op->newline() << "for (i=0; i<" << probes_by_module.size() << "; i++) {";
   s.op->newline(1) << "struct stap_dwarf_probe *sdp = & stap_dwarf_probes[i];";
   s.op->newline() << "struct stap_dwarf_kprobe *kp = & stap_dwarf_kprobes[i];";
-  s.op->newline() << "unsigned long relocated_addr = _stp_module_relocate (sdp->module, sdp->section, sdp->address);";
-  s.op->newline() << "if (relocated_addr == 0) continue;"; // quietly; assume module is absent
   s.op->newline() << "probe_point = sdp->pp;";
   s.op->newline() << "if (sdp->return_p) {";
-  s.op->newline(1) << "kp->u.krp.kp.addr = (void *) relocated_addr;";
+  s.op->newline(1) << "kp->u.krp.kp.symbol_name = sdp->symbol_name;";
+  s.op->newline(1) << "kp->u.krp.kp.offset = sdp->offset;";
   s.op->newline() << "if (sdp->maxactive_p) {";
   s.op->newline(1) << "kp->u.krp.maxactive = sdp->maxactive_val;";
   s.op->newline(-1) << "} else {";
@@ -4748,7 +4762,8 @@ dwarf_derived_probe_group::emit_module_init (systemtap_session& s)
   s.op->newline() << "kp->u.krp.handler = &enter_kretprobe_probe;";
   s.op->newline() << "rc = register_kretprobe (& kp->u.krp);";
   s.op->newline(-1) << "} else {";
-  s.op->newline(1) << "kp->u.kp.addr = (void *) relocated_addr;";
+  s.op->newline(1) << "kp->u.krp.kp.symbol_name = sdp->symbol_name;";
+  s.op->newline(1) << "kp->u.krp.kp.offset = sdp->offset;";
   s.op->newline() << "kp->u.kp.pre_handler = &enter_kprobe_probe;";
   s.op->newline() << "rc = register_kprobe (& kp->u.kp);";
   s.op->newline(-1) << "}";
@@ -4885,12 +4900,20 @@ dwarf_builder::build(systemtap_session & sess,
           throw semantic_error ("absolute statement probe in unprivileged script", q.base_probe->tok);
         }
 
+      struct module_info *mi = dw->mod_info;
+      if (!mi->sym_table)
+	mi->get_symtab(&q);
+      struct symbol_table *sym_tab = mi->sym_table;
+      func_info *symbol = sym_tab->get_func_containing_address(q.statement_num_val, true);
+
       // For kernel.statement(NUM).absolute probe points, we bypass
       // all the debuginfo stuff: We just wire up a
       // dwarf_derived_probe right here and now.
       dwarf_derived_probe* p =
         new dwarf_derived_probe ("", "", 0, "kernel", "",
-                                 q.statement_num_val, q.statement_num_val,
+                                 q.statement_num_val,
+				 symbol->name,
+				 (unsigned int)(q.statement_num_val - symbol->addr),
                                  q, 0);
       finished_results.push_back (p);
       sess.unwindsym_modules.insert ("kernel");
@@ -4908,8 +4931,8 @@ symbol_table::~symbol_table()
 }
 
 void
-symbol_table::add_symbol(const char *name, bool weak, Dwarf_Addr addr,
-                                                      Dwarf_Addr *high_addr)
+symbol_table::add_symbol(const char *name, bool weak, bool global, 
+			 Dwarf_Addr addr, Dwarf_Addr *high_addr)
 {
 #ifdef __powerpc__
   // Map ".sys_foo" to "sys_foo".
@@ -4920,10 +4943,13 @@ symbol_table::add_symbol(const char *name, bool weak, Dwarf_Addr addr,
   fi->addr = addr;
   fi->name = name;
   fi->weak = weak;
+  fi->global = global;
   map_by_name[fi->name] = fi;
   // TODO: Use a multimap in case there are multiple static
   // functions with the same name?
   list_by_addr.push_back(fi);
+  if (global)
+    global_list_by_addr.push_back(fi);
 }
 
 enum info_status
@@ -4961,7 +4987,8 @@ symbol_table::read_symbols(FILE *f, const string& path)
           break;
         }
       if (type == 'T' || type == 't' || type == 'W')
-        add_symbol(name, (type == 'W'), (Dwarf_Addr) addr, &high_addr);
+        add_symbol(name, (type == 'W'), (type == 'T'),
+		   (Dwarf_Addr) addr, &high_addr);
     }
 
   if (list_by_addr.size() < 1)
@@ -5080,7 +5107,8 @@ symbol_table::get_from_elf()
       if (name && GELF_ST_TYPE(sym.st_info) == STT_FUNC &&
                                                    !reject_section(section))
         add_symbol(name, (GELF_ST_BIND(sym.st_info) == STB_WEAK),
-                                              sym.st_value, &high_addr);
+		   GELF_ST_BIND(sym.st_info) == STB_GLOBAL,
+		   sym.st_value, &high_addr);
     }
   sort();
   return info_present;
@@ -5121,14 +5149,29 @@ symbol_table::mark_dwarf_redundancies(dwflpp *dw)
 }
 
 func_info *
-symbol_table::get_func_containing_address(Dwarf_Addr addr)
+symbol_table::get_func_containing_address(Dwarf_Addr addr, bool global)
 {
-  iterator_t iter = upper_bound(list_by_addr.begin(), list_by_addr.end(), addr,
-                              func_info::Compare());
-  if (iter == list_by_addr.begin())
-    return NULL;
+  iterator_t iter;
+
+  if (global)
+    {
+      iter = upper_bound(global_list_by_addr.begin(),
+			 global_list_by_addr.end(), addr,
+			 func_info::Compare());
+      if (iter == global_list_by_addr.begin())
+	return NULL;
+      else
+	return *(iter - 1);
+    }
   else
-    return *(iter - 1);
+    {
+      iter = upper_bound(list_by_addr.begin(), list_by_addr.end(), addr,
+			 func_info::Compare());
+      if (iter == list_by_addr.begin())
+	return NULL;
+      else
+	return *(iter - 1);
+    }
 }
 
 func_info *
@@ -5181,12 +5224,16 @@ symbol_table::purge_syscall_stubs()
   list_by_addr.erase(remove(purge_range.first, purge_range.second,
                             (func_info*)0),
                      purge_range.second);
+  // NOTE: At the moment global_list_by_addr has no weak symbols
+  // so nothing needs to be removed from it.
 }
 
 void
 symbol_table::sort()
 {
   stable_sort(list_by_addr.begin(), list_by_addr.end(), func_info::Compare());
+  stable_sort(global_list_by_addr.begin(), global_list_by_addr.end(),
+	      func_info::Compare());
 }
 
 void
-- 
1.5.6



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