Merge pull request #1670 from joelagnel/submit-fix-1628

bcc/syms: Fix shared library symbol lookup

src/cc/bcc_elf.c

@@ -500,6 +500,41 @@ int bcc_elf_foreach_sym(const char *path, bcc_elf_symcb callback,
       path, callback, (struct bcc_symbol_option*)option, payload, 0);
 }
 
+int bcc_elf_get_text_scn_info(const char *path, uint64_t *addr,
+				   uint64_t *offset) {
+  Elf *e = NULL;
+  int fd = -1, err;
+  Elf_Scn *section = NULL;
+  GElf_Shdr header;
+  size_t stridx;
+  char *name;
+
+  if ((err = openelf(path, &e, &fd)) < 0 ||
+      (err = elf_getshdrstrndx(e, &stridx)) < 0)
+    goto exit;
+
+  err = -1;
+  while ((section = elf_nextscn(e, section)) != 0) {
+    if (!gelf_getshdr(section, &header))
+      continue;
+
+    name = elf_strptr(e, stridx, header.sh_name);
+    if (name && !strcmp(name, ".text")) {
+      *addr = (uint64_t)header.sh_addr;
+      *offset = (uint64_t)header.sh_offset;
+      err = 0;
+      break;
+    }
+  }
+
+exit:
+  if (e)
+    elf_end(e);
+  if (fd >= 0)
+    close(fd);
+  return err;
+}
+
 int bcc_elf_foreach_load_section(const char *path,
                                  bcc_elf_load_sectioncb callback,
                                  void *payload) {

src/cc/bcc_elf.h

@@ -61,6 +61,9 @@ int bcc_elf_foreach_sym(const char *path, bcc_elf_symcb callback, void *option,
 // Returns -1 on error, and 0 on success or stopped by callback
 int bcc_elf_foreach_vdso_sym(bcc_elf_symcb callback, void *payload);
 
+int bcc_elf_get_text_scn_info(const char *path, uint64_t *addr,
+                              uint64_t *offset);
+
 int bcc_elf_get_type(const char *path);
 int bcc_elf_is_shared_obj(const char *path);
 int bcc_elf_is_exe(const char *path);

src/cc/bcc_syms.cc

@@ -158,6 +158,18 @@ int ProcSyms::_add_module(const char *modname, uint64_t start, uint64_t end,
     auto module = Module(
         modname, check_mount_ns ? ps->mount_ns_instance_.get() : nullptr,
         &ps->symbol_option_);
+
+    // pid/maps doesn't account for file_offset of text within the ELF.
+    // It only gives the mmap offset. We need the real offset for symbol
+    // lookup.
+    if (module.type_ == ModuleType::SO) {
+      if (bcc_elf_get_text_scn_info(modname, &module.elf_so_addr_,
+                                    &module.elf_so_offset_) < 0) {
+        fprintf(stderr, "WARNING: Couldn't find .text section in %s\n", modname);
+        fprintf(stderr, "WARNING: BCC can't handle sym look ups for %s", modname);
+      }
+    }
+
     if (!bcc_is_perf_map(modname) || module.type_ != ModuleType::UNKNOWN)
       // Always add the module even if we can't read it, so that we could
       // report correct module name. Unless it's a perf map that we only
@@ -251,6 +263,10 @@ ProcSyms::Module::Module(const char *name, ProcMountNS *mount_ns,
     type_ = ModuleType::PERF_MAP;
   else if (bcc_elf_is_vdso(name_.c_str()) == 1)
     type_ = ModuleType::VDSO;
+
+  // Will be stored later
+  elf_so_offset_ = 0;
+  elf_so_addr_ = 0;
 }
 
 int ProcSyms::Module::_add_symbol(const char *symname, uint64_t start,
@@ -282,13 +298,22 @@ void ProcSyms::Module::load_sym_table() {
 }
 
 bool ProcSyms::Module::contains(uint64_t addr, uint64_t &offset) const {
-  for (const auto &range : ranges_)
+  for (const auto &range : ranges_) {
     if (addr >= range.start && addr < range.end) {
-      offset = (type_ == ModuleType::SO || type_ == ModuleType::VDSO)
-                   ? (addr - range.start + range.file_offset)
-                   : addr;
+      if (type_ == ModuleType::SO || type_ == ModuleType::VDSO) {
+        // Offset within the mmap
+        offset = addr - range.start + range.file_offset;
+
+        // Offset within the ELF for SO symbol lookup
+        offset += (elf_so_addr_ - elf_so_offset_);
+      } else {
+        offset = addr;
+      }
+
       return true;
     }
+  }
+
   return false;
 }
 

src/cc/syms.h

@@ -108,6 +108,10 @@ class ProcSyms : SymbolCache {
     bcc_symbol_option *symbol_option_;
     ModuleType type_;
 
+    // The file offset within the ELF of the SO's first text section.
+    uint64_t elf_so_offset_;
+    uint64_t elf_so_addr_;
+
     std::unordered_set<std::string> symnames_;
     std::vector<Symbol> syms_;