[PATCH] elf: Remove fallback to the start of DT_STRTAB for dladdr

[Fangrui Song <maskray@google.com>]

When neither DT_HASH nor DT_GNU_HASH is present, the code scans
[DT_SYMTAB, DT_STRTAB). However, there is no guarantee that .dynstr
immediately follows .dynsym (e.g. lld typically places .gnu.version
after .dynsym).

In the absence of a hash table, symbol lookup will always fail
(map->l_nbuckets == 0), so it seems appropriate to just bail out.
---
 elf/dl-addr.c | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/elf/dl-addr.c b/elf/dl-addr.c
index e3c5598e1a..6f5f8eac5c 100644
--- a/elf/dl-addr.c
+++ b/elf/dl-addr.c
@@ -71,18 +71,10 @@ determine_info (const ElfW(Addr) addr, struct link_map *match, Dl_info *info,
 	    }
 	}
     }
-  else
+  else if (match->l_info[DT_HASH] != NULL)
     {
-      const ElfW(Sym) *symtabend;
-      if (match->l_info[DT_HASH] != NULL)
-	symtabend = (symtab
-		     + ((Elf_Symndx *) D_PTR (match, l_info[DT_HASH]))[1]);
-      else
-	/* There is no direct way to determine the number of symbols in the
-	   dynamic symbol table and no hash table is present.  The ELF
-	   binary is ill-formed but what shall we do?  Use the beginning of
-	   the string table which generally follows the symbol table.  */
-	symtabend = (const ElfW(Sym) *) strtab;
+      const ElfW (Sym) *symtabend
+	  = (symtab + ((Elf_Symndx *) D_PTR (match, l_info[DT_HASH]))[1]);
 
       for (; (void *) symtab < (void *) symtabend; ++symtab)
 	if ((ELFW(ST_BIND) (symtab->st_info) == STB_GLOBAL
@@ -96,6 +88,7 @@ determine_info (const ElfW(Addr) addr, struct link_map *match, Dl_info *info,
 	    && symtab->st_name < strtabsize)
 	  matchsym = (ElfW(Sym) *) symtab;
     }
+  /* In the absence of a hash table, as if the object has no symbol.  */
 
   if (mapp)
     *mapp = match;
-- 
2.36.0.464.gb9c8b46e94-goog