[PATCH] libdwl: Add validate_strdata to limit Elf_Data d_size to valid strings.

[Mark Wielaard]

dwfl_module_getsym returns the name of a symbol as found in the
corresponding (symbol) string section. Make sure all names are
correctly zero terminated by making sure the last valid index in a
section/segment Elf_Data contains a zero character.

        * libdwfl/dwfl_module_getdwarf.c (validate_strdata): New
        function taking Elf_Data and restricting d_size to last zero
        char.
        (translate_offs): Call validate_strdata.
        (find_symtab): Likewise for both symstrdata and aux_symstrdata.

https://sourceware.org/bugzilla/show_bug.cgi?id=33099

Signed-off-by: Mark Wielaard <m...@klomp.org>
---
 libdwfl/dwfl_module_getdwarf.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/libdwfl/dwfl_module_getdwarf.c b/libdwfl/dwfl_module_getdwarf.c
index 7fd0d3aa3b17..135132d69178 100644
--- a/libdwfl/dwfl_module_getdwarf.c
+++ b/libdwfl/dwfl_module_getdwarf.c
@@ -1,5 +1,6 @@
 /* Find debugging and symbol information for a module in libdwfl.
    Copyright (C) 2005-2012, 2014, 2015, 2025 Red Hat, Inc.
+   Copyright (C) 2025 Mark J. Wielaard <m...@klomp.org>
    This file is part of elfutils.
 
    This file is free software; you can redistribute it and/or modify
@@ -692,6 +693,19 @@ find_offsets (Elf *elf, GElf_Addr main_bias, size_t phnum, 
size_t n,
     }
 }
 
+/* This is a string section/segment, so we want to make sure the last
+   valid index contains a zero character to terminate a string.  */
+static void
+validate_strdata (Elf_Data *symstrdata)
+{
+  size_t size = symstrdata->d_size;
+  const char *buf = symstrdata->d_buf;
+  while (size > 0 && *(buf + size - 1) != '\0')
+    --size;
+  symstrdata->d_size = size;
+}
+
+
 /* Various addresses we might want to pull from the dynamic segment.  */
 enum
 {
@@ -816,6 +830,8 @@ translate_offs (GElf_Addr adjust,
                                                  ELF_T_BYTE);
          if (mod->symstrdata == NULL)
            mod->symdata = NULL;
+         else
+           validate_strdata (mod->symstrdata);
        }
       if (mod->symdata == NULL)
        mod->symerr = DWFL_E (LIBELF, elf_errno ());
@@ -1181,6 +1197,8 @@ find_symtab (Dwfl_Module *mod)
   mod->symstrdata = elf_getdata (symstrscn, NULL);
   if (mod->symstrdata == NULL || mod->symstrdata->d_buf == NULL)
     goto elferr;
+  else
+    validate_strdata (mod->symstrdata);
 
   if (xndxscn == NULL)
     mod->symxndxdata = NULL;
@@ -1264,6 +1282,8 @@ find_symtab (Dwfl_Module *mod)
       mod->aux_symstrdata = elf_getdata (aux_strscn, NULL);
       if (mod->aux_symstrdata == NULL || mod->aux_symstrdata->d_buf == NULL)
        goto aux_cleanup;
+      else
+       validate_strdata (mod->aux_symstrdata);
 
       if (aux_xndxscn == NULL)
        mod->aux_symxndxdata = NULL;
-- 
2.49.0