libelf: Handle elf_strptr on section without any data

In the unlikely situation that elf_strptr was called on a section with
sh_size already set, but that doesn't have any data yet we could crash
trying to verify the string to return.

This could happen for example when a new section was created with
elf_newscn, but no data having been added yet.

	* libelf/elf_strptr.c (elf_strptr): Check strscn->rawdata_base
	is not NULL.

https://sourceware.org/bugzilla/show_bug.cgi?id=32672

Signed-off-by: Mark Wielaard <mark@klomp.org>

diff --git a/libelf/elf_strptr.c b/libelf/elf_strptr.c
index c5a94f82aa55167c7c7c348c2f80f757bd6f63b4..7be7f5e8fb1848bf9f9f057c734637ac3ad0e4da 100644 (file)
--- a/libelf/elf_strptr.c
+++ b/libelf/elf_strptr.c
@@ -1,5 +1,6 @@
 /* Return string pointer from string section.
    Copyright (C) 1998-2002, 2004, 2008, 2009, 2015 Red Hat, Inc.
+   Copyright (C) 2025 Mark J. Wielaard <mark@klomp.org>
    This file is part of elfutils.
    Contributed by Ulrich Drepper <drepper@redhat.com>, 1998.
 
@@ -183,9 +184,12 @@ elf_strptr (Elf *elf, size_t idx, size_t offset)
       // initialized yet (when data_read is zero). So we cannot just
       // look at the rawdata.d.d_size.
 
-      /* Make sure the string is NUL terminated.  Start from the end,
-        which very likely is a NUL char.  */
-      if (likely (validate_str (strscn->rawdata_base, offset, sh_size)))
+      /* First check there actually is any data.  This could be a new
+         section which hasn't had any data set yet.  Then make sure
+         the string is at a valid offset and NUL terminated.  */
+      if (unlikely (strscn->rawdata_base == NULL))
+       __libelf_seterrno (ELF_E_INVALID_SECTION);
+      else if (likely (validate_str (strscn->rawdata_base, offset, sh_size)))
        result = &strscn->rawdata_base[offset];
       else
        __libelf_seterrno (ELF_E_INVALID_INDEX);