diff --git a/NEWS b/NEWS
index acb3a4e18..ed33d27aa 100644
--- a/NEWS
+++ b/NEWS
@@ -4,6 +4,10 @@ GNU coreutils NEWS                                    -*- outline -*-
 
 ** Bug fixes
 
+  du and ls no longer modify strings returned by getenv.
+  POSIX says this is not portable.
+  [bug introduced in fileutils-4.1.6]
+
   md5sum --text correctly translates CRLF line endings with the MSYS2 runtime.
   This also applies to the sha*sum and b2sum utilities.
   [This bug was present in "the beginning".]
@@ -16,6 +20,9 @@ GNU coreutils NEWS                                    -*- outline -*-
   indicate the line count acceleration being used.
   [bug introduced in coreutils-9.0]
 
+  Programs now port to C23 platforms that strictly check types when
+  qualifier-generic functions like strchr are used.
+
 ** New Features
 
   configure accepts a new --enable-single-binary=hardlinks mode to build the
diff --git a/bootstrap.conf b/bootstrap.conf
index ec68ac8bf..356aaabef 100644
--- a/bootstrap.conf
+++ b/bootstrap.conf
@@ -69,8 +69,8 @@ gnulib_modules="
   crc-x86_64
   crypto/md5
   crypto/sha1
-  crypto/sha3
   crypto/sha256
+  crypto/sha3
   crypto/sha512
   crypto/sm3
   cycle-check
@@ -215,14 +215,14 @@ gnulib_modules="
   pipe2
   posix-shell
   posix_spawn
-  posix_spawnattr_destroy
-  posix_spawnattr_init
-  posix_spawnattr_setflags
-  posix_spawnattr_setsigdefault
   posix_spawn_file_actions_addclose
   posix_spawn_file_actions_adddup2
   posix_spawn_file_actions_destroy
   posix_spawn_file_actions_init
+  posix_spawnattr_destroy
+  posix_spawnattr_init
+  posix_spawnattr_setflags
+  posix_spawnattr_setsigdefault
   posix_spawnp
   posixtm
   posixver
@@ -325,6 +325,7 @@ gnulib_modules="
   xgetgroups
   xgethostname
   xmemcoll
+  xmemdup0
   xnanosleep
   xprintf
   xprintf-posix
diff --git a/src/du.c b/src/du.c
index c6d6d7b33..105acb2df 100644
--- a/src/du.c
+++ b/src/du.c
@@ -40,6 +40,7 @@
 #include "stat-time.h"
 #include "stdio--.h"
 #include "xfts.h"
+#include "xmemdup0.h"
 #include "xstrtol.h"
 #include "xstrtol-error.h"
 
@@ -962,9 +963,9 @@ main (int argc, char **argv)
             {
               /* Ignore anything after a newline, for compatibility
                  with ls.  */
-              char *p = strchr (time_style, '\n');
+              char const *p = strchr (time_style, '\n');
               if (p)
-                *p = '\0';
+                time_style = xmemdup0 (time_style, p - time_style);
             }
           else
             {
diff --git a/src/ls.c b/src/ls.c
index a81bab9c6..6b5f2b053 100644
--- a/src/ls.c
+++ b/src/ls.c
@@ -70,20 +70,22 @@
 
 #include <fnmatch.h>
 
+/* Gnulib includes.  */
 #include "acl.h"
+#include "areadlink.h"
 #include "argmatch.h"
-#include "system.h"
 #include "assure.h"
+#include "c-ctype.h"
 #include "c-strcase.h"
+#include "canonicalize.h"
 #include "dev-ino.h"
+#include "filemode.h"
 #include "filenamecat.h"
+#include "filevercmp.h"
 #include "hard-locale.h"
 #include "hash.h"
 #include "human.h"
-#include "filemode.h"
-#include "filevercmp.h"
 #include "idcache.h"
-#include "ls.h"
 #include "mbswidth.h"
 #include "mpsort.h"
 #include "obstack.h"
@@ -91,16 +93,18 @@
 #include "stat-size.h"
 #include "stat-time.h"
 #include "strftime.h"
-#include "term-sig.h"
 #include "xdectoint.h"
-#include "xstrtol.h"
-#include "xstrtol-error.h"
-#include "areadlink.h"
-#include "dircolors.h"
 #include "xgethostname.h"
-#include "c-ctype.h"
-#include "canonicalize.h"
+#include "xmemdup0.h"
+#include "xstrtol-error.h"
+#include "xstrtol.h"
+
+#include "system.h"
+
+#include "dircolors.h"
+#include "ls.h"
 #include "statx.h"
+#include "term-sig.h"
 
 /* Include <sys/capability.h> last to avoid a clash of <sys/types.h>
    include guards with some premature versions of libcap.
@@ -1904,7 +1908,7 @@ stdout_isatty (void)
 static int
 decode_switches (int argc, char **argv)
 {
-  char const *time_style_option = nullptr;
+  char *time_style_option = nullptr;
 
   /* These variables are false or -1 unless a switch says otherwise.  */
   bool kibibytes_specified = false;
@@ -2151,7 +2155,7 @@ decode_switches (int argc, char **argv)
 
         case FULL_TIME_OPTION:
           format_opt = long_format;
-          time_style_option = "full-iso";
+          time_style_option = (char *) "full-iso";
           break;
 
         case COLOR_OPTION:
@@ -2377,35 +2381,39 @@ decode_switches (int argc, char **argv)
 
   if (format == long_format)
     {
-      char const *style = time_style_option;
-      static char const posix_prefix[] = "posix-";
+      char *envstyle = nullptr;
+      char *style = time_style_option;
+      if (! style)
+        style = envstyle = getenv ("TIME_STYLE");
 
       if (! style)
+        style = (char *) "locale";
+      else
         {
-          style = getenv ("TIME_STYLE");
-          if (! style)
-            style = "locale";
-        }
-
-      while (STREQ_LEN (style, posix_prefix, sizeof posix_prefix - 1))
-        {
-          if (! hard_locale (LC_TIME))
-            return optind;
-          style += sizeof posix_prefix - 1;
+          static char const posix_prefix[] = "posix-";
+          while (STREQ_LEN (style, posix_prefix, sizeof posix_prefix - 1))
+            {
+              if (! hard_locale (LC_TIME))
+                return optind;
+              style += sizeof posix_prefix - 1;
+            }
         }
 
       if (*style == '+')
         {
-          char const *p0 = style + 1;
+          char *p0 = style + 1;
           char *p0nl = strchr (p0, '\n');
           char const *p1 = p0;
           if (p0nl)
             {
-              if (strchr (p0nl + 1, '\n'))
+              p1 = p0nl + 1;
+              if (strchr (p1, '\n'))
                 error (LS_FAILURE, 0, _("invalid time style format %s"),
                        quote (p0));
-              *p0nl++ = '\0';
-              p1 = p0nl;
+              if (envstyle)
+                p0 = xmemdup0 (p0, p0nl - p0);
+              else
+                *p0nl = '\0';
             }
           long_time_format[0] = p0;
           long_time_format[1] = p1;