[v2] ungetc: Guarantee single char pushback

diff --git a/libio/bits/types/struct_FILE.h b/libio/bits/types/struct_FILE.h
index d8d26639d1..5d08509078 100644
--- a/libio/bits/types/struct_FILE.h
+++ b/libio/bits/types/struct_FILE.h
@@ -1,4 +1,5 @@ 
 /* Copyright (C) 1991-2024 Free Software Foundation, Inc.
+   Copyright The GNU Toolchain Authors.
    This file is part of the GNU C Library.
 
    The GNU C Library is free software; you can redistribute it and/or
@@ -94,8 +95,9 @@  struct _IO_FILE_complete
   void *_freeres_buf;
   struct _IO_FILE **_prevchain;
   int _mode;
+  char _short_backupbuf[1];
   /* Make sure we don't get into trouble again.  */
-  char _unused2[15 * sizeof (int) - 5 * sizeof (void *)];
+  char _unused2[15 * sizeof (int) - 5 * sizeof (void *) - sizeof (char)];
 };
 
 /* These macros are used by bits/stdio.h and internal headers.  */
diff --git a/libio/fileops.c b/libio/fileops.c
index 759d737ec7..d49e489f55 100644
--- a/libio/fileops.c
+++ b/libio/fileops.c
@@ -1,4 +1,5 @@ 
 /* Copyright (C) 1993-2024 Free Software Foundation, Inc.
+   Copyright The GNU Toolchain Authors.
    This file is part of the GNU C Library.
 
    The GNU C Library is free software; you can redistribute it and/or
@@ -480,7 +481,7 @@  _IO_new_file_underflow (FILE *fp)
       /* Maybe we already have a push back pointer.  */
       if (fp->_IO_save_base != NULL)
 	{
-	  free (fp->_IO_save_base);
+	  _IO_free_backup_buf (fp, fp->_IO_save_base);
 	  fp->_flags &= ~_IO_IN_BACKUP;
 	}
       _IO_doallocbuf (fp);
@@ -932,7 +933,7 @@  _IO_new_file_seekoff (FILE *fp, off64_t offset, int dir, int mode)
       /* It could be that we already have a pushback buffer.  */
       if (fp->_IO_read_base != NULL)
 	{
-	  free (fp->_IO_read_base);
+	  _IO_free_backup_buf (fp, fp->_IO_read_base);
 	  fp->_flags &= ~_IO_IN_BACKUP;
 	}
       _IO_doallocbuf (fp);
@@ -1282,7 +1283,7 @@  _IO_file_xsgetn (FILE *fp, void *data, size_t n)
       /* Maybe we already have a push back pointer.  */
       if (fp->_IO_save_base != NULL)
 	{
-	  free (fp->_IO_save_base);
+	  _IO_free_backup_buf (fp, fp->_IO_save_base);
 	  fp->_flags &= ~_IO_IN_BACKUP;
 	}
       _IO_doallocbuf (fp);
diff --git a/libio/genops.c b/libio/genops.c
index d7e35e67d5..996daf0ad8 100644
--- a/libio/genops.c
+++ b/libio/genops.c
@@ -1,4 +1,5 @@ 
 /* Copyright (C) 1993-2024 Free Software Foundation, Inc.
+   Copyright The GNU Toolchain Authors.
    This file is part of the GNU C Library.
 
    The GNU C Library is free software; you can redistribute it and/or
@@ -48,6 +49,13 @@  flush_cleanup (void *not_used)
 }
 #endif
 
+void
+_IO_free_backup_buf (FILE *fp, char *ptr)
+{
+  if (ptr != fp->_short_backupbuf)
+    free (ptr);
+}
+
 /* Fields in struct _IO_FILE after the _lock field are internal to
    glibc and opaque to applications.  We can change them as long as
    the size of struct _IO_FILE is unchanged, which is checked as the
@@ -212,7 +220,7 @@  _IO_free_backup_area (FILE *fp)
 {
   if (_IO_in_backup (fp))
     _IO_switch_to_main_get_area (fp);  /* Just in case. */
-  free (fp->_IO_save_base);
+  _IO_free_backup_buf (fp, fp->_IO_save_base);
   fp->_IO_save_base = NULL;
   fp->_IO_save_end = NULL;
   fp->_IO_backup_base = NULL;
@@ -260,7 +268,7 @@  save_for_backup (FILE *fp, char *end_p)
 	memcpy (new_buffer + avail,
 		fp->_IO_read_base + least_mark,
 		needed_size);
-      free (fp->_IO_save_base);
+      _IO_free_backup_buf (fp, fp->_IO_save_base);
       fp->_IO_save_base = new_buffer;
       fp->_IO_save_end = new_buffer + avail + needed_size;
     }
@@ -636,7 +644,7 @@  _IO_default_finish (FILE *fp, int dummy)
 
   if (fp->_IO_save_base)
     {
-      free (fp->_IO_save_base);
+      _IO_free_backup_buf (fp, fp->_IO_save_base);
       fp->_IO_save_base = NULL;
     }
 
@@ -998,11 +1006,17 @@  _IO_default_pbackfail (FILE *fp, int c)
 	  else if (!_IO_have_backup (fp))
 	    {
 	      /* No backup buffer: allocate one. */
-	      /* Use nshort buffer, if unused? (probably not)  FIXME */
 	      int backup_size = 128;
 	      char *bbuf = (char *) malloc (backup_size);
 	      if (bbuf == NULL)
-		return EOF;
+		{
+		  /* Guarantee a 1-char pushback, except for legacy code where
+		     we don't have the extended part of FILE.  */
+		  if (__glibc_unlikely (_IO_vtable_offset (fp) != 0))
+		    return EOF;
+		  bbuf = fp->_short_backupbuf;
+		  backup_size = 1;
+		}
 	      fp->_IO_save_base = bbuf;
 	      fp->_IO_save_end = fp->_IO_save_base + backup_size;
 	      fp->_IO_backup_base = fp->_IO_save_end;
@@ -1022,7 +1036,7 @@  _IO_default_pbackfail (FILE *fp, int c)
 	    return EOF;
 	  memcpy (new_buf + (new_size - old_size), fp->_IO_read_base,
 		  old_size);
-	  free (fp->_IO_read_base);
+	  _IO_free_backup_buf (fp, fp->_IO_read_base);
 	  _IO_setg (fp, new_buf, new_buf + (new_size - old_size),
 		    new_buf + new_size);
 	  fp->_IO_backup_base = fp->_IO_read_ptr;
diff --git a/libio/libioP.h b/libio/libioP.h
index 34bf91fcd8..90ef8e90be 100644
--- a/libio/libioP.h
+++ b/libio/libioP.h
@@ -1,4 +1,5 @@ 
 /* Copyright (C) 1993-2024 Free Software Foundation, Inc.
+   Copyright The GNU Toolchain Authors.
    This file is part of the GNU C Library.
 
    The GNU C Library is free software; you can redistribute it and/or
@@ -357,6 +358,8 @@  typedef FILE *_IO_ITER;
 
 /* Generic functions */
 
+extern void _IO_free_backup_buf (FILE *, char *);
+libc_hidden_proto (_IO_free_backup_buf)
 extern void _IO_switch_to_main_get_area (FILE *) __THROW;
 extern void _IO_switch_to_backup_area (FILE *) __THROW;
 extern int _IO_switch_to_get_mode (FILE *);
diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index e76e40e587..b1a04fd064 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -1,4 +1,5 @@ 
 # Copyright (C) 1991-2024 Free Software Foundation, Inc.
+# Copyright The GNU Toolchain Authors.
 # This file is part of the GNU C Library.
 
 # The GNU C Library is free software; you can redistribute it and/or
@@ -303,6 +304,7 @@  tests := \
   tst-tmpnam \
   tst-ungetc \
   tst-ungetc-leak \
+  tst-ungetc-nomem \
   tst-unlockedio \
   tst-vfprintf-mbs-prec \
   tst-vfprintf-user-type \
diff --git a/stdio-common/tst-ungetc-nomem.c b/stdio-common/tst-ungetc-nomem.c
new file mode 100644
index 0000000000..73b88e2e1c
--- /dev/null
+++ b/stdio-common/tst-ungetc-nomem.c
@@ -0,0 +1,98 @@ 
+/* Test ungetc behavior with malloc failures.
+   Copyright The GNU Toolchain Authors.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/temp_file.h>
+#include <support/xstdio.h>
+
+extern void *__libc_malloc (size_t)
+     __attribute__ ((malloc)) __attribute__ ((alloc_size (1)));
+
+static volatile bool fail = false;
+
+void *
+malloc (size_t sz)
+{
+  if (fail)
+    return NULL;
+
+  return __libc_malloc (sz);
+}
+
+static int
+do_test (void)
+{
+  char *filename = NULL;
+  struct stat props = {};
+  size_t bufsz = 0;
+
+  create_temp_file ("tst-ungetc-nomem.", &filename);
+  if (stat (filename, &props) != 0)
+    FAIL_EXIT1 ("Could not get file status: %m\n");
+
+  FILE *fp = fopen (filename, "w");
+
+  /* The libio buffer sizes are the same as block size.  */
+  bufsz = props.st_blksize + 2;
+
+  char *buf = xmalloc (bufsz);
+  memset (buf, 'a', bufsz);
+
+  if (fwrite (buf, sizeof (char), bufsz, fp) != bufsz)
+    FAIL_EXIT1 ("fwrite failed: 5m\n");
+  xfclose (fp);
+
+  /* Begin test.  */
+  fp = xfopen (filename, "r");
+
+
+  /* The standard requires the first ungetc to always work.  */
+  fail = true;
+  TEST_COMPARE (ungetc ('y', fp), 'y');
+
+  /* Now let the buffers get allocated to allow for subsequent tests.  */
+  fail = false;
+  TEST_COMPARE (fgetc (fp), 'y');
+  TEST_COMPARE (ungetc ('y', fp), 'y');
+  TEST_COMPARE (fgetc (fp), 'y');
+
+  while (!feof (fp))
+    {
+      fail = true;
+      TEST_COMPARE (ungetc ('y', fp), 'y');
+      /* This will result in resizing, which should fail.  */
+      TEST_COMPARE (ungetc ('y', fp), EOF);
+      fail = false;
+      TEST_COMPARE (fgetc (fp), 'y');
+      /* The second fgetc should return a char from the file or EOF.  */
+      char c = fgetc (fp);
+      if (!feof (fp))
+	TEST_COMPARE (c, 'a');
+    }
+
+  /* Final sanity check before we're done.  */
+  TEST_COMPARE (ferror (fp), 0);
+  xfclose (fp);
+
+  return 0;
+}
+
+#include <support/test-driver.c>