stdio-common: Handle -1 buffer size in __sprintf_chk & co (bug 30039)

This shows up as an assertion failure when sprintf is called with
a specifier like "%.8g" and libquadmath is linked in:

Fatal glibc error: printf_buffer_as_file.c:31
  (__printf_buffer_as_file_commit): assertion failed:
  file->stream._IO_write_ptr <= file->next->write_end

Fix this by detecting pointer wraparound in __vsprintf_internal
and saturate the addition to the end of the address space instead.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>

debug/Makefile

@@ -200,6 +200,10 @@ LDFLAGS-tst-backtrace6 = -rdynamic
 
 CFLAGS-tst-ssp-1.c += -fstack-protector-all
 
+# Disable compiler optimizations around vsprintf (the function under test).
+CFLAGS-tst-sprintf-fortify-unchecked.c = \
+  -fno-builtin-vsprintf -fno-builtin-__vsprintf_chk
+
 tests = backtrace-tst \
 	tst-longjmp_chk \
 	test-strcpy_chk \
@@ -211,6 +215,7 @@ tests = backtrace-tst \
 	tst-backtrace5 \
 	tst-backtrace6 \
 	tst-realpath-chk \
+	tst-sprintf-fortify-unchecked \
 	$(tests-all-chk)
 
 tests-time64 += \

debug/tst-sprintf-fortify-unchecked.c

@@ -0,0 +1,126 @@
+/* Tests for fortified sprintf with unknown buffer bounds (bug 30039).
+   Copyright (C) 2023 Free Software Foundation, Inc.
+   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 <printf.h>
+#include <stdarg.h>
+#include <stdio.h>
+#include <string.h>
+#include <support/check.h>
+
+/* This test is not built with _FORTIFY_SOURCE.  Instead it calls the
+   appropriate implementation directly.  The fortify mode is specified
+   in this variable.  */
+static int fortify_mode;
+
+/* This does not handle long-double redirects etc., but we test only
+   format strings that stay within the confines of the base
+   implementation.  */
+int __vsprintf_chk (char *s, int flag, size_t slen, const char *format,
+                    va_list ap);
+
+/* Invoke vsprintf or __vsprintf_chk according to fortify_mode.  */
+static int
+my_vsprintf (char *buf, const char *format, va_list ap)
+{
+  int result;
+  if (fortify_mode == 0)
+    result = vsprintf (buf, format, ap);
+  else
+    /* Call the fortified version with an unspecified length.  */
+    result = __vsprintf_chk (buf, fortify_mode - 1, -1, format, ap);
+  return result;
+}
+
+/* Run one test, with the specified expected output.  */
+static void __attribute ((format (printf, 2, 3)))
+do_check (const char *expected, const char *format, ...)
+{
+  va_list ap;
+  va_start (ap, format);
+
+  char buf_expected[24];
+  memset (buf_expected, '@', sizeof (buf_expected));
+  TEST_VERIFY (strlen (expected) < sizeof (buf_expected));
+  strcpy (buf_expected, expected);
+
+  char buf[sizeof (buf_expected)];
+  memset (buf, '@', sizeof (buf));
+
+  int ret = my_vsprintf (buf, format, ap);
+  TEST_COMPARE_BLOB (buf_expected, sizeof (buf_expected), buf, sizeof (buf));
+  TEST_COMPARE (ret, strlen (expected));
+
+  va_end (ap);
+}
+
+/* Run the tests in all fortify modes.  */
+static void
+do_tests (void)
+{
+  for (fortify_mode = 0; fortify_mode <= 3; ++fortify_mode)
+    {
+      do_check ("0", "%d", 0);
+      do_check ("-2147483648", "%d", -2147483647 - 1);
+      do_check ("-9223372036854775808", "%lld", -9223372036854775807LL - 1);
+      do_check ("", "%s", "");
+      do_check ("                      ", "%22s", "");
+      do_check ("XXXXXXXXXXXXXXXXXXXXXX", "%s", "XXXXXXXXXXXXXXXXXXXXXX");
+      do_check ("1.125000", "%f", 1.125);
+      do_check ("1.125", "%g", 1.125);
+      do_check ("1.125", "%.8g", 1.125);
+    }
+}
+
+/* printf callback that falls back to the glibc-supplied
+   implementation.  */
+static int
+dummy_printf_function (FILE *__stream,
+                       const struct printf_info *__info,
+                       const void *const *__args)
+{
+  return -2;                    /* Request fallback.  */
+}
+
+/* Likewise for the type information.  */
+static int
+dummy_arginfo_function (const struct printf_info *info,
+                        size_t n, int *argtypes, int *size)
+{
+  return -1;                    /* Request fallback.  */
+}
+
+static int
+do_test (void)
+{
+  do_tests ();
+
+  /* Activate __printf_function_invoke mode.  */
+  register_printf_specifier ('d', dummy_printf_function,
+                             dummy_arginfo_function);
+  register_printf_specifier ('g', dummy_printf_function,
+                             dummy_arginfo_function);
+  register_printf_specifier ('s', dummy_printf_function,
+                             dummy_arginfo_function);
+
+  /* Rerun the tests with callback functions.  */
+  do_tests ();
+
+  return 0;
+}
+
+#include <support/test-driver.c>

include/printf_buffer.h

@@ -115,17 +115,26 @@ __printf_buffer_has_failed (struct __printf_buffer *buf)
   return buf->mode == __printf_buffer_mode_failed;
 }
 
+/* Initialization of a buffer, using the memory region from [BASE,
+   END) as the initial buffer contents.  */
+static inline void
+__printf_buffer_init_end (struct __printf_buffer *buf, char *base, char *end,
+                          enum __printf_buffer_mode mode)
+{
+  buf->write_base = base;
+  buf->write_ptr = base;
+  buf->write_end = end;
+  buf->written = 0;
+  buf->mode = mode;
+}
+
 /* Initialization of a buffer, using the memory region from [BASE, BASE +LEN)
    as the initial buffer contents.  LEN can be zero.  */
 static inline void
 __printf_buffer_init (struct __printf_buffer *buf, char *base, size_t len,
                       enum __printf_buffer_mode mode)
 {
-  buf->write_base = base;
+  __printf_buffer_init_end (buf, base, base + len, mode);
-  buf->write_ptr = base;
-  buf->write_end = base + len;
-  buf->written = 0;
-  buf->mode = mode;
 }
 
 /* Called by printf_buffer_putc for a full buffer.  */

libio/iovsprintf.c

@@ -45,14 +45,19 @@ __vsprintf_internal (char *string, size_t maxlen,
   if ((mode_flags & PRINTF_CHK) != 0)
     {
       string[0] = '\0';
-      __printf_buffer_init (&buf, string, maxlen,
+      /* In some cases, __sprintf_chk is called with an unknown buffer
+	 size (the special value -1).  Prevent pointer wraparound in
+	 this case and saturate to the end of the address space.  */
+      uintptr_t end;
+      if (__builtin_add_overflow ((uintptr_t) string, maxlen, &end))
+	end = -1;
+      __printf_buffer_init_end (&buf, string, (char *) end,
 			    __printf_buffer_mode_sprintf_chk);
     }
   else
-    {
+    /* Use end of address space.  */
-      __printf_buffer_init (&buf, string, 0, __printf_buffer_mode_sprintf);
+    __printf_buffer_init_end (&buf, string, (char *) ~(uintptr_t) 0,
-      buf.write_end = (char *) ~(uintptr_t) 0; /* End of address space.  */
+			      __printf_buffer_mode_sprintf);
-    }
 
   __printf_buffer (&buf, format, args, mode_flags);