Enable support for GCC 11 -Wmismatched-dealloc.

To help detect common kinds of memory (and other resource) management
bugs, GCC 11 adds support for the detection of mismatched calls to
allocation and deallocation functions.  At each call site to a known
deallocation function GCC checks the set of allocation functions
the former can be paired with and, if the two don't match, issues
a -Wmismatched-dealloc warning (something similar happens in C++
for mismatched calls to new and delete).  GCC also uses the same
mechanism to detect attempts to deallocate objects not allocated
by any allocation function (or pointers past the first byte into
allocated objects) by -Wfree-nonheap-object.

This support is enabled for built-in functions like malloc and free.
To extend it beyond those, GCC extends attribute malloc to designate
a deallocation function to which pointers returned from the allocation
function may be passed to deallocate the allocated objects.  Another,
optional argument designates the positional argument to which
the pointer must be passed.

This change is the first step in enabling this extended support for
Glibc.

3 files changed, 95 insertions, 6 deletions

diff --git a/stdlib/Makefile b/stdlib/Makefile
index f5755a1654..ec30011b4c 100644
--- a/stdlib/Makefile
+++ b/stdlib/Makefile
@@ -87,7 +87,8 @@ tests		:= tst-strtol tst-strtod testmb testrand testsort testdiv   \
 		   tst-makecontext-align test-bz22786 tst-strtod-nan-sign \
 		   tst-swapcontext1 tst-setcontext4 tst-setcontext5 \
 		   tst-setcontext6 tst-setcontext7 tst-setcontext8 \
-		   tst-setcontext9 tst-bz20544 tst-canon-bz26341
+		   tst-setcontext9 tst-bz20544 tst-canon-bz26341 \
+		   tst-realpath
 
 tests-internal	:= tst-strtod1i tst-strtod3 tst-strtod4 tst-strtod5i \
 		   tst-tls-atexit tst-tls-atexit-nodelete
diff --git a/stdlib/stdlib.h b/stdlib/stdlib.h
index 6360845d98..0481c12355 100644
--- a/stdlib/stdlib.h
+++ b/stdlib/stdlib.h
@@ -550,6 +550,9 @@ extern void *calloc (size_t __nmemb, size_t __size)
 extern void *realloc (void *__ptr, size_t __size)
      __THROW __attribute_warn_unused_result__ __attribute_alloc_size__ ((2));
 
+/* Free a block allocated by `malloc', `realloc' or `calloc'.  */
+extern void free (void *__ptr) __THROW;
+
 #ifdef __USE_MISC
 /* Re-allocate the previously allocated block in PTR, making the new
    block large enough for NMEMB elements of SIZE bytes each.  */
@@ -558,11 +561,13 @@ extern void *realloc (void *__ptr, size_t __size)
    between objects pointed by the old and new pointers.  */
 extern void *reallocarray (void *__ptr, size_t __nmemb, size_t __size)
      __THROW __attribute_warn_unused_result__
-     __attribute_alloc_size__ ((2, 3));
-#endif
+     __attribute_alloc_size__ ((2, 3))
+    __attr_dealloc_free;
 
-/* Free a block allocated by `malloc', `realloc' or `calloc'.  */
-extern void free (void *__ptr) __THROW;
+/* Add reallocarray as its own deallocator.  */
+extern void *reallocarray (void *__ptr, size_t __nmemb, size_t __size)
+     __THROW __attr_dealloc (reallocarray, 1);
+#endif
 
 #ifdef __USE_MISC
 # include <alloca.h>
@@ -788,7 +793,8 @@ extern int system (const char *__command) __wur;
 /* Return a malloc'd string containing the canonical absolute name of the
    existing named file.  */
 extern char *canonicalize_file_name (const char *__name)
-     __THROW __nonnull ((1)) __wur;
+     __THROW __nonnull ((1)) __attribute_malloc__
+     __attr_dealloc_free __wur;
 #endif
 
 #if defined __USE_MISC || defined __USE_XOPEN_EXTENDED
diff --git a/stdlib/tst-realpath.c b/stdlib/tst-realpath.c
new file mode 100644
index 0000000000..2ae5e4fb2b
--- /dev/null
+++ b/stdlib/tst-realpath.c
@@ -0,0 +1,82 @@
+/* Test to verify that realpath() doesn't cause false positives due
+   to GCC attribute malloc.
+
+   Test failure exposes the presence of the attribute in the following
+   declaration:
+
+   __attribute__ ((__malloc__ (free, 1))) char*
+   realpath (const char *, char *);
+
+   Copyright (C) 2021 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 <stdio.h>
+#include <stdlib.h>
+#include <malloc.h>
+
+#if defined __GNUC__ && __GNUC__ >= 11
+/* Turn GCC -Wmismatched-dealloc warnings into errors to expose false
+   positives.  */
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic error "-Wmismatched-dealloc"
+
+/* Associate dealloc as the only deallocator suitable for pointers
+   returned from alloc.
+   GCC automatically disables inlining of allocator and deallocator
+   functions marked with the argument form of attribute malloc but
+   it doesn't hurt to disable it explicitly.  */
+__attribute  ((noipa)) void dealloc (void *);
+__attribute ((malloc (dealloc, 1))) char* alloc (void);
+#endif
+
+void dealloc (void *p)
+{
+  free (p);
+}
+
+char* alloc (void)
+{
+  return (char *)malloc (8);
+}
+
+static int
+do_test (void)
+{
+  char *resolved_path = alloc ();
+  char *ret = realpath ("/", resolved_path);
+  dealloc (ret);
+
+  resolved_path = alloc ();
+  ret = realpath ("/", resolved_path);
+  dealloc (resolved_path);
+
+  /* The following should emit a warning (but doesn't with GCC 11):
+     resolved_path = alloc ();
+     ret = realpath ("/", resolved_path);
+     free (ret);   // expect -Wmismatched-dealloc
+  */
+
+  return 0;
+}
+
+#if defined __GNUC__ && __GNUC__ >= 11
+/* Restore -Wmismatched-dealloc setting.  */
+#  pragma GCC diagnostic pop
+#endif
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"