free: preserve errno [BZ#17924]

In the next release of POSIX, free must preserve errno
<https://www.austingroupbugs.net/view.php?id=385>.
Modify __libc_free to save and restore errno, so that
any internal munmap etc. syscalls do not disturb the caller's errno.
Add a test malloc/tst-free-errno.c (almost all by Bruno Haible),
and document that free preserves errno.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

malloc/Makefile

@@ -34,6 +34,7 @@ tests := mallocbug tst-malloc tst-valloc tst-calloc tst-obstack \
 	 tst-interpose-nothread \
 	 tst-interpose-thread \
 	 tst-alloc_buffer \
+	 tst-free-errno \
 	 tst-malloc-tcache-leak \
 	 tst-malloc_info tst-mallinfo2 \
 	 tst-malloc-too-large \

malloc/malloc.c

@@ -3278,6 +3278,8 @@ __libc_free (void *mem)
   *(volatile char *)mem;
 #endif
 
+  int err = errno;
+
   p = mem2chunk (mem);
 
   /* Mark the chunk as belonging to the library again.  */
@@ -3298,13 +3300,16 @@ __libc_free (void *mem)
                       mp_.mmap_threshold, mp_.trim_threshold);
         }
       munmap_chunk (p);
-      return;
+    }
+  else
+    {
+      MAYBE_INIT_TCACHE ();
+
+      ar_ptr = arena_for_chunk (p);
+      _int_free (ar_ptr, p, 0);
     }
 
-  MAYBE_INIT_TCACHE ();
-
-  ar_ptr = arena_for_chunk (p);
-  _int_free (ar_ptr, p, 0);
+  __set_errno (err);
 }
 libc_hidden_def (__libc_free)
 

malloc/tst-free-errno.c

@@ -0,0 +1,131 @@
+/* Test that free preserves errno.
+   Copyright (C) 2020 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 <errno.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <stdint.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/temp_file.h>
+#include <support/xunistd.h>
+
+/* The __attribute__ ((weak)) prevents a GCC optimization.  Without
+   it, GCC would "know" that errno is unchanged by calling free (ptr),
+   when ptr was the result of a malloc call in the same function.  */
+int __attribute__ ((weak))
+get_errno (void)
+{
+  return errno;
+}
+
+static int
+do_test (void)
+{
+  /* Check that free() preserves errno.  */
+  {
+    errno = 1789; /* Liberté, égalité, fraternité.  */
+    free (NULL);
+    TEST_VERIFY (get_errno () == 1789);
+  }
+  { /* Large memory allocations, to force mmap.  */
+    enum { N = 2 };
+    void * volatile ptrs[N];
+    size_t i;
+    for (i = 0; i < N; i++)
+      ptrs[i] = xmalloc (5318153);
+    for (i = 0; i < N; i++)
+      {
+        errno = 1789;
+        free (ptrs[i]);
+        TEST_VERIFY (get_errno () == 1789);
+      }
+  }
+
+  /* Test a less common code path.
+     When malloc() is based on mmap(), free() can sometimes call munmap().
+     munmap() usually succeeds, but fails in a particular situation: when
+       - it has to unmap the middle part of a VMA, and
+       - the number of VMAs of a process is limited and the limit is
+         already reached.
+     The latter condition is fulfilled on Linux, when the file
+     /proc/sys/vm/max_map_count exists.  For all known Linux versions
+     the default limit is at most 65536.
+   */
+  #if defined __linux__
+  if (xopen ("/proc/sys/vm/max_map_count", O_RDONLY, 0) >= 0)
+    {
+      /* Preparations.  */
+      size_t pagesize = getpagesize ();
+      void *firstpage_backup = xmalloc (pagesize);
+      void *lastpage_backup = xmalloc (pagesize);
+      /* Allocate a large memory area, as a bumper, so that the MAP_FIXED
+         allocation later will not overwrite parts of the memory areas
+         allocated to ld.so or libc.so.  */
+      xmmap (NULL, 0x1000000, PROT_READ, MAP_ANONYMOUS | MAP_PRIVATE, -1);
+      /* A file descriptor pointing to a regular file.  */
+      int fd = create_temp_file ("tst-free-errno", NULL);
+      if (fd < 0)
+	FAIL_EXIT1 ("cannot create temporary file");
+
+      /* Do a large memory allocation.  */
+      size_t big_size = 0x1000000;
+      void * volatile ptr = xmalloc (big_size - 0x100);
+      char *ptr_aligned = (char *) ((uintptr_t) ptr & ~(pagesize - 1));
+      /* This large memory allocation allocated a memory area
+	 from ptr_aligned to ptr_aligned + big_size.
+	 Enlarge this memory area by adding a page before and a page
+	 after it.  */
+      memcpy (firstpage_backup, ptr_aligned, pagesize);
+      memcpy (lastpage_backup, ptr_aligned + big_size - pagesize,
+	      pagesize);
+      xmmap (ptr_aligned - pagesize, pagesize + big_size + pagesize,
+	     PROT_READ | PROT_WRITE,
+	     MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, -1);
+      memcpy (ptr_aligned, firstpage_backup, pagesize);
+      memcpy (ptr_aligned + big_size - pagesize, lastpage_backup,
+	      pagesize);
+
+      /* Now add as many mappings as we can.
+	 Stop at 65536, in order not to crash the machine (in case the
+	 limit has been increased by the system administrator).  */
+      for (int i = 0; i < 65536; i++)
+	if (mmap (NULL, pagesize, PROT_READ, MAP_FILE | MAP_PRIVATE, fd, 0)
+	    == MAP_FAILED)
+	  break;
+      /* Now the number of VMAs of this process has hopefully attained
+	 its limit.  */
+
+      errno = 1789;
+      /* This call to free() is supposed to call
+	   munmap (ptr_aligned, big_size);
+	 which increases the number of VMAs by 1, which is supposed
+	 to fail.  */
+      free (ptr);
+      TEST_VERIFY (get_errno () == 1789);
+    }
+  #endif
+
+  return 0;
+}
+
+#include <support/test-driver.c>

manual/memory.texi

@@ -738,6 +738,12 @@ later call to @code{malloc} to reuse the space.  In the meantime, the
 space remains in your program as part of a free-list used internally by
 @code{malloc}.
 
+The @code{free} function preserves the value of @code{errno}, so that
+cleanup code need not worry about saving and restoring @code{errno}
+around a call to @code{free}.  Although neither @w{ISO C} nor
+POSIX.1-2017 requires @code{free} to preserve @code{errno}, a future
+version of POSIX is planned to require it.
+
 There is no point in freeing blocks at the end of a program, because all
 of the program's space is given back to the system when the process
 terminates.
@@ -1935,6 +1941,9 @@ linking against @code{libc.a} (explicitly or implicitly).
 functions (that is, all the functions used by the application,
 @theglibc{}, and other linked-in libraries) can lead to static linking
 failures, and, at run time, to heap corruption and application crashes.
+Replacement functions should implement the behavior documented for
+their counterparts in @theglibc{}; for example, the replacement
+@code{free} should also preserve @code{errno}.
 
 The minimum set of functions which has to be provided by a custom
 @code{malloc} is given in the table below.