[PATCH] free: preserve errno [BZ#17924]
Paul Eggert
eggert@cs.ucla.edu
Sun Dec 20 20:25:56 GMT 2020
* malloc/Makefile (tests): Add tst-free-errno.
* malloc/malloc.c (tcache_init): Preserve errno when initializing,
since 'free' might be calling us.
(__libc_free): Preserve errno when calling munmap.
* malloc/tst-free-errno.c: New file, almost all from Bruno Haible.
* manual/memory.texi (Freeing after Malloc, Replacing malloc):
Document that free preserves errno.
---
malloc/Makefile | 1 +
malloc/malloc.c | 6 ++
malloc/tst-free-errno.c | 169 ++++++++++++++++++++++++++++++++++++++++
manual/memory.texi | 9 +++
4 files changed, 185 insertions(+)
create mode 100644 malloc/tst-free-errno.c
diff --git a/malloc/Makefile b/malloc/Makefile
index ab64dcfd73..4b3975f90d 100644
--- a/malloc/Makefile
+++ b/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 \
diff --git a/malloc/malloc.c b/malloc/malloc.c
index 326075e704..14bc55f96d 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -3003,6 +3003,8 @@ tcache_init(void)
if (tcache_shutting_down)
return;
+ int err = errno;
+
arena_get (ar_ptr, bytes);
victim = _int_malloc (ar_ptr, bytes);
if (!victim && ar_ptr != NULL)
@@ -3015,6 +3017,8 @@ tcache_init(void)
if (ar_ptr != NULL)
__libc_lock_unlock (ar_ptr->mutex);
+ __set_errno (err);
+
/* In a low memory situation, we may not be able to allocate memory
- in which case, we just keep trying later. However, we
typically do this very early, so either there is sufficient
@@ -3140,7 +3144,9 @@ __libc_free (void *mem)
LIBC_PROBE (memory_mallopt_free_dyn_thresholds, 2,
mp_.mmap_threshold, mp_.trim_threshold);
}
+ int err = errno;
munmap_chunk (p);
+ __set_errno (err);
return;
}
diff --git a/malloc/tst-free-errno.c b/malloc/tst-free-errno.c
new file mode 100644
index 0000000000..6243cb6e0b
--- /dev/null
+++ b/malloc/tst-free-errno.c
@@ -0,0 +1,169 @@
+/* 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/>. */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2020. */
+
+#include <errno.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#if defined __linux__
+# include <fcntl.h>
+# include <stdint.h>
+# include <string.h>
+# include <sys/mman.h>
+#endif
+
+#define ASSERT_NO_STDIO(expr) \
+ do \
+ { \
+ if (!(expr)) \
+ { \
+ WRITE_TO_STDERR (__FILE__); \
+ WRITE_TO_STDERR (":"); \
+ WRITE_MACROEXPANDED_INTEGER_TO_STDERR (__LINE__); \
+ WRITE_TO_STDERR (": assertion '"); \
+ WRITE_TO_STDERR (#expr); \
+ WRITE_TO_STDERR ("' failed\n"); \
+ abort (); \
+ } \
+ } \
+ while (0)
+#define WRITE_MACROEXPANDED_INTEGER_TO_STDERR(integer) \
+ WRITE_INTEGER_TO_STDERR(integer)
+#define WRITE_INTEGER_TO_STDERR(integer) \
+ WRITE_TO_STDERR (#integer)
+#define WRITE_TO_STDERR(string_literal) \
+ { \
+ const char *s = string_literal; \
+ int ret = write (2, s, strlen (s)); \
+ (void) ret; \
+ }
+
+/* The indirection through a volatile function pointer is necessary to prevent
+ a GCC optimization. Without it, when optimizing, GCC would "know" that errno
+ is unchanged by calling free(ptr), when ptr was the result of a malloc(...)
+ call in the same function. */
+static int
+get_errno (void)
+{
+ volatile int err = errno;
+ return err;
+}
+
+static int (* volatile get_errno_func) (void) = get_errno;
+
+static int
+do_test (void)
+{
+ /* Check that free() preserves errno. */
+ {
+ errno = 1789; /* Liberté, égalité, fraternité. */
+ free (NULL);
+ ASSERT_NO_STDIO (get_errno_func () == 1789);
+ }
+ { /* Large memory allocations. */
+ #define N 2
+ void * volatile ptrs[N];
+ size_t i;
+ for (i = 0; i < N; i++)
+ ptrs[i] = malloc (5318153);
+ for (i = 0; i < N; i++)
+ {
+ errno = 1789;
+ free (ptrs[i]);
+ ASSERT_NO_STDIO (get_errno_func () == 1789);
+ }
+ #undef N
+ }
+
+ /* 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. This file contains the limit
+ - for Linux >= 2.4.19: 65536 (DEFAULT_MAX_MAP_COUNT in linux/include/linux/sched.h)
+ - for Linux >= 2.6.31: 65530 (DEFAULT_MAX_MAP_COUNT in linux/include/linux/mm.h).
+ */
+ #if defined __linux__
+ if (open ("/proc/sys/vm/max_map_count", O_RDONLY) >= 0)
+ {
+ /* Preparations. */
+ size_t pagesize = getpagesize ();
+ void *firstpage_backup = malloc (pagesize);
+ void *lastpage_backup = malloc (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. */
+ void *bumper_region =
+ mmap (NULL, 0x1000000, PROT_READ, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+ /* A file descriptor pointing to a regular file. */
+ int fd = open ("/etc/hosts", O_RDONLY);
+
+ if (firstpage_backup != NULL && lastpage_backup != NULL
+ && bumper_region != (void *)(-1)
+ && fd >= 0)
+ {
+ /* Do a large memory allocation. */
+ size_t big_size = 0x1000000;
+ void * volatile ptr = malloc (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);
+ if (mmap (ptr_aligned - pagesize, pagesize + big_size + pagesize,
+ PROT_READ | PROT_WRITE,
+ MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, -1, 0)
+ != (void *)(-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). */
+ size_t i;
+ for (i = 0; i < 65536; i++)
+ if (mmap (NULL, pagesize, PROT_READ, MAP_FILE | MAP_PRIVATE, fd, 0)
+ == (void *)(-1))
+ 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);
+ ASSERT_NO_STDIO (get_errno_func () == 1789);
+ }
+ }
+ }
+ #endif
+
+ return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/manual/memory.texi b/manual/memory.texi
index c132261084..b2cc65228a 100644
--- a/manual/memory.texi
+++ b/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.
--
2.29.2
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