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authorPaul Eggert <eggert@cs.ucla.edu>2020-12-29 00:45:49 -0800
committerPaul Eggert <eggert@cs.ucla.edu>2020-12-29 00:46:46 -0800
commit69fda43b8dd795c3658869633ca0708ed3134006 (patch)
treee86b216293d450bfbecaaf8230c3247444b668c4
parent016c64236dee6e28f09c10ba38f274aad7205f95 (diff)
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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>
-rw-r--r--malloc/Makefile1
-rw-r--r--malloc/malloc.c13
-rw-r--r--malloc/tst-free-errno.c131
-rw-r--r--manual/memory.texi9
4 files changed, 150 insertions, 4 deletions
diff --git a/malloc/Makefile b/malloc/Makefile
index 37173b29ea..39ffaa7161 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 a3e914fa8a..3b151f44f7 100644
--- a/malloc/malloc.c
+++ b/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 ();
- MAYBE_INIT_TCACHE ();
+ ar_ptr = arena_for_chunk (p);
+ _int_free (ar_ptr, p, 0);
+ }
- ar_ptr = arena_for_chunk (p);
- _int_free (ar_ptr, p, 0);
+ __set_errno (err);
}
libc_hidden_def (__libc_free)
diff --git a/malloc/tst-free-errno.c b/malloc/tst-free-errno.c
new file mode 100644
index 0000000000..7781c62bff
--- /dev/null
+++ b/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>
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.