free: preserve errno [BZ#17924]
Commit Message
* 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
Comments
On 12/21/20 1:55 AM, Paul Eggert wrote:
> * 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;
> }
The _int_free call after this may trigger a trim, which could result in
an mmap, madvise or brk, all of which set errno. In practice I think
only mmap may do this but it's reason enough to perhaps just save errno
at the top and restore it on exit.
The mmap is called when /proc/sys/vm/overcommit_memory has 2.
>
> 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. */
We don't write these anymore.
> +
> +#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.
>
On 12/20/20 3:25 PM, Paul Eggert wrote:
> * 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.
Please provide a reasonable commit message.
On 12/20/20 8:27 PM, Carlos O'Donell wrote:
> Please provide a reasonable commit message.
Thanks, unfortunately I slipped up and used the old-style commits still
common in other projects. Revised patch attached. This revision also
addresses Siddhesh's comments (he found another way errno could be munged).
On 12/21/20 12:50 PM, Paul Eggert wrote:
> From e4fec6f28270c9b0979f9fe8920dc52f8f7d70cd Mon Sep 17 00:00:00 2001
> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Sat, 19 Dec 2020 12:52:09 -0800
> Subject: [PATCH] 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.
Thanks, this version looks good to me.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
* Paul Eggert:
> +/* 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)
Long line (80 characters).
I think you need to use this:
int __attribute__ ((weak))
get_errno (void)
{
return errno;
}
With that volatile construct, the read of errno is not actually behind a
compiler barrier.
Thanks,
Florian
On 12/21/20 3:03 PM, Florian Weimer via Libc-alpha wrote:
> * Paul Eggert:
>
>> +/* 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)
>
> Long line (80 characters).
>
> I think you need to use this:
>
> int __attribute__ ((weak))
> get_errno (void)
> {
> return errno;
> }
>
> With that volatile construct, the read of errno is not actually behind a
> compiler barrier.
Wouldn't it be a bug in the compiler if it delays load of the TLS var
beyond a function call boundary?
Siddhesh
On 12/21/20 3:33 PM, Siddhesh Poyarekar wrote:
> Wouldn't it be a bug in the compiler if it delays load of the TLS var
> beyond a function call boundary?
Sorry scratch that, I wasn't thinking straight.
Siddhesh
Thanks for the comment about the test case's compiler barrier, and about the
test case's too-long lines. I fixed those, and also fixed the BZ# in the
Subject line. Revised patch attached.
On 23/12/2020 02:30, Paul Eggert wrote:
> Thanks for the comment about the test case's compiler barrier, and about the test case's too-long lines. I fixed those, and also fixed the BZ# in the Subject line. Revised patch attached.
> From a994c467ce04abfe102809812156cb30810eaa95 Mon Sep 17 00:00:00 2001
> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Tue, 22 Dec 2020 21:17:20 -0800
> Subject: [PATCH] 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.
> ---
> malloc/Makefile | 1 +
> malloc/malloc.c | 13 +++-
> malloc/tst-free-errno.c | 168 ++++++++++++++++++++++++++++++++++++++++
> manual/memory.texi | 9 +++
> 4 files changed, 187 insertions(+), 4 deletions(-)
> 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 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)
>
I am not very found on adding a errno set/restore on *every* free, specially
because it is extra overhead on small allocations that won't touch mmap/munmap
(specially for tcache case).
Fixing in a more fine grained would require a lot of more work to check if
the shared routines that calls mmap, madvise or brk won't interfere with other
symbols; so maybe it should be ok to use this large hammer for now.
> diff --git a/malloc/tst-free-errno.c b/malloc/tst-free-errno.c
> new file mode 100644
> index 0000000000..a612bbb3bf
> --- /dev/null
> +++ b/malloc/tst-free-errno.c
> @@ -0,0 +1,168 @@
> +/* 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>
> +#if defined __linux__
> +# include <fcntl.h>
> +# include <stdint.h>
> +# include <string.h>
> +# include <sys/mman.h>
> +#endif
Why do you need to include this solely for Linux? I think all of the headers
should be provided by glibc on all supported systems.
> +
> +#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; \
> + }
Why can't you use support/check.h instead?
> +
> +/* 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);
> + ASSERT_NO_STDIO (get_errno () == 1789);
> + }
> + { /* Large memory allocations. */
Maybe add a comment that it is forcing a mmap with the large allocation.
> + #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 () == 1789);
> + }
> + #undef N
> + }
No need to set/unset N here.
> +
> + /* 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).
I think there is no need to reference old and unsupported kernel such as
2.4.
> + */
> + #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);
Use xmalloc here, so there is no need to check if the allocation has succeded
(they are small enough we can assume a failure means something wrong).
> + /* 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. */
Use xmmap.
> + int fd = open ("/etc/hosts", O_RDONLY);
> +
I think it is better to use a temporary file from support/create_temp_file.h.
> + 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);
User xmalloc.
> + 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))
Use xmmap.
> + {
> + 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 () == 1789);
> + }
> + }
> + }
> + #endif
> +
> + return 0;
> +}
> +
> +#include <support/test-driver.c>
Ok.
> 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.
Not sure if this is worth to add, since we will need to update the manual
once the POSIX does require it.
> @@ -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
Thanks for the comments about the patch's test case. I modified the test
case to reflect nearly all the comments, resulting in the attached revised
patch. I'm replying below only to the comments that didn't result in a
change to the patch.
On 12/23/20 11:19 AM, Adhemerval Zanella wrote:
> Fixing in a more fine grained would require a lot of more work to check if
> the shared routines that calls mmap, madvise or brk won't interfere with other
> symbols; so maybe it should be ok to use this large hammer for now.
Yes, that was my thought as well.
>> +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.
...
> Not sure if this is worth to add, since we will need to update the manual
> once the POSIX does require it.
I'll volunteer to update the manual. :-)
It's worth mentioning that preserving errno is not something that portable C
or POSIX code should assume for 'free'. If there's a better way for the
manual to warn its readers about this, that'd be fine of course.
I did consider changing "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" to "Neither @w{ISO C} nor POSIX requires @code{free}
to preserve @code{errno}", but that wording would be less informative and
would still need updating once POSIX does require 'free' to preserve errno.
On 23/12/2020 22:03, Paul Eggert wrote:
> Thanks for the comments about the patch's test case. I modified the test case to reflect nearly all the comments, resulting in the attached revised patch. I'm replying below only to the comments that didn't result in a change to the patch.
>
> On 12/23/20 11:19 AM, Adhemerval Zanella wrote:
>
>> Fixing in a more fine grained would require a lot of more work to check if
>> the shared routines that calls mmap, madvise or brk won't interfere with other
>> symbols; so maybe it should be ok to use this large hammer for now.
>
> Yes, that was my thought as well.
I will try to revise this for 2.33.
>
>>> +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.
> ...
>> Not sure if this is worth to add, since we will need to update the manual
>> once the POSIX does require it.
>
> I'll volunteer to update the manual. :-)
>
> It's worth mentioning that preserving errno is not something that portable C or POSIX code should assume for 'free'. If there's a better way for the manual to warn its readers about this, that'd be fine of course.
>
> I did consider changing "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" to "Neither @w{ISO C} nor POSIX requires @code{free} to preserve @code{errno}", but that wording would be less informative and would still need updating once POSIX does require 'free' to preserve errno.
Thanks.
> From afbf4ff042cf3a5c8f983d5aa3bd0de3fb696dd3 Mon Sep 17 00:00:00 2001
> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Wed, 23 Dec 2020 11:27:25 -0800
> Subject: [PATCH] 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.
LGTM with the small test change below.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> ---
> malloc/Makefile | 1 +
> malloc/malloc.c | 13 ++--
> malloc/tst-free-errno.c | 131 ++++++++++++++++++++++++++++++++++++++++
> manual/memory.texi | 9 +++
> 4 files changed, 150 insertions(+), 4 deletions(-)
> 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 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..89629751e8
> --- /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] = malloc (5318153);
Use xmalloc here.
> + 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 (open ("/proc/sys/vm/max_map_count", O_RDONLY) >= 0)
I think we can assume for tests /proc should be mounted, otherwise this
only partially test this interface. So I think we can use xopen here.
> + {
> + /* 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>
Ok.
> 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.
Ok.
> @@ -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
Ok.
On Mon, Dec 28, 2020 at 11:24 AM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
>
>
> On 23/12/2020 22:03, Paul Eggert wrote:
> > Thanks for the comments about the patch's test case. I modified the test case to reflect nearly all the comments, resulting in the attached revised patch. I'm replying below only to the comments that didn't result in a change to the patch.
> >
> > On 12/23/20 11:19 AM, Adhemerval Zanella wrote:
> >
> >> Fixing in a more fine grained would require a lot of more work to check if
> >> the shared routines that calls mmap, madvise or brk won't interfere with other
> >> symbols; so maybe it should be ok to use this large hammer for now.
> >
> > Yes, that was my thought as well.
>
> I will try to revise this for 2.33.
>
> >
> >>> +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.
> > ...
> >> Not sure if this is worth to add, since we will need to update the manual
> >> once the POSIX does require it.
> >
> > I'll volunteer to update the manual. :-)
> >
> > It's worth mentioning that preserving errno is not something that portable C or POSIX code should assume for 'free'. If there's a better way for the manual to warn its readers about this, that'd be fine of course.
> >
> > I did consider changing "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" to "Neither @w{ISO C} nor POSIX requires @code{free} to preserve @code{errno}", but that wording would be less informative and would still need updating once POSIX does require 'free' to preserve errno.
>
> Thanks.
>
> > From afbf4ff042cf3a5c8f983d5aa3bd0de3fb696dd3 Mon Sep 17 00:00:00 2001
> > From: Paul Eggert <eggert@cs.ucla.edu>
> > Date: Wed, 23 Dec 2020 11:27:25 -0800
> > Subject: [PATCH] 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.
>
> LGTM with the small test change below.
>
> Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
>
On a machine with 192 GB RAM, I got
[hjl@gnu-clx-1 build-x86_64-linux]$ cat malloc/tst-free-errno-mcheck.out
error: tst-free-errno.c:124: not true: get_errno () == 1789
error: 1 test failures
[hjl@gnu-clx-1 build-x86_64-linux]$
On 29/12/2020 10:38, H.J. Lu wrote:
> On Mon, Dec 28, 2020 at 11:24 AM Adhemerval Zanella via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
>>
>>
>>
>> On 23/12/2020 22:03, Paul Eggert wrote:
>>> Thanks for the comments about the patch's test case. I modified the test case to reflect nearly all the comments, resulting in the attached revised patch. I'm replying below only to the comments that didn't result in a change to the patch.
>>>
>>> On 12/23/20 11:19 AM, Adhemerval Zanella wrote:
>>>
>>>> Fixing in a more fine grained would require a lot of more work to check if
>>>> the shared routines that calls mmap, madvise or brk won't interfere with other
>>>> symbols; so maybe it should be ok to use this large hammer for now.
>>>
>>> Yes, that was my thought as well.
>>
>> I will try to revise this for 2.33.
>>
>>>
>>>>> +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.
>>> ...
>>>> Not sure if this is worth to add, since we will need to update the manual
>>>> once the POSIX does require it.
>>>
>>> I'll volunteer to update the manual. :-)
>>>
>>> It's worth mentioning that preserving errno is not something that portable C or POSIX code should assume for 'free'. If there's a better way for the manual to warn its readers about this, that'd be fine of course.
>>>
>>> I did consider changing "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" to "Neither @w{ISO C} nor POSIX requires @code{free} to preserve @code{errno}", but that wording would be less informative and would still need updating once POSIX does require 'free' to preserve errno.
>>
>> Thanks.
>>
>>> From afbf4ff042cf3a5c8f983d5aa3bd0de3fb696dd3 Mon Sep 17 00:00:00 2001
>>> From: Paul Eggert <eggert@cs.ucla.edu>
>>> Date: Wed, 23 Dec 2020 11:27:25 -0800
>>> Subject: [PATCH] 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.
>>
>> LGTM with the small test change below.
>>
>> Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
>>
>
> On a machine with 192 GB RAM, I got
>
> [hjl@gnu-clx-1 build-x86_64-linux]$ cat malloc/tst-free-errno-mcheck.out
> error: tst-free-errno.c:124: not true: get_errno () == 1789
> error: 1 test failures
> [hjl@gnu-clx-1 build-x86_64-linux]$
>
We need to fix it for the malloc check hooks as well:
diff --git a/malloc/hooks.c b/malloc/hooks.c
index 6474ba8b38..336ff497e9 100644
--- a/malloc/hooks.c
+++ b/malloc/hooks.c
@@ -260,6 +260,8 @@ free_check (void *mem, const void *caller)
if (!mem)
return;
+ int err = errno;
+
#ifdef USE_MTAG
/* Quickly check that the freed pointer matches the tag for the memory.
This gives a useful double-free detection. */
@@ -274,12 +276,16 @@ free_check (void *mem, const void *caller)
{
__libc_lock_unlock (main_arena.mutex);
munmap_chunk (p);
- return;
}
- /* Mark the chunk as belonging to the library again. */
- (void)TAG_REGION (chunk2rawmem (p), CHUNK_AVAILABLE_SIZE (p) - CHUNK_HDR_SZ);
- _int_free (&main_arena, p, 1);
- __libc_lock_unlock (main_arena.mutex);
+ else
+ {
+ /* Mark the chunk as belonging to the library again. */
+ (void)TAG_REGION (chunk2rawmem (p), CHUNK_AVAILABLE_SIZE (p)
+ - CHUNK_HDR_SZ);
+ _int_free (&main_arena, p, 1);
+ __libc_lock_unlock (main_arena.mutex);
+ }
+ __set_errno (err);
}
static void *
@@ -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 \
@@ -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;
}
new file mode 100644
@@ -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>
@@ -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.