If ashmem is not available, fall back to wrapping vanilla SYSV shared memory

.gitignore

@@ -2,3 +2,4 @@
 *.so
 tags
 test/test-*
+.vscode

Makefile

@@ -1,11 +1,11 @@
 CFLAGS += -fpic -shared -std=c11 -Wall -Wextra
 LDFLAGS += -Wl,--version-script=exports.txt
 
-libandroid-shmem.a: shmem.o
-	$(AR) rcu $@ shmem.o
+libandroid-shmem.a: shmem.o shmem_vanilla.o
+	$(AR) rcu $@ shmem.o shmem_vanilla.o
 
-libandroid-shmem.so: shmem.o
-	$(CC) $(LDFLAGS) -shared shmem.o -o $@ -llog -landroid
+libandroid-shmem.so: shmem.o shmem_vanilla.o
+	$(CC) $(LDFLAGS) -shared shmem.o shmem_vanilla.o -o $@ -llog -landroid
 
 shmem.o: shmem.c shm.h
 	$(CC) $(CFLAGS) -c shmem.c -o $@

shm_vanilla.h

@@ -0,0 +1,51 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/** fork of https://android.googlesource.com/platform/bionic/+/02ce401d1e2b31586c94c8b6999364bbbce27388/libc/include/sys/shm.h */
+
+#pragma once
+
+#include <sys/cdefs.h>
+#include <sys/ipc.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <linux/shm.h>
+
+__BEGIN_DECLS
+
+/** Not useful on normal Android; disallowed by SELinux. */
+/** useful for partial Android userspace components running inside the unmodified kernel of a */
+/** different operating system that doesn't have ashmem, ion, dma-buf or SELinux, */
+/** for example using a container, or other method */
+
+void* _Nonnull shmat_vanilla(int __shm_id, const void* _Nullable __addr, int __flags);
+int shmctl_vanilla(int __shm_id, int __op, struct shmid64_ds* _Nullable __buf);
+int shmdt_vanilla(const void* _Nonnull __addr);
+int shmget_vanilla(key_t __key, size_t __size, int __flags);
+
+__END_DECLS

shmem.c

@@ -21,6 +21,7 @@
 #endif
 
 #include "shm.h"
+#include "shm_vanilla.h"
 
 #define DBG(...) __android_log_print(ANDROID_LOG_INFO, "shmem", __VA_ARGS__)
 #define ASHV_KEY_SYMLINK_PATH _PATH_TMP "ashv_key_%d"
@@ -153,6 +154,21 @@ static int ashmem_create_region(char const* name, size_t size)
 #endif
 }
 
+static bool ashmem_is_available(void) {
+#if __ANDROID_API__ >= 26
+	int fd = ASharedMemory_create("libandroid-shmem-test", 1);
+	if (fd == -1) {
+#else
+	if (access("/dev/ashmem", F_OK) != 0) {
+#endif
+	   return false;
+	}
+#if __ANDROID_API__ >= 26
+	close(fd);
+#endif
+	return true;
+}
+
 static void ashv_check_pid()
 {
 	pid_t mypid = getpid();
@@ -296,7 +312,9 @@ static int ashv_read_remote_segment(int shmid)
 /* Get shared memory area identifier. */
 int shmget(key_t key, size_t size, int flags)
 {
-	(void) flags;
+	if (!ashmem_is_available()) {
+		return shmget_vanilla(key, size, flags);
+	}
 
 	ashv_check_pid();
 
@@ -441,6 +459,10 @@ int shmget(key_t key, size_t size, int flags)
 /* Attach shared memory segment. */
 void* shmat(int shmid, void const* shmaddr, int shmflg)
 {
+	if (!ashmem_is_available()) {
+		return shmat_vanilla(shmid, shmaddr, shmflg);
+	}
+
 	ashv_check_pid();
 
 	int socket_id = ashv_socket_id_from_shmid(shmid);
@@ -479,6 +501,10 @@ void* shmat(int shmid, void const* shmaddr, int shmflg)
 /* Detach shared memory segment. */
 int shmdt(void const* shmaddr)
 {
+	if (!ashmem_is_available()) {
+		return shmdt_vanilla(shmaddr);
+	}
+
 	ashv_check_pid();
 
 	pthread_mutex_lock(&mutex);
@@ -561,6 +587,10 @@ int libandroid_shmdt_fd(int fd)
 /* Shared memory control operation. */
 int shmctl(int shmid, int cmd, struct shmid_ds *buf)
 {
+	if (!ashmem_is_available()) {
+		return shmctl_vanilla(shmid, cmd, buf);
+	}
+
 	ashv_check_pid();
 
 	if (cmd == IPC_RMID) {

shmem_vanilla.c

@@ -0,0 +1,62 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/** fork of https://android.googlesource.com/platform/bionic/+/02ce401d1e2b31586c94c8b6999364bbbce27388/libc/bionic/sys_shm.cpp */
+
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#include "shm_vanilla.h"
+
+void* shmat_vanilla(int id, const void* address, int flags) {
+  return (void*)(syscall(SYS_shmat, id, address, flags));
+}
+
+int shmctl_vanilla(int id, int cmd, struct shmid64_ds* buf) {
+  // upstream bionic libc and also other libc implementations contain
+  // this code that ORs cmd with IPC_64, but unfortunately, for some reason
+  // in 32-bit termux-docker, an error always occurs during test-with-key
+  // 'shmctl: Invalid argument' unless the ORing with IPC_64 is disabled.
+  // when that is disabled, all tests pass. This result holds for both arm and i686
+  // termux-docker running in both fully 32-bit kernels and the 32-bit mode of
+  // 64-bit kernels.
+#if 0
+//#if !defined(__LP64__)
+  // Annoyingly, the kernel requires this for 32-bit but rejects it for 64-bit.
+  cmd |= IPC_64;
+#endif
+  return syscall(SYS_shmctl, id, cmd, buf);
+}
+
+int shmdt_vanilla(const void* address) {
+  return syscall(SYS_shmdt, address);
+}
+
+int shmget_vanilla(key_t key, size_t size, int flags) {
+  return syscall(SYS_shmget, key, size, flags);
+}

test/Makefile

@@ -4,11 +4,11 @@ OS := $(shell uname -o 2> /dev/null)
 
 ifeq ($(OS), Android)
 CFLAGS += -I.. -llog
-EXTRA_CFILE = ../shmem.c
+EXTRA_CFILES = ../shmem.c ../shmem_vanilla.c
 cleanup-memory: test ;
 else
 CFLAGS += -DSYSV_ASHMEM_TEST_SYSTEM
-EXTRA_CFILE =
+EXTRA_CFILES =
 cleanup-memory: test
 	./cleanup-shared-memory.sh
 endif
@@ -18,7 +18,7 @@ test: $(addprefix testcase-,$(patsubst %.c,%,$(wildcard *.c))) ;
 testcase-% : test-%
 	./$^
 
-test-%: %.c $(EXTRA_CFILE)
+test-%: %.c $(EXTRA_CFILES)
 	$(CC) $(CFLAGS) $+ -o $@
 
 clean: