This reverts commit 732bf609b670631731765a585a68d14ed3fdc9b7.

Bring back `CONFIG_HAVE_MEMFD_CREATE` and the associated memfd test
to resolve Android build failures caused by:

  93d3a7a70b4a ("examples/zcrx: udmabuf backed areas")

It added a call to `memfd_create()`, which is unavailable on some
Android toolchains, leading to the following build error:

```
  zcrx.c:111:10: error: call to undeclared function 'memfd_create'; ISO C99 and \
  later do not support implicit function declarations \
  [-Wimplicit-function-declaration]
    111 |         memfd = memfd_create("udmabuf-test", MFD_ALLOW_SEALING);
        |                 ^
```

This reversion is a preparation step for a proper fix by ensuring
`memfd_create()` usage is guarded and portable. Issue #620 was
initially unclear, but we now suspect it stemmed from improper
compiler/linker flag combinations.

Co-authored-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
---
 configure                |  19 +++++++
 test/io_uring_register.c | 119 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 138 insertions(+)

diff --git a/configure b/configure
index 552f8ae..3c95214 100755
--- a/configure
+++ b/configure
@@ -379,6 +379,22 @@ if compile_prog "" "" "has_ucontext"; then
 fi
 print_config "has_ucontext" "$has_ucontext"
 
+##########################################
+# check for memfd_create(2)
+has_memfd_create="no"
+cat > $TMPC << EOF
+#include <sys/mman.h>
+int main(int argc, char **argv)
+{
+  int memfd = memfd_create("test", 0);
+  return 0;
+}
+EOF
+if compile_prog "-Werror=implicit-function-declaration" "" "has_memfd_create"; then
+  has_memfd_create="yes"
+fi
+print_config "has_memfd_create" "$has_memfd_create"
+
 ##########################################
 # Check NVME_URING_CMD support
 nvme_uring_cmd="no"
@@ -539,6 +555,9 @@ fi
 if test "$array_bounds" = "yes"; then
   output_sym "CONFIG_HAVE_ARRAY_BOUNDS"
 fi
+if test "$has_memfd_create" = "yes"; then
+  output_sym "CONFIG_HAVE_MEMFD_CREATE"
+fi
 if test "$nvme_uring_cmd" = "yes"; then
   output_sym "CONFIG_HAVE_NVME_URING"
 fi
diff --git a/test/io_uring_register.c b/test/io_uring_register.c
index b53a67d..f08f0ca 100644
--- a/test/io_uring_register.c
+++ b/test/io_uring_register.c
@@ -32,6 +32,17 @@ static int pagesize;
 static rlim_t mlock_limit;
 static int devnull;
 
+#if !defined(CONFIG_HAVE_MEMFD_CREATE)
+#include <sys/syscall.h>
+#include <linux/memfd.h>
+
+static int memfd_create(const char *name, unsigned int flags)
+{
+	return (int)syscall(SYS_memfd_create, name, flags);
+}
+#endif
+
+
 static int expect_fail(int fd, unsigned int opcode, void *arg,
 		       unsigned int nr_args, int error, int error2)
 {
@@ -466,6 +477,113 @@ static int __test_poll_ringfd(int ring_flags)
 	return status;
 }
 
+static int test_shmem(void)
+{
+	const char pattern = 0xEA;
+	const int len = 4096;
+	struct io_uring_sqe *sqe;
+	struct io_uring_cqe *cqe;
+	struct io_uring ring;
+	struct iovec iov;
+	int memfd, ret, i;
+	char *mem;
+	int pipefd[2] = {-1, -1};
+
+	ret = io_uring_queue_init(8, &ring, 0);
+	if (ret)
+		return 1;
+
+	if (pipe(pipefd)) {
+		perror("pipe");
+		return 1;
+	}
+	memfd = memfd_create("uring-shmem-test", 0);
+	if (memfd < 0) {
+		fprintf(stderr, "memfd_create() failed %i\n", -errno);
+		return 1;
+	}
+	if (ftruncate(memfd, len)) {
+		fprintf(stderr, "can't truncate memfd\n");
+		return 1;
+	}
+	mem = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, memfd, 0);
+	if (!mem) {
+		fprintf(stderr, "mmap failed\n");
+		return 1;
+	}
+	for (i = 0; i < len; i++)
+		mem[i] = pattern;
+
+	iov.iov_base = mem;
+	iov.iov_len = len;
+	ret = io_uring_register_buffers(&ring, &iov, 1);
+	if (ret) {
+		if (ret == -EOPNOTSUPP) {
+			fprintf(stdout, "memfd registration isn't supported, "
+					"skip\n");
+			goto out;
+		}
+
+		fprintf(stderr, "buffer reg failed: %d\n", ret);
+		return 1;
+	}
+
+	/* check that we can read and write from/to shmem reg buffer */
+	sqe = io_uring_get_sqe(&ring);
+	io_uring_prep_write_fixed(sqe, pipefd[1], mem, 512, 0, 0);
+	sqe->user_data = 1;
+
+	ret = io_uring_submit(&ring);
+	if (ret != 1) {
+		fprintf(stderr, "submit write failed\n");
+		return 1;
+	}
+	ret = io_uring_wait_cqe(&ring, &cqe);
+	if (ret < 0 || cqe->user_data != 1 || cqe->res != 512) {
+		fprintf(stderr, "reading from shmem failed\n");
+		return 1;
+	}
+	io_uring_cqe_seen(&ring, cqe);
+
+	/* clean it, should be populated with the pattern back from the pipe */
+	memset(mem, 0, 512);
+	sqe = io_uring_get_sqe(&ring);
+	io_uring_prep_read_fixed(sqe, pipefd[0], mem, 512, 0, 0);
+	sqe->user_data = 2;
+
+	ret = io_uring_submit(&ring);
+	if (ret != 1) {
+		fprintf(stderr, "submit write failed\n");
+		return 1;
+	}
+	ret = io_uring_wait_cqe(&ring, &cqe);
+	if (ret < 0 || cqe->user_data != 2 || cqe->res != 512) {
+		fprintf(stderr, "reading from shmem failed\n");
+		return 1;
+	}
+	io_uring_cqe_seen(&ring, cqe);
+
+	for (i = 0; i < 512; i++) {
+		if (mem[i] != pattern) {
+			fprintf(stderr, "data integrity fail\n");
+			return 1;
+		}
+	}
+
+	ret = io_uring_unregister_buffers(&ring);
+	if (ret) {
+		fprintf(stderr, "buffer unreg failed: %d\n", ret);
+		return 1;
+	}
+out:
+	io_uring_queue_exit(&ring);
+	close(pipefd[0]);
+	close(pipefd[1]);
+	munmap(mem, len);
+	close(memfd);
+	return 0;
+}
+
 static int test_poll_ringfd(void)
 {
 	int ret;
@@ -526,6 +644,7 @@ int main(int argc, char **argv)
 	/* uring poll on the uring fd */
 	status |= test_poll_ringfd();
 
+	status |= test_shmem();
 	if (status)
 		fprintf(stderr, "FAIL\n");
 
-- 
Alviro Iskandar Setiawan

On 7/14/25 11:06 PM, Alviro Iskandar Setiawan wrote:
> This reverts commit 732bf609b670631731765a585a68d14ed3fdc9b7.
> 
> Bring back `CONFIG_HAVE_MEMFD_CREATE` and the associated memfd test
> to resolve Android build failures caused by:
> 
>   93d3a7a70b4a ("examples/zcrx: udmabuf backed areas")
> 
> It added a call to `memfd_create()`, which is unavailable on some
> Android toolchains, leading to the following build error:
> 
> ```
>   zcrx.c:111:10: error: call to undeclared function 'memfd_create'; ISO C99 and \
>   later do not support implicit function declarations \
>   [-Wimplicit-function-declaration]
>     111 |         memfd = memfd_create("udmabuf-test", MFD_ALLOW_SEALING);
>         |                 ^
> ```
> 
> This reversion is a preparation step for a proper fix by ensuring
> `memfd_create()` usage is guarded and portable. Issue #620 was
> initially unclear, but we now suspect it stemmed from improper
> compiler/linker flag combinations.

Maybe just bring back the configure parts? The test, as mentioned in
that commit, is pretty useless.

-- 
Jens Axboe

On Tue, Jul 15, 2025 at 9:10 PM Jens Axboe wrote:
> Maybe just bring back the configure parts? The test, as mentioned in
> that commit, is pretty useless.

Ok, I'll send a v2 with the test omitted.

-- Viro