Bring back CONFIG_HAVE_MEMFD_CREATE to fix Android build error

Commit 732bf60 ("test/io_uring_register: kill old memfd test")
removed the `CONFIG_HAVE_MEMFD_CREATE`, which was previously used to
conditionally provide `memfd_create()` for an old test in
`test/io_uring_register.c`.

Reintroduce `CONFIG_HAVE_MEMFD_CREATE` to resolve Android build error
caused by commit:

  93d3a7a ("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);
        |                 ^
```
Then, provide memfd_create() in examples/ if `CONFIG_HAVE_MEMFD_CREATE`
is not set to fix the build error.

v2 -> v3:
  - Don't bother touching the test/ dir.
    Link: https://lore.kernel.org/io-uring/3b28fddb-2171-4f2f-9729-0c0ed14d20cc@kernel.dk

v1 -> v2:
  - Omit the old memfd test because it's not needed anymore.
    Link: https://lore.kernel.org/io-uring/4bc75566-9cb5-42ec-a6b7-16e04062e0c6@kernel.dk

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>
Link: https://lore.kernel.org/r/20250716144610.3938579-1-alviro.iskandar@gnuweeb.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: 2 people

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

examples/helpers.c

@@ -13,6 +13,14 @@
 
 #include "helpers.h"
 
+#ifndef CONFIG_HAVE_MEMFD_CREATE
+#include <sys/syscall.h>
+int memfd_create(const char *name, unsigned int flags)
+{
+	return (int)syscall(SYS_memfd_create, name, flags);
+}
+#endif
+
 int setup_listening_socket(int port, int ipv6)
 {
 	struct sockaddr_in srv_addr = { };

examples/helpers.h

@@ -17,4 +17,9 @@ void *t_aligned_alloc(size_t alignment, size_t size);
 
 void t_error(int status, int errnum, const char *format, ...);
 
+#ifndef CONFIG_HAVE_MEMFD_CREATE
+#include <linux/memfd.h>
+#endif
+int memfd_create(const char *name, unsigned int flags);
+
 #endif