libbpf: Fix implicit memfd_create() for bionic

Since memfd_create() is not consistently available across different
bionic libc implementations, using memfd_create() directly can break
some Android builds:

  tools/lib/bpf/linker.c:576:7: error: implicit declaration of function 'memfd_create' [-Werror,-Wimplicit-function-declaration]
    576 |         fd = memfd_create(filename, 0);
        |              ^

To fix this, relocate and inline the sys_memfd_create() helper so that
it can be used in "linker.c". Similar issues were previously fixed by
commit 9fa5e1a ("libbpf: Call memfd_create() syscall directly").

Fixes: 6d5e5e5 ("libbpf: Extend linker API to support in-memory ELF files")
Signed-off-by: Carlos Llamas <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: Carlos Llamas

tools/lib/bpf/libbpf.c

@@ -1725,15 +1725,6 @@ static Elf64_Sym *find_elf_var_sym(const struct bpf_object *obj, const char *nam
 	return ERR_PTR(-ENOENT);
 }
 
-/* Some versions of Android don't provide memfd_create() in their libc
- * implementation, so avoid complications and just go straight to Linux
- * syscall.
- */
-static int sys_memfd_create(const char *name, unsigned flags)
-{
-	return syscall(__NR_memfd_create, name, flags);
-}
-
 #ifndef MFD_CLOEXEC
 #define MFD_CLOEXEC 0x0001U
 #endif

tools/lib/bpf/libbpf_internal.h

@@ -667,6 +667,15 @@ static inline int sys_dup3(int oldfd, int newfd, int flags)
 	return syscall(__NR_dup3, oldfd, newfd, flags);
 }
 
+/* Some versions of Android don't provide memfd_create() in their libc
+ * implementation, so avoid complications and just go straight to Linux
+ * syscall.
+ */
+static inline int sys_memfd_create(const char *name, unsigned flags)
+{
+	return syscall(__NR_memfd_create, name, flags);
+}
+
 /* Point *fixed_fd* to the same file that *tmp_fd* points to.
  * Regardless of success, *tmp_fd* is closed.
  * Whatever *fixed_fd* pointed to is closed silently.

tools/lib/bpf/linker.c

@@ -573,7 +573,7 @@ int bpf_linker__add_buf(struct bpf_linker *linker, void *buf, size_t buf_sz,
 
 	snprintf(filename, sizeof(filename), "mem:%p+%zu", buf, buf_sz);
 
-	fd = memfd_create(filename, 0);
+	fd = sys_memfd_create(filename, 0);
 	if (fd < 0) {
 		ret = -errno;
 		pr_warn("failed to create memfd '%s': %s\n", filename, errstr(ret));