dlfcn: add macro for exporting dlopen() module names in ELF sections

This allows code to declare "weak" dlopen() style deps via an ELF
section following the just added specification.

The idea is that any user of dlopen() will place ELF_NOTE_DLOPEN(…)
somewhere close which will synthesize the note.

Tools such as rpm/dpkg package builders as well as initrd generators
(such as dracut) can then automatically pick up these weak deps of
suggested dependencies for their purposes.

Co-authored-by: Luca Boccassi <bluca@debian.org>

diff --git a/src/basic/dlfcn-util.h b/src/basic/dlfcn-util.h
index 050f1e2da748a9047aef34136e0cb50e4e9c60cd..b395d4ca04c9c410b34f7f2c70f5604b29f18cb7 100644 (file)
--- a/src/basic/dlfcn-util.h
+++ b/src/basic/dlfcn-util.h
@@ -39,3 +39,44 @@ int dlopen_many_sym_or_warn_sentinel(void **dlp, const char *filename, int log_l
 /* libbpf is a bit confused about type-safety and API compatibility. Provide a macro that can tape over that mess. Sad. */
 #define DLSYM_ARG_FORCE(arg) \
         &sym_##arg, STRINGIFY(arg)
+
+#define ELF_NOTE_DLOPEN_VENDOR "FDO"
+#define ELF_NOTE_DLOPEN_TYPE UINT32_C(0x407c0c0a)
+#define ELF_NOTE_DLOPEN_PRIORITY_REQUIRED "required"
+#define ELF_NOTE_DLOPEN_PRIORITY_RECOMMENDED "recommended"
+#define ELF_NOTE_DLOPEN_PRIORITY_SUGGESTED "suggested"
+
+/* Add an ".note.dlopen" ELF note to our binary that declares our weak dlopen() dependency. This
+ * information can be read from an ELF file via "readelf -p .note.dlopen" or an equivalent command. */
+#define _ELF_NOTE_DLOPEN(json, variable_name)                              \
+        __attribute__((used, section(".note.dlopen"))) _Alignas(sizeof(uint32_t)) static const struct { \
+                struct {                                                   \
+                        uint32_t n_namesz, n_descsz, n_type;               \
+                } nhdr;                                                    \
+                char name[sizeof(ELF_NOTE_DLOPEN_VENDOR)];                 \
+                _Alignas(sizeof(uint32_t)) char dlopen_json[sizeof(json)]; \
+        } variable_name = {                                                \
+                .nhdr = {                                                  \
+                        .n_namesz = sizeof(ELF_NOTE_DLOPEN_VENDOR),        \
+                        .n_descsz = sizeof(json),                          \
+                        .n_type   = ELF_NOTE_DLOPEN_TYPE,                  \
+                },                                                         \
+                .name = ELF_NOTE_DLOPEN_VENDOR,                            \
+                .dlopen_json = json,                                       \
+        }
+
+#define _SONAME_ARRAY1(a) "[\""a"\"]"
+#define _SONAME_ARRAY2(a, b) "[\""a"\",\""b"\"]"
+#define _SONAME_ARRAY3(a, b, c) "[\""a"\",\""b"\",\""c"\"]"
+#define _SONAME_ARRAY4(a, b, c, d) "[\""a"\",\""b"\",\""c"\"",\""d"\"]"
+#define _SONAME_ARRAY5(a, b, c, d, e) "[\""a"\",\""b"\",\""c"\"",\""d"\",\""e"\"]"
+#define _SONAME_ARRAY_GET(_1,_2,_3,_4,_5,NAME,...) NAME
+#define _SONAME_ARRAY(...) _SONAME_ARRAY_GET(__VA_ARGS__, _SONAME_ARRAY5, _SONAME_ARRAY4, _SONAME_ARRAY3, _SONAME_ARRAY2, _SONAME_ARRAY1)(__VA_ARGS__)
+
+/* The 'priority' must be one of 'required', 'recommended' or 'suggested' as per specification, use the
+ * macro defined above to specify it.
+ * Multiple sonames can be passed and they will be automatically contructed into a json array (but note that
+ * due to preprocessor language limitations if more than the limit defined above is used, a new
+ * _SONAME_ARRAY<X+1> will need to be added). */
+#define ELF_NOTE_DLOPEN(feature, description, priority, ...) \
+        _ELF_NOTE_DLOPEN("[{\"feature\":\"" feature "\",\"description\":\"" description "\",\"priority\":\"" priority "\",\"soname\":" _SONAME_ARRAY(__VA_ARGS__) "}]", UNIQ_T(s, UNIQ))