sd-dlopen: make macros to generate .notes.dlopen sections public API (#41047)

If this new scheme of adding dependencies is supposed to be used more
widely we need to start making it easy to add them. So add a new
self-contained header that projects can simply include without the need
to link against libsystemd itself. This will allow them to generate
`.notes.dlopen` sections:

```
> readelf -p .note.dlopen ./l2md

String dump of section '.note.dlopen':
  [     a]  |@FDO
  [    10]  [{"feature":"manifest-json","description":"Manifest-based change detection via gzip and JSON parsing","priority":"suggested","soname":["libz.so.1","libsystemd.so.0"]}]
  [    c2]  |@FDO
  [    c8]  [{"feature":"manifest-http","description":"HTTP transport for lore.kernel.org manifest fetch","priority":"suggested","soname":["libcurl.so.4"]}]
```

src/basic/compress.c

@@ -20,6 +20,8 @@
 #include <zstd_errors.h>
 #endif
 
+#include "sd-dlopen.h"
+
 #include "alloc-util.h"
 #include "bitfield.h"
 #include "compress.h"
@@ -148,7 +150,8 @@ bool compression_supported(Compression c) {
 
 int dlopen_lzma(void) {
 #if HAVE_XZ
-        ELF_NOTE_DLOPEN("lzma",
+        SD_ELF_NOTE_DLOPEN(
+                        "lzma",
                         "Support lzma compression in journal and coredump files",
                         COMPRESSION_PRIORITY_XZ,
                         "liblzma.so.5");
@@ -219,7 +222,8 @@ int compress_blob_xz(const void *src, uint64_t src_size,
 
 int dlopen_lz4(void) {
 #if HAVE_LZ4
-        ELF_NOTE_DLOPEN("lz4",
+        SD_ELF_NOTE_DLOPEN(
+                        "lz4",
                         "Support lz4 compression in journal and coredump files",
                         COMPRESSION_PRIORITY_LZ4,
                         "liblz4.so.1");
@@ -286,7 +290,8 @@ int compress_blob_lz4(const void *src, uint64_t src_size,
 
 int dlopen_zstd(void) {
 #if HAVE_ZSTD
-        ELF_NOTE_DLOPEN("zstd",
+        SD_ELF_NOTE_DLOPEN(
+                        "zstd",
                         "Support zstd compression in journal and coredump files",
                         COMPRESSION_PRIORITY_ZSTD,
                         "libzstd.so.1");

src/basic/dlfcn-util.h

@@ -33,47 +33,6 @@ int dlopen_many_sym_or_warn_sentinel(void **dlp, const char *filename, int log_l
 #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 constructed 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))
-
 /* If called dlopen_many_sym_or_warn() will fail with EPERM. This can be used to block lazy loading of shared
  * libs, if we transfer a process into a different namespace. Note that this does not work for all calls of
  * dlopen(), just those through our dlopen_safe() wrapper (which we use comprehensively in our

src/basic/gcrypt-util.c

@@ -2,6 +2,8 @@
 
 #include <sys/syslog.h>
 
+#include "sd-dlopen.h"
+
 #include "gcrypt-util.h"
 
 #if HAVE_GCRYPT
@@ -44,9 +46,10 @@ DLSYM_PROTOTYPE(gcry_strerror) = NULL;
 
 int dlopen_gcrypt(void) {
 #if HAVE_GCRYPT
-        ELF_NOTE_DLOPEN("gcrypt",
+        SD_ELF_NOTE_DLOPEN(
+                        "gcrypt",
                         "Support for journald forward-sealing",
-                        ELF_NOTE_DLOPEN_PRIORITY_SUGGESTED,
+                        SD_ELF_NOTE_DLOPEN_PRIORITY_SUGGESTED,
                         "libgcrypt.so.20");
 
         return dlopen_many_sym_or_warn(