From 2f6bd11c1a0147f6bd24bce3ed00c3d91c0884e6 Mon Sep 17 00:00:00 2001 From: Lennart Poettering Date: Thu, 11 Apr 2024 17:45:17 +0200 Subject: [PATCH] dlfcn: add macro for exporting dlopen() module names in ELF sections MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 --- src/basic/dlfcn-util.h | 41 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 41 insertions(+) diff --git a/src/basic/dlfcn-util.h b/src/basic/dlfcn-util.h index 050f1e2da74..b395d4ca04c 100644 --- 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 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))