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

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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>
This commit is contained in:
Lennart Poettering 2024-04-11 17:45:17 +02:00 committed by Luca Boccassi
parent cb7e3126b9
commit 2f6bd11c1a
1 changed files with 41 additions and 0 deletions
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41
src/basic/dlfcn-util.h
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@ -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))