sd-dlopen: introduce SD_ELF_NOTE_DLOPEN_ANCHORED() macro
SD_ELF_NOTE_DLOPEN_ANCHORED() emits a .note.dlopen ELF note that is
"anchored" to a dummy symbol via the SHF_LINK_ORDER ('o') section flag,
in addition to SHF_GROUP ('G') for folding identical notes together.
Unlike the plain SD_ELF_NOTE_DLOPEN() macro, this variant ties the
note's lifetime to a dummy symbol named with the specified tag: if the
linker's --gc-sections removes the function that calls the macro (e.g.
because the function is never referenced), the associated .note.dlopen
entry is garbage-collected along with it.
Currently, the new macro is unused in our code, but all dlopen notes
will be generated with this in later commits.
This commit is contained in:
Yu Watanabe
+86
-1
@@ -23,6 +23,8 @@
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#include <stdint.h>
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#include "_sd-common.h"
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#ifdef __cplusplus
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extern "C" {
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#endif
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@@ -94,6 +96,69 @@ extern "C" {
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".popsection\n" \
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".endif\n")
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/*
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* SD_ELF_NOTE_DLOPEN_ANCHORED() emits a .note.dlopen ELF note that is "anchored" to a dummy symbol via the
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* SHF_LINK_ORDER ('o') section flag, in addition to SHF_GROUP ('G') for folding identical notes together.
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*
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* Unlike the plain SD_ELF_NOTE_DLOPEN() macro, this variant ties the note's lifetime to a dummy symbol named
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* with the specified tag: if the linker's --gc-sections removes the function that calls the macro (e.g.
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* because the function is never referenced), the associated .note.dlopen entry is garbage-collected along
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* with it.
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*
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* USAGE NOTE:
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* - The 'tag' argument must be a unique symbol name to characterize the dlopen note (e.g. generated from
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* feature and its priority).
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*
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* Example:
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* void dlopen_libfoo(int log_level) {
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* SD_ELF_NOTE_DLOPEN_ANCHORED(foo_suggested, "foo", "description of foo", "suggested", "libfoo.so.0");
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* ...
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* }
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*
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* TOOLCHAIN REQUIREMENTS:
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* - GNU as (binutils) >= 2.35: this is when support for specifying the SHF_LINK_ORDER ('o') associated
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* symbol as a direct argument to .section/.pushsection was added.
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* - LLVM (clang -integrated-as) >= 18: matching support for the 'o' flag argument combined with 'G'
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* (SHF_GROUP) landed around LLVM 18.
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* - Do NOT add 'R' (SHF_GNU_RETAIN) to these flags: combined with SHF_LINK_ORDER, it would prevent the
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* linker from ever garbage-collecting the note, defeating the whole purpose of anchoring it to a symbol in
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* the first place.
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*/
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#define _SD_ELF_NOTE_DLOPEN_ANCHORED(tag, uniq_var, json) \
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_Pragma("GCC diagnostic push") \
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_Pragma("GCC diagnostic ignored \"-Wnested-externs\"") \
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__attribute__((visibility("hidden"))) \
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extern volatile const char uniq_var __asm__("__sd_dlopen_anchor_" #tag); \
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_Pragma("GCC diagnostic pop") \
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__asm__ ( \
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".ifndef \"sd_dlopen:emitted:" #tag "\"\n" \
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_SD_ELF_NOTE_DLOPEN_GUARD " \"sd_dlopen:emitted:" #tag "\", 1\n" \
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".pushsection .data.sd_dlopen_dummy." #tag ", \"awG\", %progbits, sd_dlopen_group_" #tag ", comdat\n" \
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".weak __sd_dlopen_anchor_" #tag "\n" \
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".hidden __sd_dlopen_anchor_" #tag "\n" \
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".type __sd_dlopen_anchor_" #tag ", %object\n" \
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".balign 8\n" \
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"__sd_dlopen_anchor_" #tag ":\n" \
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".byte 0\n" \
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".popsection\n" \
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".pushsection .note.dlopen, \"aGo\", %note, __sd_dlopen_anchor_" #tag ", sd_dlopen_group_" #tag ", comdat\n" \
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".balign 4\n" \
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".long 884f - 883f\n" \
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".long 882f - 881f\n" \
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".long 0x407c0c0a\n" \
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"883:\n" \
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".asciz \"" SD_ELF_NOTE_DLOPEN_VENDOR "\"\n" \
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"884:\n" \
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".balign 4\n" \
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"881:\n" \
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".asciz \"" json "\"\n" \
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"882:\n" \
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".balign 4\n" \
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".popsection\n" \
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".endif\n" \
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); \
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__asm__ volatile ("" : : "r" (&uniq_var))
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#define _SD_SONAME_ARRAY1(a) "[\\\"" a "\\\"]"
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#define _SD_SONAME_ARRAY2(a, b) "[\\\"" a "\\\",\\\"" b "\\\"]"
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#define _SD_SONAME_ARRAY3(a, b, c) "[\\\"" a "\\\",\\\"" b "\\\",\\\"" c "\\\"]"
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@@ -102,8 +167,28 @@ extern "C" {
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#define _SD_SONAME_ARRAY_GET(_1,_2,_3,_4,_5,NAME,...) NAME
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#define _SD_SONAME_ARRAY(...) _SD_SONAME_ARRAY_GET(__VA_ARGS__, _SD_SONAME_ARRAY5, _SD_SONAME_ARRAY4, _SD_SONAME_ARRAY3, _SD_SONAME_ARRAY2, _SD_SONAME_ARRAY1)(__VA_ARGS__)
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#define _SD_DLOPEN_UNIQ_VAR(tag) \
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_SD_CONCATENATE(_SD_CONCATENATE(__sd_dlopen_anchor_, tag), _SD_CONCATENATE(_, _SD_UNIQ))
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#define _SD_DLOPEN_JSON(feature, description, priority, ...) \
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"[{" \
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"\\\"feature\\\":\\\"" feature "\\\"," \
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"\\\"description\\\":\\\"" description "\\\"," \
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"\\\"priority\\\":\\\"" priority "\\\"," \
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"\\\"soname\\\":" _SD_SONAME_ARRAY(__VA_ARGS__) \
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"}]"
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#define SD_ELF_NOTE_DLOPEN(feature, description, priority, ...) \
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_SD_ELF_NOTE_DLOPEN("[{\\\"feature\\\":\\\"" feature "\\\",\\\"description\\\":\\\"" description "\\\",\\\"priority\\\":\\\"" priority "\\\",\\\"soname\\\":" _SD_SONAME_ARRAY(__VA_ARGS__) "}]")
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_SD_ELF_NOTE_DLOPEN(_SD_DLOPEN_JSON(feature, description, priority, __VA_ARGS__))
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/* The anchored note requires LLVM >= 18 (see above). Fall back to the non-anchored note on older clang. */
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#if defined(__clang__) && !defined(__apple_build_version__) && __clang_major__ < 18
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# define SD_ELF_NOTE_DLOPEN_ANCHORED(tag, feature, description, priority, ...) \
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SD_ELF_NOTE_DLOPEN(feature, description, priority, __VA_ARGS__)
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#else
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# define SD_ELF_NOTE_DLOPEN_ANCHORED(tag, feature, description, priority, ...) \
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_SD_ELF_NOTE_DLOPEN_ANCHORED(tag, _SD_DLOPEN_UNIQ_VAR(tag), _SD_DLOPEN_JSON(feature, description, priority, __VA_ARGS__))
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#endif
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#ifdef __cplusplus
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}
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