Embed ELF metadata about dynamic runtime dependencies #1482

cgzones · 2024-05-18T21:24:44Z

Embed metadata about runtime dependencies of dynamic libraries used via dlopen(3) in the binary so that distributions can automatically generate package dependencies.

Specification: https://systemd.io/ELF_DLOPEN_METADATA/ (TODO: link not yet available)
Alternative link: https://github.com/systemd/systemd/blob/main/docs/ELF_DLOPEN_METADATA.md

Inspired-by: systemd/systemd#32234

cgzones · 2024-05-18T21:25:40Z

See https://salsa.debian.org/debian/htop/-/merge_requests/5 for the packaging update.

Embed metadata about runtime dependencies of dynamic libraries used via dlopen(3) in the binary so that distributions can automatically generate package dependencies. Specification: https://systemd.io/ELF_DLOPEN_METADATA/ (TODO: link not yet available) Inspired-by: systemd/systemd#32234

BenBE · 2024-05-19T00:20:02Z

Macros.h

+   };                                                                                              \
+   IGNORE_W11EXTENSIONS_END
+#else
+#define DECLARE_ELF_NOTE_DLOPEN()


Should name the content parameter for consistency.

BenBE · 2024-05-19T00:32:57Z

Macros.h

+
+#define DECLARE_ELF_NOTE_DLOPEN(content) \
+   IGNORE_W11EXTENSIONS_BEGIN                                                                      \
+   __attribute__((used, section(".note.dlopen"))) alignas(sizeof(uint32_t)) static const struct {  \


Suggested change

__attribute__((used, section(".note.dlopen"))) alignas(sizeof(uint32_t)) static const struct { \

__attribute__((packed, used, section(".note.dlopen"))) static const struct { \

Just byte-align ourselves and round up with some little magic later

Another problem: C99 doesn't support anonymous struct yet.

Another problem: C99 doesn't support anonymous struct yet.

The workaround for that is kinda easy though, too …

BenBE · 2024-05-19T00:34:43Z

Macros.h

+      struct {                                                                                     \
+         uint32_t n_namesz, n_descsz, n_type;                                                      \
+      } nhdr;                                                                                      \
+      char name[sizeof(ELF_NOTE_DLOPEN_OWNER)];                                                    \


Suggested change

char name[sizeof(ELF_NOTE_DLOPEN_OWNER)]; \

char name[(sizeof(ELF_NOTE_DLOPEN_OWNER)+3)&(~3)]; \

BenBE · 2024-05-19T00:37:35Z

Macros.h

+         uint32_t n_namesz, n_descsz, n_type;                                                      \
+      } nhdr;                                                                                      \
+      char name[sizeof(ELF_NOTE_DLOPEN_OWNER)];                                                    \
+      alignas(sizeof(uint32_t)) char dlopen_content[sizeof(content)];                              \


Suggested change

alignas(sizeof(uint32_t)) char dlopen_content[sizeof(content)]; \

char dlopen_content[(sizeof(content)+3)&(~3)]; \

Although one could argue if this last expansion is still required at the end of the attribute section.

BenBE · 2024-05-19T00:40:04Z

configure.ac

+AC_MSG_CHECKING(for alignas support)
+AC_COMPILE_IFELSE([
+   AC_LANG_SOURCE(
+      [[
+         #include <stdalign.h>
+         alignas(128) char buffer[128];
+      ]]
+   )],
+   AC_DEFINE([HAVE_ALIGNAS], 1, [The alignas C11 feature is supported.])
+   AC_MSG_RESULT(yes),
+   AC_MSG_RESULT(no))
+


Cf. Macros.h for a way on how to do this without C11 support. If we throw around GCC/Clang compiler extensions, we also could just use packed, which at least is even kinda portable to other compilers too …

BenBE · 2024-05-19T00:43:18Z

linux/LibNl.c

+   DECLARE_ELF_NOTE_DLOPEN("[{\"soname\":[\"libnl-3.so\",\"libnl-3.so.200\"],\"feature\":\"delay-accounting\"," \
+      "\"description:\":\"Enables delay accounting support\",\"priority\":\"recommended\"},{\"soname\":[" \
+      "\"libnl-genl-3.so\",\"libnl-genl-3.so.200\"],\"feature\":\"delay-accounting\",\"description:\":"\
+      "\"Enables delay accounting support\",\"priority\":\"recommended\"}]")


Any objections to have this literal span multiple lines for better maintainability?

Please add proper meta-commands for astyle to leave this block alone afterwards.

Also, this note probably should only contain the optional dependencies we actually have enabled; i.e. if configured without delay accounting, there's no need to declare this as an optional runtime dependency if we would never try to load that library anyway.

Suggested change

DECLARE_ELF_NOTE_DLOPEN("[{\"soname\":[\"libnl-3.so\",\"libnl-3.so.200\"],\"feature\":\"delay-accounting\"," \

"\"description:\":\"Enables delay accounting support\",\"priority\":\"recommended\"},{\"soname\":[" \

"\"libnl-genl-3.so\",\"libnl-genl-3.so.200\"],\"feature\":\"delay-accounting\",\"description:\":"\

"\"Enables delay accounting support\",\"priority\":\"recommended\"}]")

DECLARE_ELF_NOTE_DLOPEN( "["\

"{"\

"\"soname\":[\"libnl-3.so\",\"libnl-3.so.200\"],"\

"\"feature\":\"delay-accounting\"," \

"\"description:\":\"Enables delay accounting support\","\

"\"priority\":\"recommended\""\

"},"\

"{"\

"\"soname\":[\"libnl-genl-3.so\",\"libnl-genl-3.so.200\"],"\

"\"feature\":\"delay-accounting\","\

"\"description:\":\"Enables delay accounting support\","\

"\"priority\":\"recommended\""\

"}"\

"]" )

I'm not a fan of having a full JSON syntax in a macro caller. Can we rework the macros so that only important fields will be used as parameters?

I expect the usage would ideally work like this:

#define ELF_NOTE_DLOPEN_DEP(sonames, feature_tag, priority) ... // Example ELF_NOTE_DLOPEN_DEP( "[\"libnl-3.so\",\"libnl-3.so.200\"]", "delay-accounting", "recommended") ELF_NOTE_DLOPEN_DEP( "[\"libnl-genl-3.so\",\"libnl-genl-3.so.200\"]", "delay-accounting", "recommended")

How do you think?

While I also thought about this, including making generating the JSON be split into different macro calls (definitively possible AFAICS), there's one minor issue with the pre-processor IMHO, that prevents this from being really nice syntax. The issue is, that you can't properly auto-terminate lists of arguments to insert the semicolon in list contexts, thus you'd most likely have to resort to something like "[" ELF_NOTE_DEP(foo…) "," ELF_NOTE_DEP(bar…) "]"; which doesn't look too terrible, but in a really great world could work without the explicit "," token in between.

Also from a practcal PoV, I'd prefer such macros to go ELF_NOTE_DLOPEN_DEP(freature, level, description, solist)`, but comparibly that's a minor change to implement.

BenBE · 2024-05-19T00:46:35Z

Macros.h

+      } nhdr;                                                                                      \
+      char name[sizeof(ELF_NOTE_DLOPEN_OWNER)];                                                    \
+      alignas(sizeof(uint32_t)) char dlopen_content[sizeof(content)];                              \
+   } variable_name = {                                                                             \


Shouldn't be variable_name be dynamic?

BenBE · 2024-05-19T00:52:48Z

linux/LibSensors.c

+   DECLARE_ELF_NOTE_DLOPEN("[{\"soname\":[\"libsensors.so\",\"libsensors.so.5\",\"libsensors.so.4\"],\"feature\":"\
+      "\"sensors\",\"description:\":\"Enables hardware sensor support\",\"priority\":\"recommended\"}]")


Multiple lines, example above …

BenBE · 2024-05-19T00:52:56Z

linux/SystemdMeter.c

+   DECLARE_ELF_NOTE_DLOPEN("[{\"soname\":[\"libsystemd.so.0\"],\"feature\":\"systemd\",\"description:\":"\
+      "\"Enables systemd support\",\"priority\":\"suggested\"}]")


Multiple lines, example above …

BenBE · 2024-05-20T00:26:37Z

Any objections to having the code for generating these annotations be its own source code file?

NB: There's also __attribute__((aligned(4)))) available to play around, which seems to have been supported by GCC for a long time (no idea re Clang) …

Explorer09 · 2024-05-20T10:03:18Z

Just my general comments:
I am personally not a fan of the specification of embedding the dlopen dependency information in ELF binaries. For me it look like increasing the binary size with little benefit, and the same information can be provided by (distros) package managers already.

Besides, this information is ELF-specific, which could potentially discriminate other OSes such as BSDs.
I wonder if the same information can be provided through a separate file, not embedding into binaries? The manifest file for Windows executable binaries can work either way - provided separately with ".manifest" extension, or embedding into resource sections of ".exe" files.

cgzones added the dependencies Pull requests that update a dependency file label May 18, 2024

cgzones assigned fasterit May 18, 2024

cgzones force-pushed the note-dlopen branch from 15513e6 to 7dc0ab1 Compare May 18, 2024 21:30

cgzones force-pushed the note-dlopen branch from 7dc0ab1 to 429d90a Compare May 18, 2024 21:34

BenBE reviewed May 19, 2024

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BenBE added the build system 🔧 Affects the build system rather then the user experience label May 19, 2024

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Embed ELF metadata about dynamic runtime dependencies #1482

Embed ELF metadata about dynamic runtime dependencies #1482

cgzones commented May 18, 2024 •

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cgzones commented May 18, 2024

BenBE May 19, 2024

BenBE May 19, 2024

Explorer09 May 19, 2024

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Explorer09 May 19, 2024 •

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BenBE commented May 20, 2024

Explorer09 commented May 20, 2024 •

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	__attribute__((used, section(".note.dlopen"))) alignas(sizeof(uint32_t)) static const struct { \
	__attribute__((packed, used, section(".note.dlopen"))) static const struct { \

	char name[sizeof(ELF_NOTE_DLOPEN_OWNER)]; \
	char name[(sizeof(ELF_NOTE_DLOPEN_OWNER)+3)&(~3)]; \

	alignas(sizeof(uint32_t)) char dlopen_content[sizeof(content)]; \
	char dlopen_content[(sizeof(content)+3)&(~3)]; \

-   DECLARE_ELF_NOTE_DLOPEN("[{\"soname\":[\"libnl-3.so\",\"libnl-3.so.200\"],\"feature\":\"delay-accounting\"," \
-      "\"description:\":\"Enables delay accounting support\",\"priority\":\"recommended\"},{\"soname\":[" \
-      "\"libnl-genl-3.so\",\"libnl-genl-3.so.200\"],\"feature\":\"delay-accounting\",\"description:\":"\
-      "\"Enables delay accounting support\",\"priority\":\"recommended\"}]")
+   DECLARE_ELF_NOTE_DLOPEN( "["\
+      "{"\
+         "\"soname\":[\"libnl-3.so\",\"libnl-3.so.200\"],"\
+         "\"feature\":\"delay-accounting\"," \
+         "\"description:\":\"Enables delay accounting support\","\
+         "\"priority\":\"recommended\""\
+      "},"\
+      "{"\
+         "\"soname\":[\"libnl-genl-3.so\",\"libnl-genl-3.so.200\"],"\
+         "\"feature\":\"delay-accounting\","\
+         "\"description:\":\"Enables delay accounting support\","\
+         "\"priority\":\"recommended\""\
+      "}"\
+   "]" )

		DECLARE_ELF_NOTE_DLOPEN("[{\"soname\":[\"libsensors.so\",\"libsensors.so.5\",\"libsensors.so.4\"],\"feature\":"\
		"\"sensors\",\"description:\":\"Enables hardware sensor support\",\"priority\":\"recommended\"}]")

		DECLARE_ELF_NOTE_DLOPEN("[{\"soname\":[\"libsystemd.so.0\"],\"feature\":\"systemd\",\"description:\":"\
		"\"Enables systemd support\",\"priority\":\"suggested\"}]")

Embed ELF metadata about dynamic runtime dependencies #1482

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Embed ELF metadata about dynamic runtime dependencies #1482

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cgzones commented May 18, 2024 • edited Loading

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