sd-bus: set retain attribute on BUS_ERROR_MAP_ELF_REGISTER #19144
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LLD 13 and GNU ld 2.37 support -z start-stop-gc which allows garbage collection of C identifier name sections despite the __start_/__stop_ references. Simply set the retain attribute so that GCC 11 (if configure-time binutils is 2.36 or newer)/Clang 13 will set the SHF_GNU_RETAIN section attribute to prevent garbage collection. Without the patch, there are linker errors like the following with -z start-stop-gc. ``` ld: error: undefined symbol: __start_SYSTEMD_BUS_ERROR_MAP >>> referenced by bus-error.c:93 (../src/libsystemd/sd-bus/bus-error.c:93) >>> sd-bus_bus-error.c.o:(bus_error_name_to_errno) in archive src/libsystemd/libsystemd_static.a ```
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LGTM. |
keszybz
reviewed
Mar 29, 2021
| */ | ||
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| #define BUS_ERROR_MAP_ELF_REGISTER \ | ||
| _Pragma("GCC diagnostic ignored \"-Wattributes\"") \ |
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This pragma will take effect until the end of the file where BUS_ERROR_MAP_ELF_REGISTER is used. That seems OK.
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bionic-i386: |
MaskRay
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Apr 16, 2021
Change the default to facilitate GC for metadata section usage, so that they don't need SHF_LINK_ORDER or SHF_GROUP just to drop the unhelpful rule (if they want to be unconditionally retained, use SHF_GNU_RETAIN (`__attribute__((retain))`) or linker script `KEEP`). The dropped SHF_GROUP special case makes the behavior of -z start-stop-gc and -z nostart-stop-gc closer to GNU ld>=2.37 (https://sourceware.org/PR27451). However, we default to -z start-stop-gc (which actually matches more closely to GNU ld before 2015-10 https://sourceware.org/PR19167), which is different from modern GNU ld (which has the unhelpful rule to work around glibc). As a compensation, we special case `__libc_` sections as a workaround for glibc<2.34 (https://sourceware.org/PR27492). Since -z start-stop-gc as the default actually matches the traditional GNU ld behavior, there isn't much to be aware of. There was a systemd usage which has been fixed by systemd/systemd#19144
arichardson
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Sep 10, 2021
Change the default to facilitate GC for metadata section usage, so that they don't need SHF_LINK_ORDER or SHF_GROUP just to drop the unhelpful rule (if they want to be unconditionally retained, use SHF_GNU_RETAIN (`__attribute__((retain))`) or linker script `KEEP`). The dropped SHF_GROUP special case makes the behavior of -z start-stop-gc and -z nostart-stop-gc closer to GNU ld>=2.37 (https://sourceware.org/PR27451). However, we default to -z start-stop-gc (which actually matches more closely to GNU ld before 2015-10 https://sourceware.org/PR19167), which is different from modern GNU ld (which has the unhelpful rule to work around glibc). As a compensation, we special case `__libc_` sections as a workaround for glibc<2.34 (https://sourceware.org/PR27492). Since -z start-stop-gc as the default actually matches the traditional GNU ld behavior, there isn't much to be aware of. There was a systemd usage which has been fixed by systemd/systemd#19144
mem-frob
pushed a commit
to draperlaboratory/hope-llvm-project
that referenced
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Oct 7, 2022
Change the default to facilitate GC for metadata section usage, so that they don't need SHF_LINK_ORDER or SHF_GROUP just to drop the unhelpful rule (if they want to be unconditionally retained, use SHF_GNU_RETAIN (`__attribute__((retain))`) or linker script `KEEP`). The dropped SHF_GROUP special case makes the behavior of -z start-stop-gc and -z nostart-stop-gc closer to GNU ld>=2.37 (https://sourceware.org/PR27451). However, we default to -z start-stop-gc (which actually matches more closely to GNU ld before 2015-10 https://sourceware.org/PR19167), which is different from modern GNU ld (which has the unhelpful rule to work around glibc). As a compensation, we special case `__libc_` sections as a workaround for glibc<2.34 (https://sourceware.org/PR27492). Since -z start-stop-gc as the default actually matches the traditional GNU ld behavior, there isn't much to be aware of. There was a systemd usage which has been fixed by systemd/systemd#19144
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LLD 13 and GNU ld 2.37 support -z start-stop-gc which allows garbage
collection of C identifier name sections despite the
__start_/__stop_references. Simply set the retain attribute so that GCC 11 (if
configure-time binutils is 2.36 or newer)/Clang 13 will set the
SHF_GNU_RETAIN section attribute to prevent garbage collection.
Without the patch, there are linker errors like the following with -z
start-stop-gc.
If you are not reluctant to read a (very long) writeup, https://maskray.me/blog/2021-01-31-metadata-sections-comdat-and-shf-link-order :)