| Message ID | 20220511185001.3269404-3-nathan@kernel.org (mailing list archive) |
|---|---|
| State | Accepted |
| Headers | show |
| Series | Link the PowerPC vDSO with ld.lld | expand |
| Context | Check | Description |
|---|---|---|
| snowpatch_ozlabs/github-powerpc_clang | success | Successfully ran 7 jobs. |
| snowpatch_ozlabs/github-powerpc_kernel_qemu | success | Successfully ran 24 jobs. |
| snowpatch_ozlabs/github-powerpc_ppctests | success | Successfully ran 8 jobs. |
| snowpatch_ozlabs/github-powerpc_selftests | success | Successfully ran 8 jobs. |
| snowpatch_ozlabs/github-powerpc_sparse | success | Successfully ran 4 jobs. |
diff --git a/arch/powerpc/kernel/vdso/Makefile b/arch/powerpc/kernel/vdso/Makefile index 954974287ee7..096b0bf1335f 100644 --- a/arch/powerpc/kernel/vdso/Makefile +++ b/arch/powerpc/kernel/vdso/Makefile @@ -48,6 +48,7 @@ UBSAN_SANITIZE := n KASAN_SANITIZE := n ccflags-y := -shared -fno-common -fno-builtin -nostdlib -Wl,--hash-style=both +ccflags-$(CONFIG_LD_IS_LLD) += $(call cc-option,--ld-path=$(LD),-fuse-ld=lld) CC32FLAGS := -Wl,-soname=linux-vdso32.so.1 -m32 AS32FLAGS := -D__VDSO32__ -s
The PowerPC vDSO uses $(CC) to link, which differs from the rest of the kernel, which uses $(LD) directly. As a result, the default linker of the compiler is used, which may differ from the linker requested by the builder. For example: $ make ARCH=powerpc LLVM=1 mrproper defconfig arch/powerpc/kernel/vdso/ ... $ llvm-readelf -p .comment arch/powerpc/kernel/vdso/vdso{32,64}.so.dbg File: arch/powerpc/kernel/vdso/vdso32.so.dbg String dump of section '.comment': [ 0] clang version 14.0.0 (Fedora 14.0.0-1.fc37) File: arch/powerpc/kernel/vdso/vdso64.so.dbg String dump of section '.comment': [ 0] clang version 14.0.0 (Fedora 14.0.0-1.fc37) LLVM=1 sets LD=ld.lld but ld.lld is not used to link the vDSO; GNU ld is because "ld" is the default linker for clang on most Linux platforms. This is a problem for Clang's Link Time Optimization as implemented in the kernel because use of GNU ld with LTO requires the LLVMgold plugin, which is not technically supported for ld.bfd per https://llvm.org/docs/GoldPlugin.html. Furthermore, if LLVMgold.so is missing from a user's system, the build will fail, even though LTO as it is implemented in the kernel requires ld.lld to avoid this dependency in the first place. Ultimately, the PowerPC vDSO should be converted to compiling and linking with $(CC) and $(LD) respectively but there were issues last time this was tried, potentially due to older but supported tool versions. To avoid regressing GCC + binutils, use the compiler option '-fuse-ld', which tells the compiler which linker to use when it is invoked as both the compiler and linker. Use '-fuse-ld=lld' when LD=ld.lld has been specified (CONFIG_LD_IS_LLD) so that the vDSO is linked with the same linker as the rest of the kernel. $ llvm-readelf -p .comment arch/powerpc/kernel/vdso/vdso{32,64}.so.dbg File: arch/powerpc/kernel/vdso/vdso32.so.dbg String dump of section '.comment': [ 0] Linker: LLD 14.0.0 [ 14] clang version 14.0.0 (Fedora 14.0.0-1.fc37) File: arch/powerpc/kernel/vdso/vdso64.so.dbg String dump of section '.comment': [ 0] Linker: LLD 14.0.0 [ 14] clang version 14.0.0 (Fedora 14.0.0-1.fc37) LD can be a full path to ld.lld, which will not be handled properly by '-fuse-ld=lld' if the full path to ld.lld is outside of the compiler's search path. '-fuse-ld' can take a path to the linker but it is deprecated in clang 12.0.0; '--ld-path' is preferred for this scenario. Use '--ld-path' if it is supported, as it will handle a full path or just 'ld.lld' properly. See the LLVM commit below for the full details of '--ld-path'. Link: https://github.com/ClangBuiltLinux/linux/issues/774 Link: https://github.com/llvm/llvm-project/commit/1bc5c84710a8c73ef21295e63c19d10a8c71f2f5 Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru> Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru> Signed-off-by: Nathan Chancellor <nathan@kernel.org> --- arch/powerpc/kernel/vdso/Makefile | 1 + 1 file changed, 1 insertion(+)