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    Subject[PATCH 5.10 23/47] riscv: Use $(LD) instead of $(CC) to link vDSO
    Date
    From: Nathan Chancellor <nathan@kernel.org>

    [ Upstream commit 7f3d349065d0c643f7f7013fbf9bc9f2c90b675f ]

    Currently, the VDSO is being linked through $(CC). This does not match
    how the rest of the kernel links objects, which is through the $(LD)
    variable.

    When linking with clang, there are a couple of warnings about flags that
    will not be used during the link:

    clang-12: warning: argument unused during compilation: '-no-pie' [-Wunused-command-line-argument]
    clang-12: warning: argument unused during compilation: '-pg' [-Wunused-command-line-argument]

    '-no-pie' was added in commit 85602bea297f ("RISC-V: build vdso-dummy.o
    with -no-pie") to override '-pie' getting added to the ld command from
    distribution versions of GCC that enable PIE by default. It is
    technically no longer needed after commit c2c81bb2f691 ("RISC-V: Fix the
    VDSO symbol generaton for binutils-2.35+"), which removed vdso-dummy.o
    in favor of generating vdso-syms.S from vdso.so with $(NM) but this also
    resolves the issue in case it ever comes back due to having full control
    over the $(LD) command. '-pg' is for function tracing, it is not used
    during linking as clang states.

    These flags could be removed/filtered to fix the warnings but it is
    easier to just match the rest of the kernel and use $(LD) directly for
    linking. See commits

    fe00e50b2db8 ("ARM: 8858/1: vdso: use $(LD) instead of $(CC) to link VDSO")
    691efbedc60d ("arm64: vdso: use $(LD) instead of $(CC) to link VDSO")
    2ff906994b6c ("MIPS: VDSO: Use $(LD) instead of $(CC) to link VDSO")
    2b2a25845d53 ("s390/vdso: Use $(LD) instead of $(CC) to link vDSO")

    for more information.

    The flags are converted to linker flags and '--eh-frame-hdr' is added to
    match what is added by GCC implicitly, which can be seen by adding '-v'
    to GCC's invocation.

    Additionally, since this area is being modified, use the $(OBJCOPY)
    variable instead of an open coded $(CROSS_COMPILE)objcopy so that the
    user's choice of objcopy binary is respected.

    Link: https://github.com/ClangBuiltLinux/linux/issues/803
    Link: https://github.com/ClangBuiltLinux/linux/issues/970
    Signed-off-by: Nathan Chancellor <nathan@kernel.org>
    Reviewed-by: Fangrui Song <maskray@google.com>
    Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/riscv/kernel/vdso/Makefile | 12 ++++--------
    1 file changed, 4 insertions(+), 8 deletions(-)

    diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
    index 71a315e73cbe..ca2b40dfd24b 100644
    --- a/arch/riscv/kernel/vdso/Makefile
    +++ b/arch/riscv/kernel/vdso/Makefile
    @@ -41,11 +41,10 @@ KASAN_SANITIZE := n
    $(obj)/vdso.o: $(obj)/vdso.so

    # link rule for the .so file, .lds has to be first
    -SYSCFLAGS_vdso.so.dbg = $(c_flags)
    $(obj)/vdso.so.dbg: $(src)/vdso.lds $(obj-vdso) FORCE
    $(call if_changed,vdsold)
    -SYSCFLAGS_vdso.so.dbg = -shared -s -Wl,-soname=linux-vdso.so.1 \
    - -Wl,--build-id=sha1 -Wl,--hash-style=both
    +LDFLAGS_vdso.so.dbg = -shared -s -soname=linux-vdso.so.1 \
    + --build-id=sha1 --hash-style=both --eh-frame-hdr

    # We also create a special relocatable object that should mirror the symbol
    # table and layout of the linked DSO. With ld --just-symbols we can then
    @@ -60,13 +59,10 @@ $(obj)/%.so: $(obj)/%.so.dbg FORCE

    # actual build commands
    # The DSO images are built using a special linker script
    -# Add -lgcc so rv32 gets static muldi3 and lshrdi3 definitions.
    # Make sure only to export the intended __vdso_xxx symbol offsets.
    quiet_cmd_vdsold = VDSOLD $@
    - cmd_vdsold = $(CC) $(KBUILD_CFLAGS) $(call cc-option, -no-pie) -nostdlib -nostartfiles $(SYSCFLAGS_$(@F)) \
    - -Wl,-T,$(filter-out FORCE,$^) -o $@.tmp && \
    - $(CROSS_COMPILE)objcopy \
    - $(patsubst %, -G __vdso_%, $(vdso-syms)) $@.tmp $@ && \
    + cmd_vdsold = $(LD) $(ld_flags) -T $(filter-out FORCE,$^) -o $@.tmp && \
    + $(OBJCOPY) $(patsubst %, -G __vdso_%, $(vdso-syms)) $@.tmp $@ && \
    rm $@.tmp

    # Extracts symbol offsets from the VDSO, converting them into an assembly file
    --
    2.30.2


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