[PATCH] riscv: Use $(LD) instead of $(CC) to link vDSO

[PATCH] riscv: Use $(LD) instead of $(CC) to link vDSO

[Nathan Chancellor]

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>
---
 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
-- 
2.31.0


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Re: [PATCH] riscv: Use $(LD) instead of $(CC) to link vDSO

[kernel test robot]

[-- Attachment #1: Type: text/plain, Size: 1851 bytes --]

Hi Nathan,

I love your patch! Yet something to improve:

[auto build test ERROR on linus/master]
[also build test ERROR on v5.12-rc4 next-20210326]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Nathan-Chancellor/riscv-Use-LD-instead-of-CC-to-link-vDSO/20210326-055421
base:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 002322402dafd846c424ffa9240a937f49b48c42
config: riscv-randconfig-r032-20210326 (attached as .config)
compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project f490a5969bd52c8a48586f134ff8f02ccbb295b3)
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # install riscv cross compiling tool for clang build
        # apt-get install binutils-riscv64-linux-gnu
        # https://github.com/0day-ci/linux/commit/dfdcaf93f40f0d15ffc3f25128442c1688e612d6
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Nathan-Chancellor/riscv-Use-LD-instead-of-CC-to-link-vDSO/20210326-055421
        git checkout dfdcaf93f40f0d15ffc3f25128442c1688e612d6
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=riscv 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

>> riscv64-linux-gnu-objcopy: 'arch/riscv/kernel/vdso/vdso.so.dbg': No such file

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org

[-- Attachment #2: .config.gz --]
[-- Type: application/gzip, Size: 33732 bytes --]

[-- Attachment #3: Type: text/plain, Size: 161 bytes --]

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Re: [PATCH] riscv: Use $(LD) instead of $(CC) to link vDSO

[Nathan Chancellor]

On Sat, Mar 27, 2021 at 12:05:34AM +0800, kernel test robot wrote:
> Hi Nathan,
> 
> I love your patch! Yet something to improve:
> 
> [auto build test ERROR on linus/master]
> [also build test ERROR on v5.12-rc4 next-20210326]
> [If your patch is applied to the wrong git tree, kindly drop us a note.
> And when submitting patch, we suggest to use '--base' as documented in
> https://git-scm.com/docs/git-format-patch]
> 
> url:    https://github.com/0day-ci/linux/commits/Nathan-Chancellor/riscv-Use-LD-instead-of-CC-to-link-vDSO/20210326-055421
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 002322402dafd846c424ffa9240a937f49b48c42
> config: riscv-randconfig-r032-20210326 (attached as .config)
> compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project f490a5969bd52c8a48586f134ff8f02ccbb295b3)
> reproduce (this is a W=1 build):
>         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>         chmod +x ~/bin/make.cross
>         # install riscv cross compiling tool for clang build
>         # apt-get install binutils-riscv64-linux-gnu
>         # https://github.com/0day-ci/linux/commit/dfdcaf93f40f0d15ffc3f25128442c1688e612d6
>         git remote add linux-review https://github.com/0day-ci/linux
>         git fetch --no-tags linux-review Nathan-Chancellor/riscv-Use-LD-instead-of-CC-to-link-vDSO/20210326-055421
>         git checkout dfdcaf93f40f0d15ffc3f25128442c1688e612d6
>         # save the attached .config to linux build tree
>         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=riscv 

For the record, I tried to use this script to reproduce but it has a
couple of bugs:

1. It does not download the right version of clang. This report says
that it is clang-13 but the one that the script downloaded is clang-12.

2. It does not download it to the right location. The script expects
~/0day/clang-latest but it is downloaded to ~/0day/clang it seems. I
symlinked it to get around it.

> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@intel.com>
> 
> All errors (new ones prefixed by >>):
> 
> >> riscv64-linux-gnu-objcopy: 'arch/riscv/kernel/vdso/vdso.so.dbg': No such file

This error only occurs because of errors before it that are not shown
due to a denylist:

ld.lld: error: arch/riscv/kernel/vdso/rt_sigreturn.o:(.text+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax
ld.lld: error: arch/riscv/kernel/vdso/getcpu.o:(.text+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax
ld.lld: error: arch/riscv/kernel/vdso/flush_icache.o:(.text+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax

My patch only adds another occurrence of this error because we move from
$(CC)'s default linker (in clang's case, ld.bfd) to $(LD), which in the
case of 0day appears to be ld.lld. ld.lld should not be used with RISC-V
in its current form due to errors of this nature, which happen without
my patch as well:

https://github.com/ClangBuiltLinux/linux/issues/1020

Linker relaxation in ld.lld for RISC-V is an ongoing debate/process.
Please give RISC-V the current treatment as s390 with ld.lld for the
time being to get meaningful reports. We will reach out once that issue
has been resolved.

TL;DR: Patch exposes existing issue with LD=ld.lld that would have
happened without it in different areas, the report can be ignored.

Cheers!
Nathan

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Re: [PATCH] riscv: Use $(LD) instead of $(CC) to link vDSO

[Fangrui Song]

On 2021-03-26, Nathan Chancellor wrote:
>On Sat, Mar 27, 2021 at 12:05:34AM +0800, kernel test robot wrote:
>> Hi Nathan,
>>
>> I love your patch! Yet something to improve:
>>
>> [auto build test ERROR on linus/master]
>> [also build test ERROR on v5.12-rc4 next-20210326]
>> [If your patch is applied to the wrong git tree, kindly drop us a note.
>> And when submitting patch, we suggest to use '--base' as documented in
>> https://git-scm.com/docs/git-format-patch]
>>
>> url:    https://github.com/0day-ci/linux/commits/Nathan-Chancellor/riscv-Use-LD-instead-of-CC-to-link-vDSO/20210326-055421
>> base:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 002322402dafd846c424ffa9240a937f49b48c42
>> config: riscv-randconfig-r032-20210326 (attached as .config)
>> compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project f490a5969bd52c8a48586f134ff8f02ccbb295b3)
>> reproduce (this is a W=1 build):
>>         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>>         chmod +x ~/bin/make.cross
>>         # install riscv cross compiling tool for clang build
>>         # apt-get install binutils-riscv64-linux-gnu
>>         # https://github.com/0day-ci/linux/commit/dfdcaf93f40f0d15ffc3f25128442c1688e612d6
>>         git remote add linux-review https://github.com/0day-ci/linux
>>         git fetch --no-tags linux-review Nathan-Chancellor/riscv-Use-LD-instead-of-CC-to-link-vDSO/20210326-055421
>>         git checkout dfdcaf93f40f0d15ffc3f25128442c1688e612d6
>>         # save the attached .config to linux build tree
>>         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=riscv
>
>For the record, I tried to use this script to reproduce but it has a
>couple of bugs:
>
>1. It does not download the right version of clang. This report says
>that it is clang-13 but the one that the script downloaded is clang-12.
>
>2. It does not download it to the right location. The script expects
>~/0day/clang-latest but it is downloaded to ~/0day/clang it seems. I
>symlinked it to get around it.
>
>> If you fix the issue, kindly add following tag as appropriate
>> Reported-by: kernel test robot <lkp@intel.com>
>>
>> All errors (new ones prefixed by >>):
>>
>> >> riscv64-linux-gnu-objcopy: 'arch/riscv/kernel/vdso/vdso.so.dbg': No such file
>
>This error only occurs because of errors before it that are not shown
>due to a denylist:
>
>ld.lld: error: arch/riscv/kernel/vdso/rt_sigreturn.o:(.text+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax
>ld.lld: error: arch/riscv/kernel/vdso/getcpu.o:(.text+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax
>ld.lld: error: arch/riscv/kernel/vdso/flush_icache.o:(.text+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax
>
>My patch only adds another occurrence of this error because we move from
>$(CC)'s default linker (in clang's case, ld.bfd) to $(LD), which in the
>case of 0day appears to be ld.lld. ld.lld should not be used with RISC-V
>in its current form due to errors of this nature, which happen without
>my patch as well:
>
>https://github.com/ClangBuiltLinux/linux/issues/1020
>
>Linker relaxation in ld.lld for RISC-V is an ongoing debate/process.
>Please give RISC-V the current treatment as s390 with ld.lld for the
>time being to get meaningful reports. We will reach out once that issue
>has been resolved.

>TL;DR: Patch exposes existing issue with LD=ld.lld that would have
>happened without it in different areas, the report can be ignored.

Yes, lkp frequently reports this error. It can be suppressed by using
-mno-relax... if ld.lld is picked.

Hmm. This motivated me to file
https://github.com/riscv/riscv-elf-psabi-doc/issues/183
R_RISCV_ALIGN friendly to linkers not supporting relaxation (riscv_relax_delete_bytes).

>Cheers!
>Nathan
>
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Re: [PATCH] riscv: Use $(LD) instead of $(CC) to link vDSO

[Fangrui Song]

On 2021-03-25, Nathan Chancellor wrote:
>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.

Looks good.

-pg affects the link action: it changes crt1.o to gcrt1.o.
Since the Makefile uses -nostdlib, crt1.o is suppressed, so -pg
is entirely unneeded.
(-nostdlib implies -nostartfiles so the previous usage has a redundant
option.)

>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.

Another minor change which may be shipped together: --hash-style=both
can be --hash-style=gnu. We don't need sysv .hash . The glibc/musl
support for .gnu.hash has been there for years. .gnu.hash is often
smaller than .hash .

Reviewed-by: Fangrui Song <maskray@google.com>

>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>
>---
> 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
>-- 
>2.31.0
>
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Re: [kbuild-all] Re: [PATCH] riscv: Use $(LD) instead of $(CC) to link vDSO

[Rong Chen]

Hi Nathan,

On 3/27/21 7:58 AM, Nathan Chancellor wrote:
> On Sat, Mar 27, 2021 at 12:05:34AM +0800, kernel test robot wrote:
>> Hi Nathan,
>>
>> I love your patch! Yet something to improve:
>>
>> [auto build test ERROR on linus/master]
>> [also build test ERROR on v5.12-rc4 next-20210326]
>> [If your patch is applied to the wrong git tree, kindly drop us a note.
>> And when submitting patch, we suggest to use '--base' as documented in
>> https://git-scm.com/docs/git-format-patch]
>>
>> url:    https://github.com/0day-ci/linux/commits/Nathan-Chancellor/riscv-Use-LD-instead-of-CC-to-link-vDSO/20210326-055421
>> base:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 002322402dafd846c424ffa9240a937f49b48c42
>> config: riscv-randconfig-r032-20210326 (attached as .config)
>> compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project f490a5969bd52c8a48586f134ff8f02ccbb295b3)
>> reproduce (this is a W=1 build):
>>          wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>>          chmod +x ~/bin/make.cross
>>          # install riscv cross compiling tool for clang build
>>          # apt-get install binutils-riscv64-linux-gnu
>>          # https://github.com/0day-ci/linux/commit/dfdcaf93f40f0d15ffc3f25128442c1688e612d6
>>          git remote add linux-review https://github.com/0day-ci/linux
>>          git fetch --no-tags linux-review Nathan-Chancellor/riscv-Use-LD-instead-of-CC-to-link-vDSO/20210326-055421
>>          git checkout dfdcaf93f40f0d15ffc3f25128442c1688e612d6
>>          # save the attached .config to linux build tree
>>          COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=riscv
> For the record, I tried to use this script to reproduce but it has a
> couple of bugs:
>
> 1. It does not download the right version of clang. This report says
> that it is clang-13 but the one that the script downloaded is clang-12.
>
> 2. It does not download it to the right location. The script expects
> ~/0day/clang-latest but it is downloaded to ~/0day/clang it seems. I
> symlinked it to get around it.

Sorry for the inconvenience, we'll fix both asap.

Best Regards,
Rong Chen

>
>> If you fix the issue, kindly add following tag as appropriate
>> Reported-by: kernel test robot <lkp@intel.com>
>>
>> All errors (new ones prefixed by >>):
>>
>>>> riscv64-linux-gnu-objcopy: 'arch/riscv/kernel/vdso/vdso.so.dbg': No such file
> This error only occurs because of errors before it that are not shown
> due to a denylist:
>
> ld.lld: error: arch/riscv/kernel/vdso/rt_sigreturn.o:(.text+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax
> ld.lld: error: arch/riscv/kernel/vdso/getcpu.o:(.text+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax
> ld.lld: error: arch/riscv/kernel/vdso/flush_icache.o:(.text+0x0): relocation R_RISCV_ALIGN requires unimplemented linker relaxation; recompile with -mno-relax
>
> My patch only adds another occurrence of this error because we move from
> $(CC)'s default linker (in clang's case, ld.bfd) to $(LD), which in the
> case of 0day appears to be ld.lld. ld.lld should not be used with RISC-V
> in its current form due to errors of this nature, which happen without
> my patch as well:
>
> https://github.com/ClangBuiltLinux/linux/issues/1020
>
> Linker relaxation in ld.lld for RISC-V is an ongoing debate/process.
> Please give RISC-V the current treatment as s390 with ld.lld for the
> time being to get meaningful reports. We will reach out once that issue
> has been resolved.
>
> TL;DR: Patch exposes existing issue with LD=ld.lld that would have
> happened without it in different areas, the report can be ignored.
>
> Cheers!
> Nathan
> _______________________________________________
> kbuild-all mailing list -- kbuild-all@lists.01.org
> To unsubscribe send an email to kbuild-all-leave@lists.01.org


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Re: [PATCH] riscv: Use $(LD) instead of $(CC) to link vDSO

[Palmer Dabbelt]

On Thu, 25 Mar 2021 14:51:56 PDT (-0700), nathan@kernel.org wrote:
> 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>
> ---
>  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

Thanks, this is on for-next.

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