[v2] Prevent dynamic relocations in vDSO

[PATCH v2 1/2] riscv: vdso: Prevent gcc from inserting calls to memset()

Posted by Alexandre Ghiti 1 year, 7 months ago

gcc is smart enough to insert a call to memset() in
riscv_vdso_get_cpus(), which generates a dynamic relocation.

So prevent gcc from doing that by using the
-fno-tree-loop-distribute-patterns option.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
---
 arch/riscv/kernel/vdso/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
index f7ef8ad9b550..c7e40bf36371 100644
--- a/arch/riscv/kernel/vdso/Makefile
+++ b/arch/riscv/kernel/vdso/Makefile
@@ -18,6 +18,7 @@ obj-vdso = $(patsubst %, %.o, $(vdso-syms)) note.o
 
 ccflags-y := -fno-stack-protector
 ccflags-y += -DDISABLE_BRANCH_PROFILING
+ccflags-y += -fno-tree-loop-distribute-patterns
 
 ifneq ($(c-gettimeofday-y),)
   CFLAGS_vgettimeofday.o += -fPIC -include $(c-gettimeofday-y)
-- 
2.39.2

Re: [PATCH v2 1/2] riscv: vdso: Prevent gcc from inserting calls to memset()

Posted by Palmer Dabbelt 1 year, 4 months ago

On Thu, 04 Jul 2024 23:09:01 PDT (-0700), alexghiti@rivosinc.com wrote:
> gcc is smart enough to insert a call to memset() in
> riscv_vdso_get_cpus(), which generates a dynamic relocation.
>
> So prevent gcc from doing that by using the
> -fno-tree-loop-distribute-patterns option.

This option doesn't exist on LLVM (at least on whatever version I'm 
using), and I'm not getting any memset()s generated locally so I'm not 
sure what to look for over there.

From poking around GCC it looks like we might want -fno-tree-ccp too?  
That seems to be able to convert assignments into builtins as well...

> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
>  arch/riscv/kernel/vdso/Makefile | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
> index f7ef8ad9b550..c7e40bf36371 100644
> --- a/arch/riscv/kernel/vdso/Makefile
> +++ b/arch/riscv/kernel/vdso/Makefile
> @@ -18,6 +18,7 @@ obj-vdso = $(patsubst %, %.o, $(vdso-syms)) note.o
>
>  ccflags-y := -fno-stack-protector
>  ccflags-y += -DDISABLE_BRANCH_PROFILING
> +ccflags-y += -fno-tree-loop-distribute-patterns
>
>  ifneq ($(c-gettimeofday-y),)
>    CFLAGS_vgettimeofday.o += -fPIC -include $(c-gettimeofday-y)

Re: [PATCH v2 1/2] riscv: vdso: Prevent gcc from inserting calls to memset()

Posted by Alexandre Ghiti 1 year, 3 months ago

Hi Palmer,

On 17/09/2024 17:41, Palmer Dabbelt wrote:
> On Thu, 04 Jul 2024 23:09:01 PDT (-0700), alexghiti@rivosinc.com wrote:
>> gcc is smart enough to insert a call to memset() in
>> riscv_vdso_get_cpus(), which generates a dynamic relocation.
>>
>> So prevent gcc from doing that by using the
>> -fno-tree-loop-distribute-patterns option.
>
> This option doesn't exist on LLVM (at least on whatever version I'm 
> using), and I'm not getting any memset()s generated locally so I'm not 
> sure what to look for over there.
>

When compiling a rv32_defconfig (still on 6.12), I get:

~/linux$ riscv64-unknown-elf-readelf -r 
build_defconfig_rv32/arch/riscv/kernel/vdso/vdso.so

Relocation section '.rela.dyn' at offset 0x5a0 contains 1 entry:
  Offset     Info    Type            Sym.Value  Sym. Name + Addend
00000314  00000205 R_RISCV_JUMP_SLOT 00000000   memset + 0


> From poking around GCC it looks like we might want -fno-tree-ccp too? 
> That seems to be able to convert assignments into builtins as well...


I gave a try at -fno-tree-ccp, but it did not remove this relocation.

I now see this relocation in llvm too, so it would be nice to find a 
common option or another way to do it, any idea is welcome!

Thanks,

Alex


>
>> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
>> ---
>>  arch/riscv/kernel/vdso/Makefile | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/riscv/kernel/vdso/Makefile 
>> b/arch/riscv/kernel/vdso/Makefile
>> index f7ef8ad9b550..c7e40bf36371 100644
>> --- a/arch/riscv/kernel/vdso/Makefile
>> +++ b/arch/riscv/kernel/vdso/Makefile
>> @@ -18,6 +18,7 @@ obj-vdso = $(patsubst %, %.o, $(vdso-syms)) note.o
>>
>>  ccflags-y := -fno-stack-protector
>>  ccflags-y += -DDISABLE_BRANCH_PROFILING
>> +ccflags-y += -fno-tree-loop-distribute-patterns
>>
>>  ifneq ($(c-gettimeofday-y),)
>>    CFLAGS_vgettimeofday.o += -fPIC -include $(c-gettimeofday-y)
>
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Re: [PATCH v2 1/2] riscv: vdso: Prevent gcc from inserting calls to memset()

Posted by Guo Ren 1 year, 7 months ago

On Fri, Jul 5, 2024 at 2:10 PM Alexandre Ghiti <alexghiti@rivosinc.com> wrote:
>
> gcc is smart enough to insert a call to memset() in
> riscv_vdso_get_cpus(), which generates a dynamic relocation.
>
> So prevent gcc from doing that by using the
> -fno-tree-loop-distribute-patterns option.
Good catch. I think we could put a comment on
"-fno-tree-loop-distribute-patterns". eg: Perform loop distribution of
patterns that can be code generated with calls to a library.

Others, LGTM!

Reviewed-by: Guo Ren <guoren@kernel.org>

>
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
>  arch/riscv/kernel/vdso/Makefile | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
> index f7ef8ad9b550..c7e40bf36371 100644
> --- a/arch/riscv/kernel/vdso/Makefile
> +++ b/arch/riscv/kernel/vdso/Makefile
> @@ -18,6 +18,7 @@ obj-vdso = $(patsubst %, %.o, $(vdso-syms)) note.o
>
>  ccflags-y := -fno-stack-protector
>  ccflags-y += -DDISABLE_BRANCH_PROFILING
> +ccflags-y += -fno-tree-loop-distribute-patterns
>
>  ifneq ($(c-gettimeofday-y),)
>    CFLAGS_vgettimeofday.o += -fPIC -include $(c-gettimeofday-y)
> --
> 2.39.2
>


-- 
Best Regards
 Guo Ren