[PATCH v2] RISC-V: Support 32_PCREL relocation type in kernel module

[PATCH v2] RISC-V: Support 32_PCREL relocation type in kernel module

[Nylon Chen]

Fix the 'unsupported relocation type' error caused by
enabling the -fasynchronous-unwind-tables flag,
which generates relocation types that are not supported.

Signed-off-by: Nylon Chen <nylon.chen@sifive.com>
Reviewed-by: Zong Li <zong.li@sifive.com>
---
Changed in v2:
- add commit message.

 arch/riscv/kernel/module.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/riscv/kernel/module.c b/arch/riscv/kernel/module.c
index 7c651d55fcbd..65be0360a494 100644
--- a/arch/riscv/kernel/module.c
+++ b/arch/riscv/kernel/module.c
@@ -310,6 +310,15 @@ static int apply_r_riscv_sub64_rela(struct module *me, u32 *location,
 	return 0;
 }
 
+static int apply_r_riscv_pcrel_32_rela(struct module *me, u32 *location,
+				       Elf_Addr v)
+{
+	ptrdiff_t offset = (void *)v - (void *)location;
+
+	*location = (*location & 0xffff0000) | (offset & 0xffff);
+	return 0;
+}
+
 static int (*reloc_handlers_rela[]) (struct module *me, u32 *location,
 				Elf_Addr v) = {
 	[R_RISCV_32]			= apply_r_riscv_32_rela,
@@ -335,6 +344,7 @@ static int (*reloc_handlers_rela[]) (struct module *me, u32 *location,
 	[R_RISCV_SUB16]			= apply_r_riscv_sub16_rela,
 	[R_RISCV_SUB32]			= apply_r_riscv_sub32_rela,
 	[R_RISCV_SUB64]			= apply_r_riscv_sub64_rela,
+	[R_RISCV_32_PCREL]		= apply_r_riscv_pcrel_32_rela,
 };
 
 int apply_relocate_add(Elf_Shdr *sechdrs, const char *strtab,
-- 
2.40.1


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Re: [PATCH v2] RISC-V: Support 32_PCREL relocation type in kernel module

[Andreas Schwab]

On Jun 15 2023, Nylon Chen wrote:

> Fix the 'unsupported relocation type' error caused by
> enabling the -fasynchronous-unwind-tables flag,
> which generates relocation types that are not supported.

arch/riscv/Makefile has
KBUILD_CFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables

-- 
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Re: [PATCH v2] RISC-V: Support 32_PCREL relocation type in kernel module

[Conor Dooley]

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Hey Nylon, thanks for the update.

On Thu, Jun 15, 2023 at 03:23:02PM +0800, Nylon Chen wrote:
> Fix the 'unsupported relocation type' error caused by
> enabling the -fasynchronous-unwind-tables flag,
> which generates relocation types that are not supported.

What commit adds the -fasynchronous-unwind-tables flag?
Should there be a Fixes: tag for that commit?

Cheers,
Conor.

> 
> Signed-off-by: Nylon Chen <nylon.chen@sifive.com>
> Reviewed-by: Zong Li <zong.li@sifive.com>
> ---
> Changed in v2:
> - add commit message.
> 
>  arch/riscv/kernel/module.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/riscv/kernel/module.c b/arch/riscv/kernel/module.c
> index 7c651d55fcbd..65be0360a494 100644
> --- a/arch/riscv/kernel/module.c
> +++ b/arch/riscv/kernel/module.c
> @@ -310,6 +310,15 @@ static int apply_r_riscv_sub64_rela(struct module *me, u32 *location,
>  	return 0;
>  }
>  
> +static int apply_r_riscv_pcrel_32_rela(struct module *me, u32 *location,
> +				       Elf_Addr v)
> +{
> +	ptrdiff_t offset = (void *)v - (void *)location;
> +
> +	*location = (*location & 0xffff0000) | (offset & 0xffff);
> +	return 0;
> +}
> +
>  static int (*reloc_handlers_rela[]) (struct module *me, u32 *location,
>  				Elf_Addr v) = {
>  	[R_RISCV_32]			= apply_r_riscv_32_rela,
> @@ -335,6 +344,7 @@ static int (*reloc_handlers_rela[]) (struct module *me, u32 *location,
>  	[R_RISCV_SUB16]			= apply_r_riscv_sub16_rela,
>  	[R_RISCV_SUB32]			= apply_r_riscv_sub32_rela,
>  	[R_RISCV_SUB64]			= apply_r_riscv_sub64_rela,
> +	[R_RISCV_32_PCREL]		= apply_r_riscv_pcrel_32_rela,
>  };
>  
>  int apply_relocate_add(Elf_Shdr *sechdrs, const char *strtab,
> -- 
> 2.40.1
> 

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Re: [PATCH v2] RISC-V: Support 32_PCREL relocation type in kernel module

[Andreas Schwab]

On Jun 15 2023, Nylon Chen wrote:

> I mean, when we use the flag "-fasynchronous-unwind-tables,"

Why?

-- 
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Re: [PATCH v2] RISC-V: Support 32_PCREL relocation type in kernel module

[Conor Dooley]

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Hey Nylon,

Firstly, no html emails please :/

On Thu, Jun 15, 2023 at 03:52:27PM +0800, Nylon Chen wrote:
> Conor Dooley <conor.dooley@microchip.com<mailto:conor.dooley@microchip.com>> 於 2023年6月15日 週四 下午3:38寫道：
> > On Thu, Jun 15, 2023 at 03:23:02PM +0800, Nylon Chen wrote:
> > > Fix the 'unsupported relocation type' error caused by
> > > enabling the -fasynchronous-unwind-tables flag,
> > > which generates relocation types that are not supported.
> >
> > What commit adds the -fasynchronous-unwind-tables flag?
> sorry my description is not correct, please allow me to add
> 
> I mean, when we use the flag "-fasynchronous-unwind-tables," it generates
> the relocation type R_RISCV_32_PCCREL. However, this type is currently not
> supported, so an error will occur.

(snip)

> > Should there be a Fixes: tag for that commit?

> yes, I will do it.

What mainline commit actually enables this flag?

Cheers,
Conor.

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Re: [PATCH v2] RISC-V: Support 32_PCREL relocation type in kernel module

[Nylon Chen]

On Thu, Jun 15, 2023 at 09:11:33AM +0100, Conor Dooley wrote:
Hi Conor,
> Hey Nylon,
> 
> Firstly, no html emails please :/
Sorry, my Gmail settings got messed up. I will be more careful in the future.
> 
> On Thu, Jun 15, 2023 at 03:52:27PM +0800, Nylon Chen wrote:
> > Conor Dooley <conor.dooley@microchip.com<mailto:conor.dooley@microchip.com>> 於 2023年6月15日 週四 下午3:38寫道：
> > > On Thu, Jun 15, 2023 at 03:23:02PM +0800, Nylon Chen wrote:
> > > > Fix the 'unsupported relocation type' error caused by
> > > > enabling the -fasynchronous-unwind-tables flag,
> > > > which generates relocation types that are not supported.
> > >
> > > What commit adds the -fasynchronous-unwind-tables flag?
> > sorry my description is not correct, please allow me to add
> > 
> > I mean, when we use the flag "-fasynchronous-unwind-tables," it generates
> > the relocation type R_RISCV_32_PCCREL. However, this type is currently not
> > supported, so an error will occur.
> 
> (snip)
> 
> > > Should there be a Fixes: tag for that commit?
> 
> > yes, I will do it.
> 
> What mainline commit actually enables this flag?
Because LLVM currently has it enabled by default(https://reviews.llvm.org/D145164), it will generate this
relocation type.

From what I know, GCC will also enable it in the future.

> 
> Cheers,
> Conor.



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Re: [PATCH v2] RISC-V: Support 32_PCREL relocation type in kernel module

[Andreas Schwab]

On Jun 15 2023, Nylon Chen wrote:

> Because LLVM currently has it enabled by default(https://reviews.llvm.org/D145164), it will generate this
> relocation type.
>
>>From what I know, GCC will also enable it in the future.

That's why the kernel explicitly disables it.

-- 
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GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
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Re: [PATCH v2] RISC-V: Support 32_PCREL relocation type in kernel module

[Nylon Chen]

On Thu, Jun 15, 2023 at 11:11:49AM +0200, Andreas Schwab wrote:
> On Jun 15 2023, Nylon Chen wrote:
>
Hi Andreas,
> > Because LLVM currently has it enabled by default(https://reviews.llvm.org/D145164), it will generate this
> > relocation type.
> >
> >>From what I know, GCC will also enable it in the future.
> 
> That's why the kernel explicitly disables it.
Ok, thanks for your feedback, after I cross-tested, there is indeed no relevant relocation type generated.

If this error no longer occurs.

I am open to the idea of adding this patch to the upstream and would like to hear your thoughts on whether it is still necessary. 
> 
> -- 
> Andreas Schwab, SUSE Labs, schwab@suse.de
> GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
> "And now for something completely different."

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Re: [PATCH v2] RISC-V: Support 32_PCREL relocation type in kernel module

[Nylon Chen]

Hi Andreas,

Currently, due to the lack of support for the unwind table in RISC-V,
we have disabled it.

If RISC-V were to support the unwind table in the future, would we
need to re-enable it? Would this require implementing related
changes(module relocation type handler) at that time?

Nylon Chen <nylon.chen@sifive.com> 於 2023年6月15日 週四 下午5:47寫道：


Nylon Chen <nylon.chen@sifive.com> 於 2023年6月15日 週四 下午5:47寫道：
>
> On Thu, Jun 15, 2023 at 11:11:49AM +0200, Andreas Schwab wrote:
> > On Jun 15 2023, Nylon Chen wrote:
> >
> Hi Andreas,
> > > Because LLVM currently has it enabled by default(https://reviews.llvm.org/D145164), it will generate this
> > > relocation type.
> > >
> > >>From what I know, GCC will also enable it in the future.
> >
> > That's why the kernel explicitly disables it.
> Ok, thanks for your feedback, after I cross-tested, there is indeed no relevant relocation type generated.
>
> If this error no longer occurs.
>
> I am open to the idea of adding this patch to the upstream and would like to hear your thoughts on whether it is still necessary.
> >
> > --
> > Andreas Schwab, SUSE Labs, schwab@suse.de
> > GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
> > "And now for something completely different."

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Re: [PATCH v2] RISC-V: Support 32_PCREL relocation type in kernel module

[Andreas Schwab]

On Aug 21 2023, Nylon Chen wrote:

> If RISC-V were to support the unwind table in the future, would we
> need to re-enable it? Would this require implementing related
> changes(module relocation type handler) at that time?

I'm not quite sure I understand your question.  Nothing in the kernel
makes use of unwind tables on RISC-V, so there is no point in generating
them.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."

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