Re: [PATCH bpf-next] riscv, bpf: fix some compiler error

[Björn Töpel]

On Tue, 2 Nov 2021 at 15:40, Tong Tiangen <tongtiangen@xxxxxxxxxx> wrote:
>
> This patch fix two compile errors:
> 1. when CONFIG_BPF_JIT and CONFIG_ARCH_32I is open, There is the following
> compiler error:
>   error: undefined symbol: rv_bpf_fixup_exception
>

Good catch for the RV32!

> 2. when CONFIG_BPF_JIT and CONFIG_ARCH_64I is open, There is the following
> compiler error (W=1):
>   error: no previous prototype for 'rv_bpf_fixup_exception'
>
> In this patch, asm/extable.h is introduced,  the rv_bpf_fixup_exception
> function declaration is added to this file. in addition, the definition of
> exception_table_entry is moved from asm-generic/extable.h to this file.
>

This is way too complicated. More below.

> Fixes: 252c765bd764 ("riscv, bpf: Add BPF exception tables")
> Signed-off-by: Tong Tiangen <tongtiangen@xxxxxxxxxx>
> ---
>  arch/riscv/include/asm/Kbuild    |  1 -
>  arch/riscv/include/asm/extable.h | 49 ++++++++++++++++++++++++++++++++
>  arch/riscv/include/asm/uaccess.h | 13 ---------
>  arch/riscv/mm/extable.c          |  8 +-----
>  4 files changed, 50 insertions(+), 21 deletions(-)
>  create mode 100644 arch/riscv/include/asm/extable.h
>
> diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
> index 445ccc97305a..57b86fd9916c 100644
> --- a/arch/riscv/include/asm/Kbuild
> +++ b/arch/riscv/include/asm/Kbuild
> @@ -1,6 +1,5 @@
>  # SPDX-License-Identifier: GPL-2.0
>  generic-y += early_ioremap.h
> -generic-y += extable.h
>  generic-y += flat.h
>  generic-y += kvm_para.h
>  generic-y += user.h
> diff --git a/arch/riscv/include/asm/extable.h b/arch/riscv/include/asm/extable.h
> new file mode 100644
> index 000000000000..aa0332b053fb
> --- /dev/null
> +++ b/arch/riscv/include/asm/extable.h
> @@ -0,0 +1,49 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ASM_EXTABLE_H
> +#define __ASM_EXTABLE_H
> +
> +/*
> + * The exception table consists of pairs of addresses: the first is the
> + * address of an instruction that is allowed to fault, and the second is
> + * the address at which the program should continue.  No registers are
> + * modified, so it is entirely up to the continuation code to figure out
> + * what to do.
> + *
> + * All the routines below use bits of fixup code that are out of line
> + * with the main instruction path.  This means when everything is well,
> + * we don't even have to jump over them.  Further, they do not intrude
> + * on our cache or tlb entries.
> + */
> +struct exception_table_entry {
> +       unsigned long insn, fixup;
> +};
> +
> +struct pt_regs;
> +int fixup_exception(struct pt_regs *regs);
> +
> +#if defined(CONFIG_MMU)
> +static inline bool rv_in_bpf_jit(struct pt_regs *regs)
> +{
> +       if (!IS_ENABLED(CONFIG_BPF_JIT) || !IS_ENABLED(CONFIG_64BIT))
> +               return false;
> +
> +       return regs->epc >= BPF_JIT_REGION_START && regs->epc < BPF_JIT_REGION_END;
> +}
> +#else
> +static inline bool rv_in_bpf_jit(struct pt_regs *regs)
> +{
> +       return false;
> +}
> +#endif
> +
> +#if defined(CONFIG_BPF_JIT) && defined(CONFIG_64BIT)
> +int rv_bpf_fixup_exception(const struct exception_table_entry *ex, struct pt_regs *regs);
> +#else
> +static inline int rv_bpf_fixup_exception(const struct exception_table_entry *ex,
> +                                        struct pt_regs *regs)
> +{
> +       return 0;
> +}
> +#endif
> +
> +#endif
> diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
> index f314ff44c48d..96ea91dc0e9c 100644
> --- a/arch/riscv/include/asm/uaccess.h
> +++ b/arch/riscv/include/asm/uaccess.h
> @@ -56,19 +56,6 @@ static inline int __access_ok(unsigned long addr, unsigned long size)
>         return size <= TASK_SIZE && addr <= TASK_SIZE - size;
>  }
>
> -/*
> - * The exception table consists of pairs of addresses: the first is the
> - * address of an instruction that is allowed to fault, and the second is
> - * the address at which the program should continue.  No registers are
> - * modified, so it is entirely up to the continuation code to figure out
> - * what to do.
> - *
> - * All the routines below use bits of fixup code that are out of line
> - * with the main instruction path.  This means when everything is well,
> - * we don't even have to jump over them.  Further, they do not intrude
> - * on our cache or tlb entries.
> - */
> -
>  #define __LSW  0
>  #define __MSW  1
>
> diff --git a/arch/riscv/mm/extable.c b/arch/riscv/mm/extable.c
> index 18bf338303b6..264f465db5bb 100644
> --- a/arch/riscv/mm/extable.c
> +++ b/arch/riscv/mm/extable.c
> @@ -11,10 +11,6 @@
>  #include <linux/module.h>
>  #include <linux/uaccess.h>
>
> -#ifdef CONFIG_BPF_JIT
> -int rv_bpf_fixup_exception(const struct exception_table_entry *ex, struct pt_regs *regs);
> -#endif
> -
>  int fixup_exception(struct pt_regs *regs)
>  {
>         const struct exception_table_entry *fixup;
> @@ -23,10 +19,8 @@ int fixup_exception(struct pt_regs *regs)
>         if (!fixup)
>                 return 0;
>
> -#ifdef CONFIG_BPF_JIT
> -       if (regs->epc >= BPF_JIT_REGION_START && regs->epc < BPF_JIT_REGION_END)
> +       if (rv_in_bpf_jit(regs))
>                 return rv_bpf_fixup_exception(fixup, regs);
> -#endif
>

The only changes that are needed are:
1. Simply gate with CONFIG_BPF_JIT && CONFIG_ARCH_RV64I, instead of of
CONFIG_BPF_JIT

2. Forward declaration of the rv_bpf_fixup_exception() in bpf_jit_comp64.c.


Björn