Re: [PATCH v2 01/13] riscv: fix jal offsets in patched alternatives

Heiko Stübner
2022-12-05T15:32:25Z

Am Sonntag, 4. Dezember 2022, 18:46:20 CET schrieb Jisheng Zhang:
> Alternatives live in a different section, so offsets used by jal
> instruction will point to wrong locations after the patch got applied.
>
> Similar to arm64, adjust the location to consider that offset.
>
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
> arch/riscv/include/asm/alternative.h | 2 ++
> arch/riscv/kernel/alternative.c | 38 ++++++++++++++++++++++++++++
> arch/riscv/kernel/cpufeature.c | 3 +++
> 3 files changed, 43 insertions(+)
>
> diff --git a/arch/riscv/include/asm/alternative.h b/arch/riscv/include/asm/alternative.h
> index c58ec3cc4bc3..33eae9541684 100644
> --- a/arch/riscv/include/asm/alternative.h
> +++ b/arch/riscv/include/asm/alternative.h
> @@ -29,6 +29,8 @@ void apply_module_alternatives(void *start, size_t length);
>
> void riscv_alternative_fix_auipc_jalr(void *alt_ptr, unsigned int len,
> int patch_offset);
> +void riscv_alternative_fix_jal(void *alt_ptr, unsigned int len,
> + int patch_offset);
>
> struct alt_entry {
> void *old_ptr; /* address of original instruciton or data */
> diff --git a/arch/riscv/kernel/alternative.c b/arch/riscv/kernel/alternative.c
> index 292cc42dc3be..9d88375624b5 100644
> --- a/arch/riscv/kernel/alternative.c
> +++ b/arch/riscv/kernel/alternative.c
> @@ -125,6 +125,44 @@ void riscv_alternative_fix_auipc_jalr(void *alt_ptr, unsigned int len,
> }
> }
>
> +#define to_jal_imm(value) \
> + (((value & (RV_J_IMM_10_1_MASK << RV_J_IMM_10_1_OFF)) << RV_I_IMM_11_0_OPOFF) | \
> + ((value & (RV_J_IMM_11_MASK << RV_J_IMM_11_OFF)) << RV_J_IMM_11_OPOFF) | \
> + ((value & (RV_J_IMM_19_12_OPOFF << RV_J_IMM_19_12_OFF)) << RV_J_IMM_19_12_OPOFF) | \
> + ((value & (1 << RV_J_IMM_SIGN_OFF)) << RV_J_IMM_SIGN_OPOFF))
> +
> +void riscv_alternative_fix_jal(void *alt_ptr, unsigned int len,
> + int patch_offset)
> +{

I think we might want to unfiy this into a common function like

riscv_alternative_fix_offsets(...)

so that we only run through the code block once

for (i = 0; i < num_instr; i++) {
if (riscv_insn_is_auipc_jalr(inst1, inst2)) {
riscv_alternative_fix_auipc_jalr(...)
continue;
}

if (riscv_insn_is_jal(inst)) {
riscv_alternative_fix_jal(...)
continue;
}
}

This would also remove the need from calling multiple functions
after patching alternatives.

Thoughts?

Heiko

> + int num_instr = len / sizeof(u32);
> + unsigned int call;
> + int i;
> + int imm;
> +
> + for (i = 0; i < num_instr; i++) {
> + u32 inst = riscv_instruction_at(alt_ptr, i);
> +
> + if (!riscv_insn_is_jal(inst))
> + continue;
> +
> + /* get and adjust new target address */
> + imm = RV_EXTRACT_JTYPE_IMM(inst);
> + imm -= patch_offset;
> +
> + /* pick the original jal */
> + call = inst;
> +
> + /* drop the old IMMs, all jal imm bits sit at 31:12 */
> + call &= ~GENMASK(31, 12);
> +
> + /* add the adapted IMMs */
> + call |= to_jal_imm(imm);
> +
> + /* patch the call place again */
> + patch_text_nosync(alt_ptr + i * sizeof(u32), &call, 4);
> + }
> +}
> +
> /*
> * This is called very early in the boot process (directly after we run
> * a feature detect on the boot CPU). No need to worry about other CPUs
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index ba62a4ff5ccd..c743f0adc794 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -324,6 +324,9 @@ void __init_or_module riscv_cpufeature_patch_func(struct alt_entry *begin,
> riscv_alternative_fix_auipc_jalr(alt->old_ptr,
> alt->alt_len,
> alt->old_ptr - alt->alt_ptr);
> + riscv_alternative_fix_jal(alt->old_ptr,
> + alt->alt_len,
> + alt->old_ptr - alt->alt_ptr);
> }
> }
> }
>