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

Jisheng Zhang
2022-12-05T16:54:32Z

On Mon, Dec 05, 2022 at 03:57:10PM +0100, Andrew Jones wrote:
> On Mon, Dec 05, 2022 at 01:46:20AM +0800, Jisheng Zhang wrote:
> > 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) | \
> ^ RV_J_IMM_10_1_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))

Hi all,

I believe there's bug in the to_jal_imm() macro implementation, the
correct one should be like this:

#define to_jal_imm(value) \
((RV_X(value, RV_J_IMM_10_1_OFF, RV_J_IMM_10_1_MASK) << RV_J_IMM_10_1_OPOFF) | \
(RV_X(value, RV_J_IMM_11_OFF, RV_J_IMM_11_MASK) << RV_J_IMM_11_OPOFF) | \
(RV_X(value, RV_J_IMM_19_12_OFF, RV_J_IMM_19_12_MASK) << RV_J_IMM_19_12_OPOFF) | \
(RV_X(value, RV_J_IMM_SIGN_OFF, 1) << RV_J_IMM_SIGN_OPOFF))

Will fix it in next version.

Thanks
>
> Should put () around value
>
> > +
> > +void riscv_alternative_fix_jal(void *alt_ptr, unsigned int len,
> > + int patch_offset)
> > +{
> > + 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);
>
> It'd be nice if this had a define.
>
> > +
> > + /* 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);
> > }
> > }
> > }
> > --
> > 2.37.2
> >
>
> Thanks,
> drew