Re: [PATCH v3 11/14] RISC-V: fix auipc-jalr addresses in patched alternatives

Andrew Jones
2022-12-01T19:39:30Z

On Wed, Nov 30, 2022 at 11:56:11PM +0100, Heiko Stuebner wrote:
> From: Heiko Stuebner <heiko.stuebner@vrull.eu>
>
> Alternatives live in a different section, so addresses used by call
> functions will point to wrong locations after the patch got applied.
>
> Similar to arm64, adjust the location to consider that offset.
>
> Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
> Signed-off-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
> ---
> arch/riscv/include/asm/alternative.h | 3 ++
> arch/riscv/kernel/alternative.c | 72 ++++++++++++++++++++++++++++
> arch/riscv/kernel/cpufeature.c | 11 ++++-
> 3 files changed, 84 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/include/asm/alternative.h b/arch/riscv/include/asm/alternative.h
> index 6511dd73e812..c58ec3cc4bc3 100644
> --- a/arch/riscv/include/asm/alternative.h
> +++ b/arch/riscv/include/asm/alternative.h
> @@ -27,6 +27,9 @@ void __init apply_boot_alternatives(void);
> void __init apply_early_boot_alternatives(void);
> void apply_module_alternatives(void *start, size_t length);
>
> +void riscv_alternative_fix_auipc_jalr(void *alt_ptr, unsigned int len,
> + int patch_offset);
> +
> struct alt_entry {
> void *old_ptr; /* address of original instruciton or data */
> void *alt_ptr; /* address of replacement instruction or data */
> diff --git a/arch/riscv/kernel/alternative.c b/arch/riscv/kernel/alternative.c
> index a7d26a00beea..292cc42dc3be 100644
> --- a/arch/riscv/kernel/alternative.c
> +++ b/arch/riscv/kernel/alternative.c
> @@ -15,6 +15,8 @@
> #include <asm/vendorid_list.h>
> #include <asm/sbi.h>
> #include <asm/csr.h>
> +#include <asm/insn.h>
> +#include <asm/patch.h>
>
> struct cpu_manufacturer_info_t {
> unsigned long vendor_id;
> @@ -53,6 +55,76 @@ static void __init_or_module riscv_fill_cpu_mfr_info(struct cpu_manufacturer_inf
> }
> }
>
> +static unsigned int riscv_instruction_at(void *p, unsigned int offset)

How about explicitly returning a u32?

> +{
> + u16 *parcel = p + (offset * sizeof(u32));

nit: I realize this is just a helper function, but I think a cleaner
interface would require the caller do this math, or at least the offset
scaling, since only the caller knows it's necessary. And, the call to
patch_text_nosync() requires all the math, so it'd be consistent for
riscv_instruction_at() to only take a pointer too.

> +
> + return (unsigned int)parcel[0] | (unsigned int)parcel[1] << 16;
> +}
> +
> +static inline bool riscv_insn_is_auipc_jalr(u32 insn1, u32 insn2)
> +{
> + return riscv_insn_is_auipc(insn1) && riscv_insn_is_jalr(insn2);
> +}
> +
> +#define JALR_SIGN_MASK BIT(RV_I_IMM_SIGN_OPOFF - RV_I_IMM_11_0_OPOFF)

We know I-type IMM is 11 bits, so we could just define this as BIT(11).

> +#define AUIPC_PAD (0x00001000)
> +
> +#define to_jalr_imm(value) \
> + ((value & RV_I_IMM_11_0_MASK) << RV_I_IMM_11_0_OPOFF)

Should put () around the macro argument, (value)

> +
> +#define to_auipc_imm(value) \
> + ((value & JALR_SIGN_MASK) ? \
> + ((value & RV_U_IMM_31_12_MASK) + AUIPC_PAD) : \
> + (value & RV_U_IMM_31_12_MASK))

I know RV_U_IMM_31_12_OPOFF is 0, but it looks odd not shifting
RV_U_IMM_31_12_MASK when we do shift RV_I_IMM_11_0_MASK.

So, it looks like to_auipc_imm() is doing

offset[31:12] + ((value & BIT(11)) ? (1 << 12) : 0)

but the spec says the auipc part of the 'call' pseudoinstruction should be

offset[31:12] + offset[11]

which I think would be written as

((((value) & RV_U_IMM_31_12_MASK) << RV_U_IMM_31_12_OPOFF) + ((value) & BIT(11)))

or what am I missing?

> +
> +void riscv_alternative_fix_auipc_jalr(void *alt_ptr, unsigned int len,
> + int patch_offset)
> +{
> + int num_instr = len / sizeof(u32);
> + unsigned int call[2];
> + int i;
> + int imm;
> + u32 rd1;
> +
> + /*
> + * stop one instruction before the end, as we're checking
> + * for auipc + jalr
> + */
> + for (i = 0; i < num_instr - 1; i++) {

If we change riscv_instruction_at() to just take a pointer then we can do
the math in the for() and actually just use pointer arithmetic.

uint32_t *p = alt_ptr;
for (i = 0; i < num_instr - 1; i++, p++) {

> + u32 inst1 = riscv_instruction_at(alt_ptr, i);
p
> + u32 inst2 = riscv_instruction_at(alt_ptr, i + 1);
p + 1
> +
> + if (!riscv_insn_is_auipc_jalr(inst1, inst2))
> + continue;
> +
> + /* call will use ra register */
> + rd1 = RV_EXTRACT_RD_REG(inst1);
> + if (rd1 != 1)
> + continue;

nit: rd1 is only used once, how about

if (RV_EXTRACT_RD_REG(inst1) != 1)

> +
> + /* get and adjust new target address */
> + imm = RV_EXTRACT_UTYPE_IMM(inst1);

Based on my understanding of a auipc part of the 'call', it seems we
should be subtracting BIT(11) here. And, since RV_EXTRACT_* does sign-
extension for I-type, then I'm not sure we should use it. So,

imm = (inst2 >> RV_I_IMM_11_0_OPOFF) & RV_I_IMM_11_0_MASK;
imm += ((inst1 >> RV_U_IMM_31_12_OPOFF) & RV_U_IMM_31_12_MASK) - (imm & BIT(11));

> + imm += RV_EXTRACT_ITYPE_IMM(inst2);
> + imm -= patch_offset;
> +
> + /* pick the original auipc + jalr */
> + call[0] = inst1;
> + call[1] = inst2;
> +
> + /* drop the old IMMs */
> + call[0] &= ~(RV_U_IMM_31_12_MASK);

Same comment as above about RV_U_IMM_31_12_OPOFF. IMO, this would be more
consistent with the shift, even though it's zero.

call[0] &= ~(RV_U_IMM_31_12_MASK << RV_U_IMM_31_12_OPOFF);

> + call[1] &= ~(RV_I_IMM_11_0_MASK << RV_I_IMM_11_0_OPOFF);
> +
> + /* add the adapted IMMs */
> + call[0] |= to_auipc_imm(imm);

As pointed out above, I'm not sure about to_auipc_imm().

> + call[1] |= to_jalr_imm(imm);
> +
> + /* patch the call place again */
> + patch_text_nosync(alt_ptr + i * sizeof(u32), call, 8);
^ ^
p sizeof(u32) * 2
> + }
> +}
> +
> /*
> * 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 694267d1fe81..ba62a4ff5ccd 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -316,8 +316,15 @@ void __init_or_module riscv_cpufeature_patch_func(struct alt_entry *begin,
> }
>
> tmp = (1U << alt->errata_id);
> - if (cpu_req_feature & tmp)
> - patch_text_nosync(alt->old_ptr, alt->alt_ptr, alt->alt_len);
> + if (cpu_req_feature & tmp) {
> + /* do the basic patching */
> + patch_text_nosync(alt->old_ptr, alt->alt_ptr,
> + alt->alt_len);

nit: I'd leave this line long and only have one wrap in the line below

> +
> + riscv_alternative_fix_auipc_jalr(alt->old_ptr,
> + alt->alt_len,
> + alt->old_ptr - alt->alt_ptr);
> + }
> }
> }
> #endif
> --
> 2.35.1
>

Thanks,
drew