Excerpts from Michael Ellerman's message of May 22, 2020 11:33 pm:
> In a few places we want to calculate the address of the next
> instruction. Previously that was simple, we just added 4 bytes, or if
> using a u32 * we incremented that pointer by 1.
>
> But prefixed instructions make it more complicated, we need to advance
> by either 4 or 8 bytes depending on the actual instruction. We also
> can't do pointer arithmetic using struct ppc_inst, because it is
> always 8 bytes in size on 64-bit, even though we might only need to
> advance by 4 bytes.
>
> So add a ppc_inst_next() helper which calculates the location of the
> next instruction, if the given instruction was located at the given
> address. Note the instruction doesn't need to actually be at the
> address in memory.
>
> Although it would seem natural for the value to be passed by value,
> that makes it too easy to write a loop that will read off the end of a
> page, eg:
>
> for (; src < end; src = ppc_inst_next(src, *src),
> dest = ppc_inst_next(dest, *dest))
>
> As noticed by Christophe and Jordan, if end is the exact end of a
> page, and the next page is not mapped, this will fault, because *dest
> will read 8 bytes, 4 bytes into the next page.
>
> So value is passed by reference, so the helper can be careful to use
> ppc_inst_read() on it.
>
> Signed-off-by: Michael Ellerman <m...@ellerman.id.au>
> ---
> arch/powerpc/include/asm/inst.h | 13 +++++++++++++
> arch/powerpc/kernel/uprobes.c | 2 +-
> arch/powerpc/lib/feature-fixups.c | 15 ++++++++-------
> arch/powerpc/xmon/xmon.c | 2 +-
> 4 files changed, 23 insertions(+), 9 deletions(-)
>
> v2: Pass the value as a pointer and use ppc_inst_read() on it.
>
> diff --git a/arch/powerpc/include/asm/inst.h b/arch/powerpc/include/asm/inst.h
> index d82e0c99cfa1..5b756ba77ed2 100644
> --- a/arch/powerpc/include/asm/inst.h
> +++ b/arch/powerpc/include/asm/inst.h
> @@ -100,6 +100,19 @@ static inline int ppc_inst_len(struct ppc_inst x)
> return ppc_inst_prefixed(x) ? 8 : 4;
> }
>
> +/*
> + * Return the address of the next instruction, if the instruction @value was
> + * located at @location.
> + */
> +static inline struct ppc_inst *ppc_inst_next(void *location, struct ppc_inst
> *value)
> +{
> + struct ppc_inst tmp;
> +
> + tmp = ppc_inst_read(value);
> +
> + return location + ppc_inst_len(tmp);
> +}
> +
> int probe_user_read_inst(struct ppc_inst *inst,
> struct ppc_inst __user *nip);
>
> diff --git a/arch/powerpc/kernel/uprobes.c b/arch/powerpc/kernel/uprobes.c
> index 83e883e1a42d..d200e7df7167 100644
> --- a/arch/powerpc/kernel/uprobes.c
> +++ b/arch/powerpc/kernel/uprobes.c
> @@ -112,7 +112,7 @@ int arch_uprobe_post_xol(struct arch_uprobe *auprobe,
> struct pt_regs *regs)
> * support doesn't exist and have to fix-up the next instruction
> * to be executed.
> */
> - regs->nip = utask->vaddr + ppc_inst_len(ppc_inst_read(&auprobe->insn));
> + regs->nip = (unsigned long)ppc_inst_next((void *)utask->vaddr,
> &auprobe->insn);
>
> user_disable_single_step(current);
> return 0;
AFAIKS except for here, there is no need for the void *location arg.
I would prefer to drop that and keep this unchanged (it's a slightly
special case anyway).
Thanks,
Nick
> diff --git a/arch/powerpc/lib/feature-fixups.c
> b/arch/powerpc/lib/feature-fixups.c
> index 80f320c2e189..4c0a7ee9fa00 100644
> --- a/arch/powerpc/lib/feature-fixups.c
> +++ b/arch/powerpc/lib/feature-fixups.c
> @@ -68,7 +68,7 @@ static int patch_alt_instruction(struct ppc_inst *src,
> struct ppc_inst *dest,
>
> static int patch_feature_section(unsigned long value, struct fixup_entry
> *fcur)
> {
> - struct ppc_inst *start, *end, *alt_start, *alt_end, *src, *dest;
> + struct ppc_inst *start, *end, *alt_start, *alt_end, *src, *dest, nop;
>
> start = calc_addr(fcur, fcur->start_off);
> end = calc_addr(fcur, fcur->end_off);
> @@ -84,14 +84,15 @@ static int patch_feature_section(unsigned long value,
> struct fixup_entry *fcur)
> src = alt_start;
> dest = start;
>
> - for (; src < alt_end; src = (void *)src +
> ppc_inst_len(ppc_inst_read(src)),
> - (dest = (void *)dest + ppc_inst_len(ppc_inst_read(dest)))) {
> + for (; src < alt_end; src = ppc_inst_next(src, src),
> + dest = ppc_inst_next(dest, dest)) {
> if (patch_alt_instruction(src, dest, alt_start, alt_end))
> return 1;
> }
>
> - for (; dest < end; dest = (void *)dest +
> ppc_inst_len(ppc_inst(PPC_INST_NOP)))
> - raw_patch_instruction(dest, ppc_inst(PPC_INST_NOP));
> + nop = ppc_inst(PPC_INST_NOP);
> + for (; dest < end; dest = ppc_inst_next(dest, &nop))
> + raw_patch_instruction(dest, nop);
>
> return 0;
> }
> @@ -405,8 +406,8 @@ static void do_final_fixups(void)
> while (src < end) {
> inst = ppc_inst_read(src);
> raw_patch_instruction(dest, inst);
> - src = (void *)src + ppc_inst_len(inst);
> - dest = (void *)dest + ppc_inst_len(inst);
> + src = ppc_inst_next(src, src);
> + dest = ppc_inst_next(dest, dest);
> }
> #endif
> }
> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> index fb135f2cd6b0..65cf853a4d26 100644
> --- a/arch/powerpc/xmon/xmon.c
> +++ b/arch/powerpc/xmon/xmon.c
> @@ -939,7 +939,7 @@ static void insert_bpts(void)
> }
>
> patch_instruction(bp->instr, instr);
> - patch_instruction((void *)bp->instr + ppc_inst_len(instr),
> + patch_instruction(ppc_inst_next(bp->instr, &instr),
> ppc_inst(bpinstr));
> if (bp->enabled & BP_CIABR)
> continue;
> --
> 2.25.1
>
>
