[PATCH v2] powerpc: Add ppc_inst_next()

[Michael Ellerman]

Nicholas Piggin <npiggin at gmail.com> writes:
> Excerpts from Nicholas Piggin's message of May 24, 2020 9:56 am:
>> 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 <mpe at 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).
>
> Ooops, I didn't read the last thread. I don't think insert_bpts needs it 
> though, only this case. Anyway it's a minor nitpick so if it's already 
> been considered, fine.

There's a few places that don't need it, eg:

+       nop = ppc_inst(PPC_INST_NOP);
+       for (; dest < end; dest = ppc_inst_next(dest, &nop))
+               raw_patch_instruction(dest, nop);


But I prefer the way that reads, it's clear we're incrementing by the
length of a nop, even though we could read the nop from dest because we
just patched it.

If we ever did execute-only kernel text, it would help to have the
location and value separate too, because then reading from the text
would require a helper, but reading from data/stack would not.

So I'll go with this version.

cheers