Re: [PATCH v2] powerpc: Add ppc_inst_next()

[Nicholas Piggin <npiggin@gmail.com>]

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@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.

Thanks,
Nick