linux-kernel - Re: [PATCH 1/2] powerpc/kprobes: Remove redundant code

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Date:   Wed, 19 Feb 2020 08:48:09 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 1/2] powerpc/kprobes: Remove redundant code



Le 18/02/2020 à 15:39, Naveen N. Rao a écrit :
> Christophe Leroy wrote:
>> At the time being we have something like
>>
>>     if (something) {
>>         p = get();
>>         if (p) {
>>             if (something_wrong)
>>                 goto out;
>>             ...
>>             return;
>>         } else if (a != b) {
>>             if (some_error)
>>                 goto out;
>>             ...
>>         }
>>         goto out;
>>     }
>>     p = get();
>>     if (!p) {
>>         if (a != b) {
>>             if (some_error)
>>                 goto out;
>>             ...
>>         }
>>         goto out;
>>     }
>>
>> This is similar to
>>
>>     p = get();
>>     if (something) {
>>         if (p) {
>>             if (something_wrong)
>>                 goto out;
>>             ...
>>             return;
>>         }
>>     }
>>     if (!p) {
>>         if (a != b) {
>>             if (some_error)
>>                 goto out;
>>             ...
>>         }
>>         goto out;
>>     }
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>> ---
>>  arch/powerpc/kernel/kprobes.c | 15 +--------------
>>  1 file changed, 1 insertion(+), 14 deletions(-)
> 
> Good cleanup, thanks.
> 
>>
>> diff --git a/arch/powerpc/kernel/kprobes.c 
>> b/arch/powerpc/kernel/kprobes.c
>> index f8b848aa65bd..7a925eb76ec0 100644
>> --- a/arch/powerpc/kernel/kprobes.c
>> +++ b/arch/powerpc/kernel/kprobes.c
>> @@ -276,8 +276,8 @@ int kprobe_handler(struct pt_regs *regs)
>>      kcb = get_kprobe_ctlblk();
>>
>>      /* Check we're not actually recursing */
>> +    p = get_kprobe(addr);
>>      if (kprobe_running()) {
>> -        p = get_kprobe(addr);
>>          if (p) {
>>              kprobe_opcode_t insn = *p->ainsn.insn;
>>              if (kcb->kprobe_status == KPROBE_HIT_SS &&
>> @@ -308,22 +308,9 @@ int kprobe_handler(struct pt_regs *regs)
>>              }
>>              prepare_singlestep(p, regs);
>>              return 1;
>> -        } else if (*addr != BREAKPOINT_INSTRUCTION) {
>> -            /* If trap variant, then it belongs not to us */
>> -            kprobe_opcode_t cur_insn = *addr;
>> -
>> -            if (is_trap(cur_insn))
>> -                goto no_kprobe;
>> -            /* The breakpoint instruction was removed by
>> -             * another cpu right after we hit, no further
>> -             * handling of this interrupt is appropriate
>> -             */
>> -            ret = 1;
>>          }
>> -        goto no_kprobe;
> 
> A minot nit -- removing the above goto makes a slight change to the 
> logic. But, see my comments for the next patch.

All legs of the (p) case are have either a return or a goto, so that 
goto no_kprobe is limited to the !p case, we have to fall_through now.

Christophe