linux-kernel - Re: [PATCH 2/6] ptrace: introduce ptrace_syscall_enter to consolidate PTRACE

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Message-ID: <20190304122331.GG3969@gate.crashing.org>
Date:   Mon, 4 Mar 2019 06:23:32 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     "Haibo Xu (Arm Technology China)" <Haibo.Xu@....com>,
        Steve Capper <Steve.Capper@....com>,
        Catalin Marinas <Catalin.Marinas@....com>,
        "jdike@...toit.com" <jdike@...toit.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Will Deacon <Will.Deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Richard Weinberger <richard@....at>,
        Ingo Molnar <mingo@...hat.com>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Bin Lu (Arm Technology China)" <Bin.Lu@....com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/6] ptrace: introduce ptrace_syscall_enter to consolidate PTRACE_SYSEMU handling

On Mon, Mar 04, 2019 at 10:46:43AM +0000, Sudeep Holla wrote:
> On Mon, Mar 04, 2019 at 08:03:47AM +0000, Haibo Xu (Arm Technology China) wrote:
> > On 2019/3/1 2:32, Sudeep Holla wrote:
> > > +long ptrace_syscall_enter(struct pt_regs *regs)
> > > +{
> > > +#ifdef TIF_SYSCALL_EMU
> > > +	if (test_thread_flag(TIF_SYSCALL_EMU)) {
> > > +		if (tracehook_report_syscall_entry(regs));
> > 
> > Shall we remove the semi-colon at end of the above line?
> 
> Added intentionally to keep GCC happy.

GCC warns because the user explicitly asked for it, with __must_check.
If you want to do things with an "if" like this, you should write e.g.

		if (tracehook_report_syscall_entry(regs))
			/*
			 * We can ignore the return code here, because of
			 * X and Y and Z.
			 */
			;

Or it probably is nicer to use a block:

		if (tracehook_report_syscall_entry(regs)) {
			/*
			 * We can ignore the return code here, because of
			 * X and Y and Z.
			 */
		}

The point is, you *always* should have a nice fat comment if you are
ignoring the return code of a __must_check function.


Segher