Re: [PATCH v2 1/1] arm64: make a single hook to syscall_trace() for all syscall features

[AKASHI Takahiro]

On 02/26/2014 12:00 AM, Will Deacon wrote:
> On Tue, Feb 25, 2014 at 09:14:43AM +0000, AKASHI Takahiro wrote:
> > Currently syscall_trace() is called only for ptrace.
> > With additional TIF_xx flags defined, it is now called in all the cases
> > of audit, ftrace and seccomp in addition to ptrace.
> >
> > Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
> > Acked-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
> > ---
> >   arch/arm64/include/asm/thread_info.h |   13 ++++++++++
> >   arch/arm64/kernel/entry.S            |    5 ++--
> >   arch/arm64/kernel/ptrace.c           |   45 +++++++++++++++++-----------------
> >   3 files changed, 38 insertions(+), 25 deletions(-)

[...]

> > diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> > index 6a8928b..c70133e 100644
> > --- a/arch/arm64/kernel/ptrace.c
> > +++ b/arch/arm64/kernel/ptrace.c
> > @@ -1062,31 +1062,30 @@ asmlinkage int syscall_trace(int dir, struct pt_regs *regs)
> >   {
> >   	unsigned long saved_reg;
> >
> > -	if (!test_thread_flag(TIF_SYSCALL_TRACE))
> > -		return regs->syscallno;
> > -
> > -	if (is_compat_task()) {
> > -		/* AArch32 uses ip (r12) for scratch */
> > -		saved_reg = regs->regs[12];
> > -		regs->regs[12] = dir;
> > -	} else {
> > -		/*
> > -		 * Save X7. X7 is used to denote syscall entry/exit:
> > -		 *   X7 = 0 -> entry, = 1 -> exit
> > -		 */
> > -		saved_reg = regs->regs[7];
> > -		regs->regs[7] = dir;
> > -	}
> > +	if (test_thread_flag(TIF_SYSCALL_TRACE)) {
> > +		if (is_compat_task()) {
> > +			/* AArch32 uses ip (r12) for scratch */
> > +			saved_reg = regs->regs[12];
> > +			regs->regs[12] = dir;
> > +		} else {
> > +			/*
> > +			 * Save X7. X7 is used to denote syscall entry/exit:
> > +			 *   X7 = 0 -> entry, = 1 -> exit
> > +			 */
> > +			saved_reg = regs->regs[7];
> > +			regs->regs[7] = dir;
> > +		}
> >
> > -	if (dir)
> > -		tracehook_report_syscall_exit(regs, 0);
> > -	else if (tracehook_report_syscall_entry(regs))
> > -		regs->syscallno = ~0UL;
> > +		if (dir)
> > +			tracehook_report_syscall_exit(regs, 0);
> > +		else if (tracehook_report_syscall_entry(regs))
> > +			regs->syscallno = ~0UL;
> >
> > -	if (is_compat_task())
> > -		regs->regs[12] = saved_reg;
> > -	else
> > -		regs->regs[7] = saved_reg;
> > +		if (is_compat_task())
> > +			regs->regs[12] = saved_reg;
> > +		else
> > +			regs->regs[7] = saved_reg;
> > +	}
>
> Aren't these changes (to ptrace.c) just a giant NOP?

Umm, the purpose of this big "if" is to run the code only if TIF_SYSCALL_TRACE is set,
and to make it easy to add additional hooks, audit and ftrace, around tracehook_report_*()
later on.

-Takahiro AKASHI

> Will
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