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

From: [email-masked] (AKASHI Takahiro)
Date: Wed, 26 Feb 2014 11:00:19 +0900

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 <[email-masked]>
>> Acked-by: Richard Guy Briggs <[email-masked]>
>> ---
>>   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