Subject: [PATCH v8 2/6] arm64: ptrace: allow tracer to skip a system call

From: [email-masked] (AKASHI Takahiro)
Date: Wed, 19 Nov 2014 17:46:19 +0900

On 11/18/2014 11:04 PM, Will Deacon wrote:
> On Tue, Nov 18, 2014 at 01:10:34AM +0000, AKASHI Takahiro wrote:
>>
>> +	if (((int)regs->syscallno == -1) && (orig_syscallno == -1)) {
>> +		/*
>> +		 * user-issued syscall(-1):
>> +		 * RESTRICTION: We always return ENOSYS whatever value is
>> +		 *   stored in x0 (a return value) at this point.
>> +		 * Normally, with ptrace off, syscall(-1) returns -ENOSYS.
>> +		 * With ptrace on, however, if a tracer didn't pay any
>> +		 * attention to user-issued syscall(-1) and just let it go
>> +		 * without a hack here, it would return a value in x0 as in
>> +		 * other system call cases. This means that this system call
>> +		 * might succeed and see any bogus return value.
>> +		 * This should be definitely avoided.
>> +		 */
>> +		regs->regs[0] = -ENOSYS;
>> +	}
>
> I'm still really uncomfortable with this, and it doesn't seem to match what
> arch/arm/ does either.

Yeah, I know but
as I mentioned before, syscall(-1) will be signaled on arm, and so we don't
have to care about a return value :)

> Doesn't it also prevent a tracer from skipping syscall(-1)?

Syscall(-1) will return -ENOSYS whether or not a syscallno is explicitly replaced with -1
by a tracer, and, in this sense, it is *skipped*.

-Takahiro AKASHI

> Will