On Wed, Nov 19, 2014 at 08:46:19AM +0000, AKASHI Takahiro wrote:

On 11/18/2014 11:04 PM, Will Deacon wrote:

quoted

On Tue, Nov 18, 2014 at 01:10:34AM +0000, AKASHI Takahiro wrote:

quoted

+	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 :)

What does x86 do?

quoted

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*.

Ok, but now userspace sees -ENOSYS for a skipped system call in that case,
whereas it would usually see whatever the trace put in x0, right?

Will