Re: [uml-devel] [RFC] PATCH 3/4 - Time virtualization : PTRACE_SYSCALL_MASK

[Heiko Carstens <heiko.carstens@de.ibm.com>]

> This sounds more complicated than what we are proposing.  
> 
> This would make the process care about the number of system calls
> implemented by the kernel, which is something that doesn't even come
> up in the normal case with the current interface.  You only care about
> it if you get a -EINVAL and want to figure out exactly why.
> 
> From a practical point of view, you would want code that looks like
> this:

Yes.

[1]
> 	n = nsyscalls();
> 	mask = malloc((n + 7)/8);
> 	if(mask == NULL)
> 		return;
> 
> 	/* Zero mask, set bits, call ptrace */
> 
> 	free(mask);
> 

[2]
> rather than code like this:
> 
> 	int mask[(BIGGEST_SYSCALL_I_CARE_ABOUT + 7) / 8];
> 
> 	/* Zero mask, set bits, call ptrace */


> That doesn't seem like an improvement to me.
> 
> The second case would be more complicated if it wanted to figure out
> what the problem was if ptrace returned -EINVAL.  However, some users

That is actually my point. If you're checking for errors you will end up
first doing [2] and later on doing something like [1] anyway...

> > In addition your proposal would already introduce a rather complicated
> > interface to figure out how many syscalls the kernel has. I'm sure this
> > will be (mis)used sooner or later.
> 
> How?  And, if so, why is that a problem?

>From the proposal:

<snip>
> Semantics:
>
> in both cases, the mask is first zero-extended to the right (for syscalls not
> known to userspace), bits for syscall not known to the kernel are checked and
> the call fails if any of them is 1, and in the failure case E2BIG or
> EOVERFLOW is returned (I want to avoid EINVAL and ENOSYS to avoid confusion)
> and the part of the mask known to the kernel is 0-ed.
<snip>

So you just need to pass a large enough bitmask with all ones and the kernel
will put zeroes in the bitmask up to the bit number NR_sycalls - 1.
Counting the zeroes should work...
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