Re: container_of in systemtap scripts

[Josh Stone <jistone at redhat dot com>]

On 01/18/2010 07:04 AM, Frank Ch. Eigler wrote:
> Andi Kleen <andi@firstfloor.org> writes:
> 
>> systemtap 1.1 works nicely.
> 
> Thanks for trying it.
> 
>> [...]  @cast doesn't support that. Are there any other clean ways to
>> write container_of() natively
> 
> Actually it this sort of thing should work, since we have "&" operators:
> 
> @cast($subfieldptr - (& @cast(0, "struct container")->subfield),
>       "struct container")
> 
>> or are there plans to extend cast to include an offset?
> 
> Sure, we can do something like that if needed.

The hardest part of such things is deciding on the syntax.  Currently it
can be written:

  @cast(ptr, "type")
  @cast(ptr, "type", "module")
  @cast(ptr, "type", "<header>")

I propose adding a qualifier on the type field, something like
"type->field" or "type:field".  The former is consistent with how fields
are specified elsewhere, but it may imply more flexibility than a
"container_of" can support.  (e.g. "mystruct->foo->bar" could only work
if there are no pointers in that chain.)  The latter with a colon looks
nice to me, but it introduces a new syntax.

Josh