probes that access userspace

[Frank Ch. Eigler]

kevinrs wrote:

> Especially when considering syscall tapset there are times when it
> is useful to access the value referenced by a user space
> pointer. [...]

> Is dwarf able to provide enough information to accurately detect a
> user space pointer reference? [...]

No, there appears to be no such qualification data in the debuginfo at
all.  IIRC, the preprocessor makes __user go away before the compiler
ever sees it.

> [...]  If this is not possible, I think it would be useful to
> enhance the language such that a probe script could explicitly
> notify the translator of a user space pointer access [...]

Perhaps.  Other than the system call interfaces, is this difficulty
likely to arise often elsewhere?

> This will be especially beneficial for scripts probing user-space
> apps. Eventually, when SystemTap supports user-space probes, the
> translator will need to know how to follow pointers in user space.

Not quite.  For user-level probes, there is no ambiguity: every
pointer dereference is in user space.

- FChE