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Re: probes that access userspace
- From: fche at redhat dot com (Frank Ch. Eigler)
- To: Kevin Stafford <kevinrs at us dot ibm dot com>
- Cc: systemtap at sources dot redhat dot com
- Date: 14 Oct 2005 12:23:30 -0400
- Subject: Re: probes that access userspace
- References: <E55D.2090005__29732.3145241991$1129302467$gmane$org@us.ibm.com>
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