[Bug runtime/4633] New: backtracing broken

[Bug runtime/4633] New: backtracing broken

[fche at redhat dot com]

Bug #3050 may have been closed but the bug did not stay dead.  The same code on
current fc7 kernels gives the usual single line of backtrace info.
The kernel backtracer always seems to do a better job than the code in the runtime.

There are several problems with the code.  It uses unprotected dereference code
like "*stack++", even though the stack values are not completely reliable.  It
does not know how to distinguish between alternative stacks such as the trap stack,
the normal kernel stack, or whatever happens to come in pt_regs.  This is key
because backtrace() should from both kprobes and from ordinary hook calls
such as timers, begin/end, and markers.  The backtrace() function should not
include the "Inexact backtrace:" string, as this breaks subsequent tokenizing 
with print_stack().

-- 
           Summary: backtracing broken
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: runtime
        AssignedTo: systemtap at sources dot redhat dot com
        ReportedBy: fche at redhat dot com
  GCC host triplet: x86-64


http://sourceware.org/bugzilla/show_bug.cgi?id=4633

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[Bug runtime/4633] backtracing broken

[fche at redhat dot com]

------- Additional Comments From fche at redhat dot com  2007-06-12 20:56 -------
Further information... an analogous problem exists even on i386.
Here, a stack traceback from a kprobe includes a lot of the kprobes
invocation path, but none actually from (above) the context of the
int3 itself.


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[Bug runtime/4633] backtracing broken

[mhiramat at redhat dot com]

------- Additional Comments From mhiramat at redhat dot com  2007-08-20 15:34 -------
Created an attachment (id=1972)
 --> (http://sourceware.org/bugzilla/attachment.cgi?id=1972&action=view)
x86-64 backtracing fix patch

This patch fixes this bug.
AFAIK, the value (not the address) of rsp is specifying the original stack
address on x86-64.


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[Bug runtime/4633] backtracing broken

[fche at redhat dot com]

------- Additional Comments From fche at redhat dot com  2007-08-20 19:50 -------
> This patch fixes this bug.
> AFAIK, the value (not the address) of rsp is specifying the original stack
> address on x86-64.

Unfortunately, it's not so easy.  Sometimes (kprobes versus other
event sources?) the &REG_SP value is more correct.  We lack a convincing
set of test cases either way.  Would you mind collecting a set?

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|systemtap at sources dot    |mhiramat at redhat dot com
                   |redhat dot com              |
             Status|NEW                         |ASSIGNED


http://sourceware.org/bugzilla/show_bug.cgi?id=4633

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