[Bug translator/10762] New: turn off userspace prologue heuristics for recent gcc

[Bug translator/10762] New: turn off userspace prologue heuristics for recent gcc

[fche at redhat dot com]

We currently force on prologue searching for userspace process
files, since widespread gcc produces poor dwarf data to locate
incoming $$parms.

If we can detect (or have a user assert) that modern (vta-based)
debuginfo is available for a given ELF file, we should disable
the heuristics.  If automatic means are unavailable, perhaps another
command line option like "-P" would be needed, or (yikes) a probe
point syntax extension.

One reason for this is that an instruction somewhat below
the prologue is more likely to be within a loop, which could
result in false "entry" probe hits at every iteration.

-- 
           Summary: turn off userspace prologue heuristics for recent gcc
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: translator
        AssignedTo: systemtap at sources dot redhat dot com
        ReportedBy: fche at redhat dot com


http://sourceware.org/bugzilla/show_bug.cgi?id=10762

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[Bug translator/10762] turn off userspace prologue heuristics for recent gcc

[fche at redhat dot com]

------- Additional Comments From fche at redhat dot com  2009-10-16 20:00 -------
mjw advises that a "readelf --debug-dump=loc a.out | egrep (stack_implicit)_value"
might be a usable heuristic that VTA was used to compile a given object file.


-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=10762

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[Bug translator/10762] turn off userspace prologue heuristics for recent gcc

[fche at redhat dot com]

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


http://sourceware.org/bugzilla/show_bug.cgi?id=10762

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[Bug translator/10762] turn off userspace prologue heuristics for recent gcc

[fche at redhat dot com]

------- Additional Comments From fche at redhat dot com  2009-10-16 21:17 -------
Parameters at the function entry seem to hit upon an existing uprobes
shortcoming: using pt_regs as it arrived from the kernel trap handler
instead of the utrace regset.  In other words, our pt_regs currently
get corrupt values (specifically for %rsp and sometimes %rax), which
precludes correct location.

So we're blocked on bug #10601.

% cat foo.c
int foo(char *p) { while(*p++); return 0; }
int main() {
  foo("hello");
}

% gcc -g -fno-inline -fomit-frmae-pointer -O3 foo.c

Probing function("foo").call vs. statement("foo") exposes the various problems
on i686.  function("foo").call will be called five times since the prologue
heuristics will fire and place the probe at the head of the loop.

On the other hand, statement("foo") will be called once, but print_regs() 
confirms that some incoming register values are wrong (compare to a gdb
breakpoint & info regs at the same address).


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|                            |10601
         AssignedTo|fche at redhat dot com      |systemtap at sources dot
                   |                            |redhat dot com


http://sourceware.org/bugzilla/show_bug.cgi?id=10762

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[Bug translator/10762] turn off userspace prologue heuristics for recent gcc

[fche at redhat dot com]

------- Additional Comments From fche at redhat dot com  2009-12-21 15:18 -------
(In reply to comment #3)
> On the other hand, statement("foo") will be called once, but print_regs() 
> confirms that some incoming register values are wrong (compare to a gdb
> breakpoint & info regs at the same address).

With the parts of bug #10601 already committed, .statement("foo") produces
correct results.


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http://sourceware.org/bugzilla/show_bug.cgi?id=10762

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[Bug translator/10762] turn off userspace prologue heuristics for recent gcc

[fche at redhat dot com]

-- 
Bug 10762 depends on bug 10601, which changed state.

Bug 10601 Summary: user-space deref/registers in loc2c
http://sourceware.org/bugzilla/show_bug.cgi?id=10601

           What    |Old Value                   |New Value
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED

http://sourceware.org/bugzilla/show_bug.cgi?id=10762

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[Bug translator/10762] turn off userspace prologue heuristics for recent gcc

[fche at redhat dot com]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
OtherBugsDependingO|                            |11179
              nThis|                            |


http://sourceware.org/bugzilla/show_bug.cgi?id=10762

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[Bug translator/10762] turn off userspace prologue heuristics for recent gcc

[fche at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=10762

Frank Ch. Eigler <fche at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |DUPLICATE

--- Comment #5 from Frank Ch. Eigler <fche at redhat dot com> 2012-06-16 16:12:26 UTC ---
See bug #6941 for some recent findings.

*** This bug has been marked as a duplicate of bug 6941 ***

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