[Crash-utility] Re: Fix for source line numbers for x86_64 modules
Dave Anderson
anderson at redhat.com
Thu Sep 24 19:00:49 UTC 2009
----- "John Wright (ALPS, Fort Collins)" <john.wright at hp.com> wrote:
> On Wed, Sep 16, 2009 at 04:44:32PM -0600, Bob Montgomery wrote:
> > John and I think that this code in gdb searches things too many times,
> > particularly with this patch, but it's a start since it seems to fix the
> > problem.
>
> I'm attaching a new version of the patch, that performs way better when
> disassembling functions that live in the kernel. (Bob found that the
> original patch made crash disassemble in-kernel functions at least 3
> times slower, but that number will be larger depending on how close the
> symbol table the function lives in is to the head of the psymtabs list.
> Module disassembly speed wasn't changed much at all.) With this updated
> patch, we found the performance penalty of "dis -l" to be marginal.
>
> The problem with the original patch is that once the address we want is
> found in a symbol table, it then looks through the rest of the symbol
> tables in that objfile for a better match. The original code would then
> return the best pst out of that objfile (and never get the next pst
> from ALL_PSYMTABS), but we want to go through the rest of the objfiles
> just in case, so I moved the return statement outside of the
> ALL_PSYMTABS loop. But the next pst from ALL_PSYMTABS will not be from
> a new objfile - so we would wind up traversing the list (minus one
> element) again, and again, and again...
>
> The new patch removes the inner list traversal, and just takes advantage
> of the fact that we already iterate through every pst via
> ALL_PSYMTABS.
Hi John
Yeah, this second patch works much better. In fact, I only noticed today -- using
your first patch -- that if I pick a text address in the kernel proper that
is very close to "_etext", the disassembly never returns from gdb.
For example, on an 2.6.29.4-167.fc11 kernel, if I do this with the
first patch applied:
crash> sym -l
... [ snip ] ...
ffffffff813afac4 (T) register_kprobes
ffffffff813afb26 (T) recycle_rp_inst
ffffffff813afbbb (T) kprobe_flush_task
ffffffff813afc75 (t) collect_one_slot
ffffffff813afd18 (t) collect_garbage_slots
ffffffff813afda9 (T) free_insn_slot
ffffffff813afe4d (T) get_insn_slot
ffffffff813aff85 (T) __kprobes_text_end
ffffffff813b02ec (t) bad_iret
ffffffff813b0313 (t) bad_gs
ffffffff813b2250 (T) bad_from_user
ffffffff813b2256 (t) bad_to_user
ffffffff813b2830 (T) __start_notes
ffffffff813b2830 (T) _etext
...
crash> dis -l register_kprobes
< never returns >
With your new patch (and unpatched for that matter) it works OK.
Dave
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