Re: [Qemu-devel] [PATCH] elf: Calculate symbol size if needed

["Edgar E. Iglesias" <edgar.iglesias@gmail.com>]

On Thu, Sep 09, 2010 at 07:36:28PM +0000, Blue Swirl wrote:
> On Thu, Sep 9, 2010 at 7:34 PM, Stefan Weil <weil@mail.berlios.de> wrote:
> > Am 09.09.2010 21:29, schrieb Blue Swirl:
> >>
> >> On Thu, Sep 9, 2010 at 7:11 PM, Stefan Weil<weil@mail.berlios.de>  wrote:
> >>
> >>>
> >>> Am 09.09.2010 20:44, schrieb Blue Swirl:
> >>>
> >>>>
> >>>> On Thu, Sep 9, 2010 at 5:42 PM, Stefan Weil<weil@mail.berlios.de>
> >>>>  wrote:
> >>>>
> >>>>>
> >>>>> Am 11.08.2010 18:21, schrieb Blue Swirl:
> >>>>>
> >>>>>>
> >>>>>> On Mon, Aug 9, 2010 at 2:43 PM, Stefan Weil<weil@mail.berlios.de>
> >>>>>>  wrote:
> >>>>>>
> >>>>>>
> >>>>>>>
> >>>>>>> Symbols with a size of 0 are unusable for the disassembler.
> >>>>>>>
> >>>>>>> Example:
> >>>>>>>
> >>>>>>> While running an arm linux kernel, no symbolic names are
> >>>>>>> used in qemu.log when the cpu is executing an assembler function.
> >>>>>>>
> >>>>>>>
> >>>>>>
> >>>>>> That is a problem of the assembler function, it should use '.size'
> >>>>>> directive like what happens when C code is compiled. And why just ARM?
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>>
> >>>>>>> Assume that the size of such symbols is the difference to the
> >>>>>>> next symbol value.
> >>>>>>>
> >>>>>>> Signed-off-by: Stefan Weil<weil@mail.berlios.de>
> >>>>>>> ---
> >>>>>>>  hw/elf_ops.h |    5 +++++
> >>>>>>>  1 files changed, 5 insertions(+), 0 deletions(-)
> >>>>>>>
> >>>>>>> diff --git a/hw/elf_ops.h b/hw/elf_ops.h
> >>>>>>> index 27d1ab9..0bd7235 100644
> >>>>>>> --- a/hw/elf_ops.h
> >>>>>>> +++ b/hw/elf_ops.h
> >>>>>>> @@ -153,6 +153,11 @@ static int glue(load_symbols, SZ)(struct elfhdr
> >>>>>>> *ehdr, int fd, int must_swab,
> >>>>>>>        syms = qemu_realloc(syms, nsyms * sizeof(*syms));
> >>>>>>>
> >>>>>>>        qsort(syms, nsyms, sizeof(*syms), glue(symcmp, SZ));
> >>>>>>> +        for (i = 0; i<    nsyms - 1; i++) {
> >>>>>>> +            if (syms[i].st_size == 0) {
> >>>>>>> +                syms[i].st_size = syms[i + 1].st_value -
> >>>>>>> syms[i].st_value;
> >>>>>>> +            }
> >>>>>>> +        }
> >>>>>>>
> >>>>>>>
> >>>>>>
> >>>>>> The size of the last symbol is not guesstimated, it could be assumed
> >>>>>> to be _etext - syms[nsyms].st_value.
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>>
> >>>>>>>    } else {
> >>>>>>>        qemu_free(syms);
> >>>>>>>        syms = NULL;
> >>>>>>> --
> >>>>>>> 1.7.1
> >>>>>>>
> >>>>>>
> >>>>>>
> >>>>>
> >>>>> The patch is still missing in qemu master.
> >>>>>  From the two feedbacks I did not read that anything needs to be
> >>>>> changed.
> >>>>> Was I wrong, or can it be applied?
> >>>>>
> >>>>
> >>>> Please fix the last symbol. Either we should fix all symbols or none,
> >>>> half fixed (OK, practically all) is not so great.
> >>>>
> >>>
> >>> The last symbol is one of several thousands, and most symbols don't need
> >>> a
> >>> fix,
> >>> so with my fix more than 99.9 or even 99.99 percent of all symbols are ok
> >>> :-)
> >>> If the last symbol happens to be wrong, there is still a high probability
> >>> that
> >>> nobody will notice this because it is unused by QEMU. The problem I faced
> >>> with
> >>> QEMU's disassembly came from symbols with an address followed by code.
> >>> Is there any code after the last symbol? I don't expect that. In a sorted
> >>> list
> >>> of symbols from the text segment, _etext should be the last symbols!
> >>>
> >>> I think that the small chance of a missing fix for the last symbol is in
> >>> no
> >>> relation
> >>> to the code needed.
> >>>
> >>> Even worse, I have no simple formula to guess a valid value for the last
> >>> symbol.
> >>> The formula you suggested (with the corrections I wrote in my reply) is
> >>> only
> >>> ok
> >>> if the last symbol is in the text segment. Usually there are also symbols
> >>> for data
> >>> in other segments, and in many cases these segments are located after the
> >>> text segment. In these cases the last symbol is not located in the text
> >>> segment
> >>> which makes guesses of its size much more complicated.
> >>>
> >>
> >> How about using _end then?
> >>
> >>
> >
> > Wouldn't _end be the last symbol then?
> 
> Right, _end should be the last one in any case. I'll apply the patch.

I'm not so sure that is the case. The load_symbols call throws away
symbols that are not typed as functions. The filtering is done
prior to the suggested size fixups so my guess is that _end is typically
gone when the suggested size fixup is done.

I'm not opposed to the patch though...

Cheers