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

[Stefan Weil <weil@mail.berlios.de>]

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.

To make it short: I don't know how to fix the last symbol in a 
reasonable way.

Sorry,
Stefan