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

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

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?