[fpc-devel] Valgrind strscan

[petr.kristan at epos.cz]

On Thu, Apr 10, 2008 at 03:09:34PM +0200, Jonas Maebe wrote:
> 
> On 10 Apr 2008, at 14:50, petr.kristan at epos.cz wrote:
> 
> >==14379== Invalid read of size 4
> >==14379==    at 0x80708F0: SYSUTILS_STRSCAN$PCHAR$CHAR$$PCHAR  
> >(strings.inc:439)
> >==14379==    by 0x804828A: main (str.pas:9)
> >==14379==  Address 0x416A494 is 4 bytes inside a block of size 5  
> >alloc'd
> >==14379==    at 0x401D38B: malloc (vg_replace_malloc.c:149)
> >==14379==    by 0x8063398: CMEM_CGETMEM$LONGWORD$$POINTER (cmem.pp:50)
> >==14379==    by 0x805AFA6: SYSTEM_GETMEM$POINTER$LONGWORD (heap.inc: 
> >235)
> >==14379==    by 0x8048276: main (str.pas:7)
> >
> >Problem is in assembler code for i386. rtl/i386/strings.inc.
> >This routine reads always 4 bytes alligned by 4 regardless string  
> >length.
> >
> >function strscan(p : pchar;c : char) : pchar;assembler;
> >...
> >// load new 4 bytes
> >       movl     (%edi),%edx   <-here valgrind report Invalid read
> >...
> >
> >Problem was discovered aleady here 
> >http://bugs.freepascal.org/view.php?id=6314
> >
> >My assembler knowledge is unfortunately not so good yet to  
> >effectively repair it.
> >Besides routine is very sophisticated. :)
> >But I think that solving this cosmetic error will save many developers
> >hours during valgrind code checking. And preparing testcases fo fpc
> >developers.
> 
> It is a false positive by Valgrind. That code intentionally does not  
> care about reading uninitialised memory from time to time, and treats  
> that situation correctly. There is similar code in glibc, but Valgrind  
> comes with standard suppression files for glibc which suppress these  
> false positives. I'm not aware of any suppression files for FPC.
I think that this is not false positive. We allocate 1 byte and read 4,
report is right. But obviously effective solution do not exists.

What about to establish valgind suppressions file in fpc tree?
Here is first record.
{
   Assembler routine always reads 4 bytes alligned by 4 regardles str length
   Memcheck:Addr4
   fun:SYSUTILS_STRSCAN$PCHAR$CHAR$$PCHAR
}

Petr

-- 
Ing. Petr Kristan
.
EPOS PRO s.r.o., Bozeny Nemcove 2625, 530 02 Pardubice
tel: +420 466335223    Czech Republic (Eastern Europe) 
fax: +420 466510709