[Crash-utility] [PATCH v4 2/4] Get the absolute value of SYMNAME_HASH_INDEX

[Philipp Rudo]

Hi Lianbo,

On Mon, 27 Sep 2021 19:05:24 +0800
lijiang <lijiang at redhat.com> wrote:

> On Mon, Sep 27, 2021 at 5:46 PM Philipp Rudo <prudo at redhat.com> wrote:
> >
> > Hi Lianbo,
> >
> > On Mon, 27 Sep 2021 11:33:31 +0800
> > lijiang <lijiang at redhat.com> wrote:
> >  
> > > > Date: Sat, 18 Sep 2021 15:59:30 +0800
> > > > From: Tao Liu <ltao at redhat.com>
> > > > To: crash-utility at redhat.com
> > > > Subject: [Crash-utility] [PATCH v4 2/4] Get the absolute value of
> > > >         SYMNAME_HASH_INDEX
> > > > Message-ID: <20210918075932.132339-3-ltao at redhat.com>
> > > > Content-Type: text/plain; charset="US-ASCII"
> > > >
> > > > SYMNAME_HASH_INDEX is used as the index of symname hash table. It will
> > > > be out of range if SYMNAME_HASH_INDEX is negative. Let's get its absolute
> > > > value to avoid such risk.
> > > >
> > > > Signed-off-by: Tao Liu <ltao at redhat.com>
> > > > Reviewed-by: Philipp Rudo <prudo at redhat.com>
> > > > ---
> > > >  defs.h | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/defs.h b/defs.h
> > > > index c10ebff..3129f06 100644
> > > > --- a/defs.h
> > > > +++ b/defs.h
> > > > @@ -2725,7 +2725,7 @@ struct downsized {
> > > >
> > > >  #define SYMNAME_HASH (512)
> > > >  #define SYMNAME_HASH_INDEX(name) \
> > > > - ((name[0] ^ (name[strlen(name)-1] * name[strlen(name)/2])) % SYMNAME_HASH)
> > > > + (abs((name[0] ^ (name[strlen(name)-1] * name[strlen(name)/2])) % SYMNAME_HASH))
> > > >  
> > >
> > > I guess this may be caused by integer overflow, the expression uses
> > > integer calculations by default.  Does the following change work for
> > > you?
> > >
> > > -#define SYMNAME_HASH (512)
> > > +#define SYMNAME_HASH (512ULL)  
> >
> > the above isn't sufficient. The sign of the result is defined by the
> > sign of the dividend, i.e. assume a % b, then the result has the same
> > sign as a. So the result of SYMNAME_HASH_INDEX will become negative
> > whenever
> >
> > name[0] ^ (name[strlen(name)-1] * name[strlen(name)/2])
> >
> > is negative. So the only alternative I see is to guarantee that 'name'
> > is an unsigned char. But that would be more complicated to implement
> > then simply adding the 'abs'.
> >  
> 
> If it still doesn't work, I would tend to remove the macro
> definition(SYMNAME_HASH_INDEX)
> from defs.h, and change the macro definition to a static function in
> the symbols.c, let's use the
> 'unsigned long long' variable to calculate it.
> 
> Currently, the macro is used twice in the symbols.c. This change seems
> not complicated. Any thoughts?

do I understand your suggestion correct, you propose to replace the 

	#define SYMNAME_HASH_INDEX(name) ...

in defs.h by something like

	static unsigned long long SYMNAME_HASH_INDEX(const unsigned char * const name) { 
		return (name[0] ^ (name[strlen(name)-1] * name[strlen(name)/2]) % SYMNAME_HASH);
	}

in symbols.c? If so, I think that should be fine.

Thanks
Philipp


> Thanks.
> Lianbo
> 
> 
> > Thanks
> > Philipp
> >  
> > >
> > > Thanks.
> > > Lianbo  
> > > >  #define PATCH_KERNEL_SYMBOLS_START  ((char *)(1))
> > > >  #define PATCH_KERNEL_SYMBOLS_STOP   ((char *)(2))
> > > > --
> > > > 2.29.2
> > > >  
> > >
> > > --
> > > Crash-utility mailing list
> > > Crash-utility at redhat.com
> > > https://listman.redhat.com/mailman/listinfo/crash-utility
> > >  
> >  
>