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

[Philipp Rudo]

Hi Lianbo,

sorry for the late response.

On Sun, 3 Oct 2021 10:25:26 +0800
lijiang <lijiang at redhat.com> wrote:

> On Sat, Oct 2, 2021 at 12:10 AM Philipp Rudo <prudo at redhat.com> wrote:
> >
> > Hi Tao,
> > Hi Lianbo,
> >
> > sorry for the late response.
> >
> > On Wed, 29 Sep 2021 15:46:19 +0800
> > lijiang <lijiang at redhat.com> wrote:
> >  
> > > > > > 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.
> > > > >  
> > > Yes, you are right, Philipp.
> > >  
> > > >
> > > > Please correct me if I'm wrong. I don't think the function can work.
> > > > Let's say name[0] ^ (name[strlen(name)-1] * name[strlen(name)/2]) ==
> > > > -1, then we will have:
> > > >
> > > > static unsigned long long SYMNAME_HASH_INDEX(const unsigned char * const name) {
> > > >         return (-1) % 512;
> > > > }
> > > >
> > > > The returned value is a very large number, and will overflow the array.  
> >
> > @Tao: I don't think that should be possible here. In the function I
> > have defined name as _unsigned_ char*. So the calculation should never
> > become negative. But to be honest I haven't tested it.
> >  
> > > No, this is a modulo operation, and its result will never exceed '512' anyway.
> > > (unsigned long long)(-1) % 512 = 511  
> >
> > @Lianbo: I don't think that's true. When the modulo is used with an
> > negative dividend the result becomes negative. When this then gets
> > casted to an unsigend integer type the result will become very large.
> > That's also what a quick test showed me
> >
> > printf("%lli\n", ((-1LL) % 512));                       --> -1
> > printf("%llu\n", (unsigned long long) ((-1LL) % 512));  --> 18446744073709551615
> >  
> 
> Thank you for the reply, Tao and Philipp. Let's go back to the problem
> itself and continue our discussion.
> Does the following change work for you?
> 
> ---
>  defs.h    |  2 --
>  symbols.c | 18 +++++++++++++++---
>  2 files changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/defs.h b/defs.h
> index cbd45e52f9da..d7b9bcc89878 100644
> --- a/defs.h
> +++ b/defs.h
> @@ -2728,8 +2728,6 @@ struct downsized {
>          (((vaddr) >> machdep->pageshift) % SYMVAL_HASH)
> 
>  #define SYMNAME_HASH (512)
> -#define SYMNAME_HASH_INDEX(name) \
> - ((name[0] ^ (name[strlen(name)-1] * name[strlen(name)/2])) % SYMNAME_HASH)
> 
>  #define PATCH_KERNEL_SYMBOLS_START  ((char *)(1))
>  #define PATCH_KERNEL_SYMBOLS_STOP   ((char *)(2))
> diff --git a/symbols.c b/symbols.c
> index 69dccdb09d5f..8a2230df7712 100644
> --- a/symbols.c
> +++ b/symbols.c
> @@ -1127,6 +1127,18 @@ symname_hash_init(void)
>   st->__per_cpu_end = sp->value;
>  }
> 
> +unsigned int symname_hash_index(unsigned char *name)
> +{
> +     unsigned int len, value;
> +
> +     len = strlen(name);
> +     if (!len)
> +         error(FATAL, "The length of the symbol name is zero!\n");
> +
> +     value = name[len-1] * name[len/2];
> +
> +     return (name[0] ^ value) % (unsigned int)SYMNAME_HASH;
> +}
>  /*
>   *  Install a single static kernel symbol into the symname_hash.
>   */
> @@ -1134,9 +1146,9 @@ static void
>  symname_hash_install(struct syment *spn)
>  {
>   struct syment *sp;
> -        int index;
> +        unsigned int index;
> 
> -        index = SYMNAME_HASH_INDEX(spn->name);
> +        index = symname_hash_index(spn->name);
>   spn->cnt = 1;
> 
>          if ((sp = st->symname_hash[index]) == NULL)
> @@ -1165,7 +1177,7 @@ symname_hash_search(char *name)
>  {
>   struct syment *sp;
> 
> -        sp = st->symname_hash[SYMNAME_HASH_INDEX(name)];
> +        sp = st->symname_hash[symname_hash_index(name)];
> 
>   while (sp) {
>   if (STREQ(sp->name, name))

the patch looks good to me. Personally I would only update the
definition of SYMNAME_HASH to be unsigned rather than doing the cast in
symname_hash_index. But that's only a personal preference.

Thanks
Philipp