Generate VDSO_HASH

[Alistair Francis]

On Tue, Nov 12, 2019 at 1:47 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Alistair Francis:
>
> > How is VDSO_HASH generated? I would like to generate one for the 5.4
> > kernel.
>
> It's the ELF hash.  Not to be confused with the GNU hash, which is used
> for symbol name lookup these days.  See <sysdeps/generic/dl-hash.h> and
> the program below.
>
> LINUX_5.4: ELF: 61765876 (0x3ae78f4), GNU: 650524491 (0x26c6374b)
>
> The ELF hash is still used for symbol versions, only the symbol names
> switched to the GNU hash.

Thanks!

Alistair

>
> Thanks,
> Florian
>
> /* Compute hash value for given string according to ELF standard.
>    Copyright (C) 1995-2019 Free Software Foundation, Inc.
>    This file is part of the GNU C Library.
>
>    The GNU C Library is free software; you can redistribute it and/or
>    modify it under the terms of the GNU Lesser General Public
>    License as published by the Free Software Foundation; either
>    version 2.1 of the License, or (at your option) any later version.
>
>    The GNU C Library is distributed in the hope that it will be useful,
>    but WITHOUT ANY WARRANTY; without even the implied warranty of
>    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>    Lesser General Public License for more details.
>
>    You should have received a copy of the GNU Lesser General Public
>    License along with the GNU C Library; if not, see
>    <https://www.gnu.org/licenses/>.  */
>
> #ifndef _DL_HASH_H
> #define _DL_HASH_H      1
>
>
> /* This is the hashing function specified by the ELF ABI.  In the
>    first five operations no overflow is possible so we optimized it a
>    bit.  */
> static unsigned int
> __attribute__ ((unused))
> _dl_elf_hash (const char *name_arg)
> {
>   const unsigned char *name = (const unsigned char *) name_arg;
>   unsigned long int hash = *name;
>   if (hash != 0 && name[1] != '\0')
>     {
>       hash = (hash << 4) + name[1];
>       if (name[2] != '\0')
>         {
>           hash = (hash << 4) + name[2];
>           if (name[3] != '\0')
>             {
>               hash = (hash << 4) + name[3];
>               if (name[4] != '\0')
>                 {
>                   hash = (hash << 4) + name[4];
>                   name += 5;
>                   while (*name != '\0')
>                     {
>                       unsigned long int hi;
>                       hash = (hash << 4) + *name++;
>                       hi = hash & 0xf0000000;
>
>                       /* The algorithm specified in the ELF ABI is as
>                          follows:
>
>                          if (hi != 0)
>                            hash ^= hi >> 24;
>
>                          hash &= ~hi;
>
>                          But the following is equivalent and a lot
>                          faster, especially on modern processors.  */
>
>                       hash ^= hi >> 24;
>                     }
>
>                   /* Second part of the modified formula.  This
>                      operation can be lifted outside the loop.  */
>                   hash &= 0x0fffffff;
>                 }
>             }
>         }
>     }
>   return hash;
> }
>
> #endif /* dl-hash.h */
>