elf: Remove extra hwcap mechanism from ldconfig

  Historically, this mechanism was used to process "nosegneg"
subdirectories, and it is still used to include the "tls"
subdirectories.  With nosegneg support gone from ld.so, this is part
no longer useful.

The entire mechanism is not well-designed because it causes the
meaning of hwcap bits in ld.so.cache to depend on the kernel version
that was used to generate the cache, which makes it difficult to use
this mechanism for anything else in the future.

Tested on x86_64-linux-gnu and i686-linux-gnu.  Manually verified that
the generated (new-format) /etc/ld.so.cache file does not change with
this patch on a reference system, with and without a shared object in a
/usr/lib64/tls subdirectory.

---
 elf/ldconfig.c | 87 +++++++++++-----------------------------------------------
 1 file changed, 16 insertions(+), 71 deletions(-)
  

On 19/05/2020 14:12, Florian Weimer via Libc-alpha wrote:
> Historically, this mechanism was used to process "nosegneg"
> subdirectories, and it is still used to include the "tls"
> subdirectories.  With nosegneg support gone from ld.so, this is part
> no longer useful.
> 
> The entire mechanism is not well-designed because it causes the
> meaning of hwcap bits in ld.so.cache to depend on the kernel version
> that was used to generate the cache, which makes it difficult to use
> this mechanism for anything else in the future.
> 
> Tested on x86_64-linux-gnu and i686-linux-gnu.  Manually verified that
> the generated (new-format) /etc/ld.so.cache file does not change with
> this patch on a reference system, with and without a shared object in a
> /usr/lib64/tls subdirectory.

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> 
> ---
>  elf/ldconfig.c | 87 +++++++++++-----------------------------------------------
>  1 file changed, 16 insertions(+), 71 deletions(-)
> 
> diff --git a/elf/ldconfig.c b/elf/ldconfig.c
> index 3b860ad732..0c090dca15 100644
> --- a/elf/ldconfig.c
> +++ b/elf/ldconfig.c
> @@ -44,11 +44,15 @@
>  
>  #include <dl-procinfo.h>
>  
> -#ifdef _DL_FIRST_PLATFORM
> -# define _DL_FIRST_EXTRA (_DL_FIRST_PLATFORM + _DL_PLATFORMS_COUNT)
> -#else
> -# define _DL_FIRST_EXTRA _DL_HWCAP_COUNT
> -#endif
> +/* This subpath in search path entries is always supported and
> +   included in the cache for backwards compatibility.  */
> +#define TLS_SUBPATH "tls"
> +
> +/* The MSB of the hwcap field is set for objects in TLS_SUBPATH
> +   directories.  There is always TLS support in glibc, so the dynamic
> +   loader does not check the bit directly.  But more hwcap bits make a
> +   an object more preferred, so the bit still has meaning.  */
> +#define TLS_HWCAP_BIT 63
>  

Ok.

>  #ifndef LD_SO_CONF
>  # define LD_SO_CONF SYSCONFDIR "/ld.so.conf"
> @@ -127,9 +131,6 @@ static const char *config_file;
>  /* Mask to use for important hardware capabilities.  */
>  static unsigned long int hwcap_mask = HWCAP_IMPORTANT;
>  
> -/* Configuration-defined capabilities defined in kernel vDSOs.  */
> -static const char *hwcap_extra[64 - _DL_FIRST_EXTRA];
> -
>  /* Name and version of program.  */
>  static void print_version (FILE *stream, struct argp_state *state);
>  void (*argp_program_version_hook) (FILE *, struct argp_state *)
> @@ -186,12 +187,9 @@ is_hwcap_platform (const char *name)
>    if (hwcap_idx != -1)
>      return 1;
>  
> -  /* Is this one of the extra pseudo-hwcaps that we map beyond
> -     _DL_FIRST_EXTRA like "tls", or "nosegneg?"  */
> -  for (hwcap_idx = _DL_FIRST_EXTRA; hwcap_idx < 64; ++hwcap_idx)
> -    if (hwcap_extra[hwcap_idx - _DL_FIRST_EXTRA] != NULL
> -	&& !strcmp (name, hwcap_extra[hwcap_idx - _DL_FIRST_EXTRA]))
> -      return 1;
> +  /* Backwards-compatibility for the "tls" subdirectory.  */
> +  if (strcmp (name, TLS_SUBPATH) == 0)
> +    return 1;
>  
>    return 0;
>  }

Ok.

> @@ -226,11 +224,9 @@ path_hwcap (const char *path)
>  	  h = _dl_string_platform (ptr + 1);
>  	  if (h == (uint64_t) -1)
>  	    {
> -	      for (h = _DL_FIRST_EXTRA; h < 64; ++h)
> -		if (hwcap_extra[h - _DL_FIRST_EXTRA] != NULL
> -		    && !strcmp (ptr + 1, hwcap_extra[h - _DL_FIRST_EXTRA]))
> -		  break;
> -	      if (h == 64)
> +	      if (strcmp (ptr + 1, TLS_SUBPATH) == 0)
> +		h = TLS_HWCAP_BIT;
> +	      else
>  		break;
>  	    }
>  	}

Ok.

> @@ -1146,52 +1142,7 @@ Warning: ignoring configuration file that cannot be opened: %s"),
>  	      parse_conf_include (filename, lineno, do_chroot, dir);
>  	}
>        else if (!strncasecmp (cp, "hwcap", 5) && isblank (cp[5]))
> -	{
> -	  cp += 6;
> -	  char *p, *name = NULL;
> -	  unsigned long int n = strtoul (cp, &cp, 0);
> -	  if (cp != NULL && isblank (*cp))
> -	    while ((p = strsep (&cp, " \t")) != NULL)
> -	      if (p[0] != '\0')
> -		{
> -		  if (name == NULL)
> -		    name = p;
> -		  else
> -		    {
> -		      name = NULL;
> -		      break;
> -		    }
> -		}
> -	  if (name == NULL)
> -	    {
> -	      error (EXIT_FAILURE, 0, _("%s:%u: bad syntax in hwcap line"),
> -		     filename, lineno);
> -	      break;
> -	    }
> -	  if (n >= (64 - _DL_FIRST_EXTRA))
> -	    error (EXIT_FAILURE, 0,
> -		   _("%s:%u: hwcap index %lu above maximum %u"),
> -		   filename, lineno, n, 64 - _DL_FIRST_EXTRA - 1);
> -	  if (hwcap_extra[n] == NULL)
> -	    {
> -	      for (unsigned long int h = 0; h < (64 - _DL_FIRST_EXTRA); ++h)
> -		if (hwcap_extra[h] != NULL && !strcmp (name, hwcap_extra[h]))
> -		  error (EXIT_FAILURE, 0,
> -			 _("%s:%u: hwcap index %lu already defined as %s"),
> -			 filename, lineno, h, name);
> -	      hwcap_extra[n] = xstrdup (name);
> -	    }
> -	  else
> -	    {
> -	      if (strcmp (name, hwcap_extra[n]))
> -		error (EXIT_FAILURE, 0,
> -		       _("%s:%u: hwcap index %lu already defined as %s"),
> -		       filename, lineno, n, hwcap_extra[n]);
> -	      if (opt_verbose)
> -		error (0, 0, _("%s:%u: duplicate hwcap %lu %s"),
> -		       filename, lineno, n, name);
> -	    }
> -	}
> +	error (0, 0, _("%s:%u: hwcap directive ignored"), filename, lineno);
>        else
>  	add_dir_1 (cp, filename, lineno);
>      }

Ok.

> @@ -1304,12 +1255,6 @@ main (int argc, char **argv)
>  	  add_dir_1 (argv[i], "<cmdline>", 0);
>      }
>  
> -  /* The last entry in hwcap_extra is reserved for the "tls" pseudo-hwcap which
> -     indicates support for TLS.  This pseudo-hwcap is only used by old versions
> -     under which TLS support was optional.  The entry is no longer needed, but
> -     must remain for compatibility.  */
> -  hwcap_extra[63 - _DL_FIRST_EXTRA] = "tls";
> -
>    set_hwcap ();
>  
>    if (opt_chroot)
> 

Ok.