Re: [PATCH] Linux: Consolidate auxiliary vector parsing (redo)

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 11/02/2022 17:33, Florian Weimer via Libc-alpha wrote:
> And optimize it slightly.
> 
> This is commit 8c8510ab2790039e58995ef3a22309582413d3ff revised.
> 
> In _dl_aux_init in elf/dl-support.c, use an explicit loop
> and -fno-tree-loop-distribute-patterns to avoid memset.
> 
> Tested on aarch64-linux-gnu, i686-linux-gnu, x86_64-linux-gnu.
> Built with build-many-glibcs.py.
> 
> ---
>  elf/Makefile                                 |   5 ++
>  elf/dl-support.c                             |  85 +++-----------------
>  sysdeps/unix/sysv/linux/alpha/dl-auxv.h      |  18 ++---
>  sysdeps/unix/sysv/linux/dl-parse_auxv.h      |  61 +++++++++++++++
>  sysdeps/unix/sysv/linux/dl-sysdep.c          | 112 ++++++++-------------------
>  sysdeps/unix/sysv/linux/powerpc/dl-auxv.h    |  14 +---
>  sysdeps/unix/sysv/linux/powerpc/dl-support.c |   4 +
>  7 files changed, 117 insertions(+), 182 deletions(-)
> 
> diff --git a/elf/Makefile b/elf/Makefile
> index bff94954c9..65ba06d8b8 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -160,6 +160,11 @@ ifeq (yes,$(have-loop-to-function))
>  CFLAGS-rtld.c += -fno-tree-loop-distribute-patterns
>  endif
>  
> +ifeq (yes,$(have-loop-to-function))
> +# Likewise, during static library startup, memset is not yet available.
> +CFLAGS-dl-support.c = -fno-tree-loop-distribute-patterns
> +endif
> +

I still think this approach is still fragile since we don't how if this
will be the only optimization option gcc will make loops to memset. We 
already have a similar problem on _dl_start, but it is slight different
since the clear memory should be done before self-relocation. Also for 
memset.c itself there is no much we can do, so we need to resort on 
-fno-tree-loop-distribute-patterns.

However in this case I think it would be better to avoid hacking the
compiler and expect it does not generate broken code and instead assume
memset *might* be generate and work around it.  We already have the
hack on sysdeps/generic/symbol-hacks.h, so I think we should do something
similiar:

  1. Add a __memset_generic internal symbol on all ports to avoid the
     iFUNC plt (which seems to be cause of this issue).

  2. Add a symbol redirection to call such symbol when required (most
     likely in the dl-start.h as H.J has suggested.

So we don't need to handle this specific issue everytime we hack into
the loader code.


>  # Compile rtld itself without stack protection.
>  # Also compile all routines in the static library that are elided from
>  # the shared libc because they are in libc.a in the same way.
> diff --git a/elf/dl-support.c b/elf/dl-support.c
> index 6d2c4baf81..153dd57ad2 100644
> --- a/elf/dl-support.c
> +++ b/elf/dl-support.c
> @@ -44,6 +44,7 @@
>  #include <dl-vdso-setup.h>
>  #include <dl-auxv.h>
>  #include <dl-find_object.h>
> +#include <array_length.h>
>  
>  extern char *__progname;
>  char **_dl_argv = &__progname;	/* This is checked for some error messages.  */
> @@ -240,93 +241,25 @@ __rtld_lock_define_initialized_recursive (, _dl_load_tls_lock)
>  
>  
>  #ifdef HAVE_AUX_VECTOR
> +#include <dl-parse_auxv.h>
> +
>  int _dl_clktck;
>  
>  void
>  _dl_aux_init (ElfW(auxv_t) *av)
>  {
> -  int seen = 0;
> -  uid_t uid = 0;
> -  gid_t gid = 0;
> -
>  #ifdef NEED_DL_SYSINFO
>    /* NB: Avoid RELATIVE relocation in static PIE.  */
>    GL(dl_sysinfo) = DL_SYSINFO_DEFAULT;
>  #endif
>  
>    _dl_auxv = av;
> -  for (; av->a_type != AT_NULL; ++av)
> -    switch (av->a_type)
> -      {
> -      case AT_PAGESZ:
> -	if (av->a_un.a_val != 0)
> -	  GLRO(dl_pagesize) = av->a_un.a_val;
> -	break;
> -      case AT_CLKTCK:
> -	GLRO(dl_clktck) = av->a_un.a_val;
> -	break;
> -      case AT_PHDR:
> -	GL(dl_phdr) = (const void *) av->a_un.a_val;
> -	break;
> -      case AT_PHNUM:
> -	GL(dl_phnum) = av->a_un.a_val;
> -	break;
> -      case AT_PLATFORM:
> -	GLRO(dl_platform) = (void *) av->a_un.a_val;
> -	break;
> -      case AT_HWCAP:
> -	GLRO(dl_hwcap) = (unsigned long int) av->a_un.a_val;
> -	break;
> -      case AT_HWCAP2:
> -	GLRO(dl_hwcap2) = (unsigned long int) av->a_un.a_val;
> -	break;
> -      case AT_FPUCW:
> -	GLRO(dl_fpu_control) = av->a_un.a_val;
> -	break;
> -#ifdef NEED_DL_SYSINFO
> -      case AT_SYSINFO:
> -	GL(dl_sysinfo) = av->a_un.a_val;
> -	break;
> -#endif
> -#ifdef NEED_DL_SYSINFO_DSO
> -      case AT_SYSINFO_EHDR:
> -	GL(dl_sysinfo_dso) = (void *) av->a_un.a_val;
> -	break;
> -#endif
> -      case AT_UID:
> -	uid ^= av->a_un.a_val;
> -	seen |= 1;
> -	break;
> -      case AT_EUID:
> -	uid ^= av->a_un.a_val;
> -	seen |= 2;
> -	break;
> -      case AT_GID:
> -	gid ^= av->a_un.a_val;
> -	seen |= 4;
> -	break;
> -      case AT_EGID:
> -	gid ^= av->a_un.a_val;
> -	seen |= 8;
> -	break;
> -      case AT_SECURE:
> -	seen = -1;
> -	__libc_enable_secure = av->a_un.a_val;
> -	__libc_enable_secure_decided = 1;
> -	break;
> -      case AT_RANDOM:
> -	_dl_random = (void *) av->a_un.a_val;
> -	break;
> -      case AT_MINSIGSTKSZ:
> -	_dl_minsigstacksize = av->a_un.a_val;
> -	break;
> -      DL_PLATFORM_AUXV
> -      }
> -  if (seen == 0xf)
> -    {
> -      __libc_enable_secure = uid != 0 || gid != 0;
> -      __libc_enable_secure_decided = 1;
> -    }
> +  dl_parse_auxv_t auxv_values;
> +  /* Use an explicit initialization loop here because memset may not
> +     be available yet.  */
> +  for (int i = 0; i < array_length (auxv_values); ++i)
> +    auxv_values[i] = 0;
> +  _dl_parse_auxv (av, auxv_values);
>  }
>  #endif
>  
> diff --git a/sysdeps/unix/sysv/linux/alpha/dl-auxv.h b/sysdeps/unix/sysv/linux/alpha/dl-auxv.h
> index 81d90da095..fcec743239 100644
> --- a/sysdeps/unix/sysv/linux/alpha/dl-auxv.h
> +++ b/sysdeps/unix/sysv/linux/alpha/dl-auxv.h
> @@ -20,16 +20,8 @@
>  
>  extern long __libc_alpha_cache_shape[4];
>  
> -#define DL_PLATFORM_AUXV				\
> -      case AT_L1I_CACHESHAPE:				\
> -	__libc_alpha_cache_shape[0] = av->a_un.a_val;	\
> -	break;						\
> -      case AT_L1D_CACHESHAPE:				\
> -	__libc_alpha_cache_shape[1] = av->a_un.a_val;	\
> -	break;						\
> -      case AT_L2_CACHESHAPE:				\
> -	__libc_alpha_cache_shape[2] = av->a_un.a_val;	\
> -	break;						\
> -      case AT_L3_CACHESHAPE:				\
> -	__libc_alpha_cache_shape[3] = av->a_un.a_val;	\
> -	break;
> +#define DL_PLATFORM_AUXV					\
> +  __libc_alpha_cache_shape[0] = auxv_values[AT_L1I_CACHESHAPE]; \
> +  __libc_alpha_cache_shape[1] = auxv_values[AT_L1D_CACHESHAPE]; \
> +  __libc_alpha_cache_shape[2] = auxv_values[AT_L2_CACHESHAPE];	\
> +  __libc_alpha_cache_shape[3] = auxv_values[AT_L3_CACHESHAPE];
> diff --git a/sysdeps/unix/sysv/linux/dl-parse_auxv.h b/sysdeps/unix/sysv/linux/dl-parse_auxv.h
> new file mode 100644
> index 0000000000..b3d82f6994
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/dl-parse_auxv.h
> @@ -0,0 +1,61 @@
> +/* Parse the Linux auxiliary vector.
> +   Copyright (C) 1995-2022 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/>.  */
> +
> +#include <elf.h>
> +#include <entry.h>
> +#include <fpu_control.h>
> +#include <ldsodefs.h>
> +#include <link.h>
> +
> +typedef ElfW(Addr) dl_parse_auxv_t[AT_MINSIGSTKSZ + 1];
> +
> +/* Copy the auxiliary vector into AUX_VALUES and set up GLRO
> +   variables.  */
> +static inline
> +void _dl_parse_auxv (ElfW(auxv_t) *av, dl_parse_auxv_t auxv_values)
> +{
> +  auxv_values[AT_ENTRY] = (ElfW(Addr)) ENTRY_POINT;
> +  auxv_values[AT_PAGESZ] = EXEC_PAGESIZE;
> +  auxv_values[AT_FPUCW] = _FPU_DEFAULT;
> +
> +  /* NB: Default to a constant CONSTANT_MINSIGSTKSZ.  */
> +  _Static_assert (__builtin_constant_p (CONSTANT_MINSIGSTKSZ),
> +                  "CONSTANT_MINSIGSTKSZ is constant");
> +  auxv_values[AT_MINSIGSTKSZ] = CONSTANT_MINSIGSTKSZ;
> +
> +  for (; av->a_type != AT_NULL; av++)
> +    if (av->a_type <= AT_MINSIGSTKSZ)
> +      auxv_values[av->a_type] = av->a_un.a_val;
> +
> +  GLRO(dl_pagesize) = auxv_values[AT_PAGESZ];
> +  __libc_enable_secure = auxv_values[AT_SECURE];
> +  GLRO(dl_platform) = (void *) auxv_values[AT_PLATFORM];
> +  GLRO(dl_hwcap) = auxv_values[AT_HWCAP];
> +  GLRO(dl_hwcap2) = auxv_values[AT_HWCAP2];
> +  GLRO(dl_clktck) = auxv_values[AT_CLKTCK];
> +  GLRO(dl_fpu_control) = auxv_values[AT_FPUCW];
> +  _dl_random = (void *) auxv_values[AT_RANDOM];
> +  GLRO(dl_minsigstacksize) = auxv_values[AT_MINSIGSTKSZ];
> +  GLRO(dl_sysinfo_dso) = (void *) auxv_values[AT_SYSINFO_EHDR];
> +#ifdef NEED_DL_SYSINFO
> +  if (GLRO(dl_sysinfo_dso) != NULL)
> +    GLRO(dl_sysinfo) = auxv_values[AT_SYSINFO];
> +#endif
> +
> +  DL_PLATFORM_AUXV
> +}
> diff --git a/sysdeps/unix/sysv/linux/dl-sysdep.c b/sysdeps/unix/sysv/linux/dl-sysdep.c
> index 8ad72c4b7b..71cf1968b5 100644
> --- a/sysdeps/unix/sysv/linux/dl-sysdep.c
> +++ b/sysdeps/unix/sysv/linux/dl-sysdep.c
> @@ -18,15 +18,15 @@
>  
>  #include <_itoa.h>
>  #include <assert.h>
> +#include <dl-auxv.h>
>  #include <dl-hwcap-check.h>
>  #include <dl-osinfo.h>
> +#include <dl-parse_auxv.h>
>  #include <dl-procinfo.h>
>  #include <dl-tunables.h>
>  #include <elf.h>
> -#include <entry.h>
>  #include <errno.h>
>  #include <fcntl.h>
> -#include <fpu_control.h>
>  #include <ldsodefs.h>
>  #include <libc-internal.h>
>  #include <libintl.h>
> @@ -45,8 +45,6 @@
>  #include <dl-machine.h>
>  
>  #ifdef SHARED
> -# include <dl-auxv.h>
> -
>  extern char **_environ attribute_hidden;
>  extern char _end[] attribute_hidden;
>  
> @@ -64,20 +62,20 @@ void *_dl_random attribute_relro = NULL;
>  # define DL_STACK_END(cookie) ((void *) (cookie))
>  #endif
>  
> -ElfW(Addr)
> -_dl_sysdep_start (void **start_argptr,
> -		  void (*dl_main) (const ElfW(Phdr) *phdr, ElfW(Word) phnum,
> -				   ElfW(Addr) *user_entry, ElfW(auxv_t) *auxv))
> +/* Arguments passed to dl_main.  */
> +struct dl_main_arguments
>  {
> -  const ElfW(Phdr) *phdr = NULL;
> -  ElfW(Word) phnum = 0;
> +  const ElfW(Phdr) *phdr;
> +  ElfW(Word) phnum;
>    ElfW(Addr) user_entry;
> -  ElfW(auxv_t) *av;
> -#ifdef NEED_DL_SYSINFO
> -  uintptr_t new_sysinfo = 0;
> -#endif
> +};
>  
> -  __libc_stack_end = DL_STACK_END (start_argptr);
> +/* Separate function, so that dl_main can be called without the large
> +   array on the stack.  */
> +static void
> +_dl_sysdep_parse_arguments (void **start_argptr,
> +			    struct dl_main_arguments *args)
> +{
>    _dl_argc = (intptr_t) *start_argptr;
>    _dl_argv = (char **) (start_argptr + 1); /* Necessary aliasing violation.  */
>    _environ = _dl_argv + _dl_argc + 1;
> @@ -89,75 +87,26 @@ _dl_sysdep_start (void **start_argptr,
>  	break;
>        }
>  
> -  user_entry = (ElfW(Addr)) ENTRY_POINT;
> -  GLRO(dl_platform) = NULL; /* Default to nothing known about the platform.  */
> +  dl_parse_auxv_t auxv_values = { 0, };
> +  _dl_parse_auxv (GLRO(dl_auxv), auxv_values);
>  
> -  /* NB: Default to a constant CONSTANT_MINSIGSTKSZ.  */
> -  _Static_assert (__builtin_constant_p (CONSTANT_MINSIGSTKSZ),
> -		  "CONSTANT_MINSIGSTKSZ is constant");
> -  GLRO(dl_minsigstacksize) = CONSTANT_MINSIGSTKSZ;
> +  args->phdr = (const ElfW(Phdr) *) auxv_values[AT_PHDR];
> +  args->phnum = auxv_values[AT_PHNUM];
> +  args->user_entry = auxv_values[AT_ENTRY];
> +}
>  
> -  for (av = GLRO(dl_auxv); av->a_type != AT_NULL; av++)
> -    switch (av->a_type)
> -      {
> -      case AT_PHDR:
> -	phdr = (void *) av->a_un.a_val;
> -	break;
> -      case AT_PHNUM:
> -	phnum = av->a_un.a_val;
> -	break;
> -      case AT_PAGESZ:
> -	GLRO(dl_pagesize) = av->a_un.a_val;
> -	break;
> -      case AT_ENTRY:
> -	user_entry = av->a_un.a_val;
> -	break;
> -      case AT_SECURE:
> -	__libc_enable_secure = av->a_un.a_val;
> -	break;
> -      case AT_PLATFORM:
> -	GLRO(dl_platform) = (void *) av->a_un.a_val;
> -	break;
> -      case AT_HWCAP:
> -	GLRO(dl_hwcap) = (unsigned long int) av->a_un.a_val;
> -	break;
> -      case AT_HWCAP2:
> -	GLRO(dl_hwcap2) = (unsigned long int) av->a_un.a_val;
> -	break;
> -      case AT_CLKTCK:
> -	GLRO(dl_clktck) = av->a_un.a_val;
> -	break;
> -      case AT_FPUCW:
> -	GLRO(dl_fpu_control) = av->a_un.a_val;
> -	break;
> -#ifdef NEED_DL_SYSINFO
> -      case AT_SYSINFO:
> -	new_sysinfo = av->a_un.a_val;
> -	break;
> -#endif
> -      case AT_SYSINFO_EHDR:
> -	GLRO(dl_sysinfo_dso) = (void *) av->a_un.a_val;
> -	break;
> -      case AT_RANDOM:
> -	_dl_random = (void *) av->a_un.a_val;
> -	break;
> -      case AT_MINSIGSTKSZ:
> -	GLRO(dl_minsigstacksize) = av->a_un.a_val;
> -	break;
> -      DL_PLATFORM_AUXV
> -      }
> +ElfW(Addr)
> +_dl_sysdep_start (void **start_argptr,
> +		  void (*dl_main) (const ElfW(Phdr) *phdr, ElfW(Word) phnum,
> +				   ElfW(Addr) *user_entry, ElfW(auxv_t) *auxv))
> +{
> +  __libc_stack_end = DL_STACK_END (start_argptr);
> +
> +  struct dl_main_arguments dl_main_args;
> +  _dl_sysdep_parse_arguments (start_argptr, &dl_main_args);
>  
>    dl_hwcap_check ();
>  
> -#ifdef NEED_DL_SYSINFO
> -  if (new_sysinfo != 0)
> -    {
> -      /* Only set the sysinfo value if we also have the vsyscall DSO.  */
> -      if (GLRO(dl_sysinfo_dso) != 0)
> -        GLRO(dl_sysinfo) = new_sysinfo;
> -    }
> -#endif
> -
>    __tunables_init (_environ);
>  
>    /* Initialize DSO sorting algorithm after tunables.  */
> @@ -188,8 +137,9 @@ _dl_sysdep_start (void **start_argptr,
>    if (__builtin_expect (__libc_enable_secure, 0))
>      __libc_check_standard_fds ();
>  
> -  (*dl_main) (phdr, phnum, &user_entry, GLRO(dl_auxv));
> -  return user_entry;
> +  (*dl_main) (dl_main_args.phdr, dl_main_args.phnum,
> +              &dl_main_args.user_entry, GLRO(dl_auxv));
> +  return dl_main_args.user_entry;
>  }
>  
>  void
> diff --git a/sysdeps/unix/sysv/linux/powerpc/dl-auxv.h b/sysdeps/unix/sysv/linux/powerpc/dl-auxv.h
> index 594371940a..ce2281cf11 100644
> --- a/sysdeps/unix/sysv/linux/powerpc/dl-auxv.h
> +++ b/sysdeps/unix/sysv/linux/powerpc/dl-auxv.h
> @@ -16,15 +16,5 @@
>     License along with the GNU C Library; if not, see
>     <https://www.gnu.org/licenses/>.  */
>  
> -#include <ldsodefs.h>
> -
> -#if IS_IN (libc) && !defined SHARED
> -int GLRO(dl_cache_line_size);
> -#endif
> -
> -/* Scan the Aux Vector for the "Data Cache Block Size" entry and assign it
> -   to dl_cache_line_size.  */
> -#define DL_PLATFORM_AUXV						      \
> -      case AT_DCACHEBSIZE:						      \
> -	GLRO(dl_cache_line_size) = av->a_un.a_val;			      \
> -	break;
> +#define DL_PLATFORM_AUXV \
> +  GLRO(dl_cache_line_size) = auxv_values[AT_DCACHEBSIZE];
> diff --git a/sysdeps/unix/sysv/linux/powerpc/dl-support.c b/sysdeps/unix/sysv/linux/powerpc/dl-support.c
> new file mode 100644
> index 0000000000..abe68a7049
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/powerpc/dl-support.c
> @@ -0,0 +1,4 @@
> +#include <elf/dl-support.c>
> +
> +/* Populated from the auxiliary vector.  */
> +int _dl_cache_line_size;
>