Re: [PATCH 06/18] linux: Only use 64-bit syscall if required for select

[Lukasz Majewski <lukma@denx.de>]

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On Thu, 17 Jun 2021 08:50:52 -0300
Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:

> For !__ASSUME_TIME64_SYSCALLS there is no need to issue a 64-bit
> syscall if the provided timeout fits in a 32-bit one.  The 64-bit
> usage should be rare since the timeout is a relative one.  This also
> avoids the need to use supports_time64() (which breaks the usage case
> of live migration like CRIU or similar).
> 
> It also fixes an issue on 32-bit select call for !__ASSUME_PSELECT
> (microblase with older kernels only) where the expected timeout
> is a 'struct timeval' instead of 'struct timespec'.
> 
> Checked on i686-linux-gnu on a 4.15 kernel and on a 5.11 kernel
> (with and without --enable-kernel=5.1) and on x86_64-linux-gnu.
> ---
>  include/sys/select.h               |  5 +++
>  misc/Makefile                      |  2 +
>  misc/tst-select.c                  | 39 +++++++++++--------
>  sysdeps/unix/sysv/linux/Makefile   |  2 +-
>  sysdeps/unix/sysv/linux/select.c   | 60
> ++++++++++-------------------- sysdeps/unix/sysv/linux/select32.c |
> 58 +++++++++++++++++++++++++++++ 6 files changed, 109 insertions(+),
> 57 deletions(-) create mode 100644 sysdeps/unix/sysv/linux/select32.c
> 
> diff --git a/include/sys/select.h b/include/sys/select.h
> index ec073deeba..a8961afbed 100644
> --- a/include/sys/select.h
> +++ b/include/sys/select.h
> @@ -21,6 +21,11 @@ extern int __pselect32 (int __nfds, fd_set
> *__readfds, const struct __timespec64 *__timeout,
>  			const __sigset_t *__sigmask)
>    attribute_hidden;
> +extern int __select32 (int __nfds, fd_set *__readfds,
> +		       fd_set *__writefds, fd_set *__exceptfds,
> +		       const struct __timespec64 *ts64,
> +		       struct __timeval64 *timeout)
> +  attribute_hidden;
>  
>  extern int __select64 (int __nfds, fd_set *__readfds,
>  		       fd_set *__writefds, fd_set *__exceptfds,
> diff --git a/misc/Makefile b/misc/Makefile
> index fa40bf0e11..66586bcc7e 100644
> --- a/misc/Makefile
> +++ b/misc/Makefile
> @@ -169,5 +169,7 @@ $(objpfx)tst-allocate_once-mem.out:
> $(objpfx)tst-allocate_once.out $(common-objpfx)malloc/mtrace
> $(objpfx)tst-allocate_once.mtrace > $@; \ $(evaluate-test)
>  
> +$(objpfx)tst-select: $(librt)
> +$(objpfx)tst-select-time64: $(librt)
>  $(objpfx)tst-pselect: $(librt)
>  $(objpfx)tst-pselect-time64: $(librt)
> diff --git a/misc/tst-select.c b/misc/tst-select.c
> index 52aa26651f..134eed99be 100644
> --- a/misc/tst-select.c
> +++ b/misc/tst-select.c
> @@ -17,6 +17,7 @@
>     <https://www.gnu.org/licenses/>.  */
>  
>  #include <errno.h>
> +#include <intprops.h>
>  #include <support/capture_subprocess.h>
>  #include <support/check.h>
>  #include <support/support.h>
> @@ -31,12 +32,6 @@ struct child_args
>    struct timeval tmo;
>  };
>  
> -static void
> -alarm_handler (int signum)
> -{
> -  /* Do nothing.  */
> -}
> -
>  static void
>  do_test_child (void *clousure)
>  {
> @@ -69,17 +64,20 @@ do_test_child (void *clousure)
>  static void
>  do_test_child_alarm (void *clousure)
>  {
> -  struct sigaction act = { .sa_handler = alarm_handler };
> -  xsigaction (SIGALRM, &act, NULL);
> -  alarm (1);
> +  struct child_args *args = (struct child_args *) clousure;
>  
> -  struct timeval tv = { .tv_sec = 10, .tv_usec = 0 };
> +  support_create_timer (0, 100000000, false, NULL);
> +  struct timeval tv = { .tv_sec = args->tmo.tv_sec, .tv_usec = 0 };
>    int r = select (0, NULL, NULL, NULL, &tv);
>    TEST_COMPARE (r, -1);
> -  TEST_COMPARE (errno, EINTR);
> -
> -  if (support_select_modifies_timeout ())
> -    TEST_VERIFY (tv.tv_sec < 10);
> +  if (args->tmo.tv_sec > INT_MAX)
> +    TEST_VERIFY (errno == EINTR || errno == EOVERFLOW);
> +  else
> +    {
> +      TEST_COMPARE (errno, EINTR);
> +      if (support_select_modifies_timeout ())
> +       TEST_VERIFY (tv.tv_sec < args->tmo.tv_sec);
> +    }
>  }
>  
>  static int
> @@ -121,13 +119,24 @@ do_test (void)
>    xclose (args.fds[0][0]);
>    xclose (args.fds[1][1]);
>  
> +  args.tmo = (struct timeval) { .tv_sec = 10, .tv_usec = 0 };
> +  {
> +    struct support_capture_subprocess result;
> +    result = support_capture_subprocess (do_test_child_alarm, &args);
> +    support_capture_subprocess_check (&result, "tst-select-child", 0,
> +				      sc_allow_none);
> +  }
> +
> +  args.tmo = (struct timeval) { .tv_sec = TYPE_MAXIMUM (time_t),
> +				.tv_usec = 0 };
>    {
>      struct support_capture_subprocess result;
> -    result = support_capture_subprocess (do_test_child_alarm, NULL);
> +    result = support_capture_subprocess (do_test_child_alarm, &args);
>      support_capture_subprocess_check (&result, "tst-select-child", 0,
>  				      sc_allow_none);
>    }
>  
> +  args.tmo = (struct timeval) { .tv_sec = 0, .tv_usec = 0 };
>    {
>      fd_set rfds;
>      FD_ZERO (&rfds);
> diff --git a/sysdeps/unix/sysv/linux/Makefile
> b/sysdeps/unix/sysv/linux/Makefile index c36ea0e494..710169a454 100644
> --- a/sysdeps/unix/sysv/linux/Makefile
> +++ b/sysdeps/unix/sysv/linux/Makefile
> @@ -61,7 +61,7 @@ sysdep_routines += adjtimex clone umount umount2
> readahead sysctl \ open_by_handle_at mlock2 pkey_mprotect pkey_set
> pkey_get \ timerfd_gettime timerfd_settime prctl \
>  		   process_vm_readv process_vm_writev clock_adjtime \
> -		   time64-support pselect32 \
> +		   time64-support pselect32 select32 \
>  		   xstat fxstat lxstat xstat64 fxstat64 lxstat64 \
>  		   fxstatat fxstatat64 \
>  		   xmknod xmknodat convert_scm_timestamps
> diff --git a/sysdeps/unix/sysv/linux/select.c
> b/sysdeps/unix/sysv/linux/select.c index dc16a816ed..2d2a7fa720 100644
> --- a/sysdeps/unix/sysv/linux/select.c
> +++ b/sysdeps/unix/sysv/linux/select.c
> @@ -21,7 +21,6 @@
>  #include <sys/select.h>
>  #include <errno.h>
>  #include <sysdep-cancel.h>
> -#include <time64-support.h>
>  
>  /* Check the first NFDS descriptors each in READFDS (if not NULL)
> for read readiness, in WRITEFDS (if not NULL) for write readiness,
> and in EXCEPTFDS @@ -65,53 +64,32 @@ __select64 (int nfds, fd_set
> *readfds, fd_set *writefds, fd_set *exceptfds, #ifndef
> __NR_pselect6_time64 # define __NR_pselect6_time64 __NR_pselect6
>  #endif
> +
> +#ifdef __ASSUME_TIME64_SYSCALLS
> +  int r = SYSCALL_CANCEL (pselect6_time64, nfds, readfds, writefds,
> exceptfds,
> +			  pts64, NULL);
> +  if (timeout != NULL)
> +    TIMESPEC_TO_TIMEVAL (timeout, pts64);
> +  return r;
> +#else
> +  bool is32bit = timeout != NULL
> +		 ? in_time_t_range (timeout->tv_sec) : true;
>    int r;
> -  if (supports_time64 ())
> +  if (!is32bit)
>      {
> -      r = SYSCALL_CANCEL (pselect6_time64, nfds, readfds, writefds,
> exceptfds,
> -			  pts64, NULL);
> -      /* Linux by default will update the timeout after a pselect6
> syscall
> -         (though the pselect() glibc call suppresses this behavior).
> -         Since select() on Linux has the same behavior as the
> pselect6
> -         syscall, we update the timeout here.  */
> -      if (r >= 0 || errno != ENOSYS)
> +      r = SYSCALL_CANCEL (pselect6_time64, nfds, readfds, writefds,
> +			      exceptfds, pts64, NULL);
> +      if ((r >= 0 || errno != ENOSYS) && timeout != NULL)
>  	{
> -	  if (timeout != NULL)
> -	    TIMESPEC_TO_TIMEVAL (timeout, &ts64);
> -	  return r;
> +	  TIMESPEC_TO_TIMEVAL (timeout, &ts64);
>  	}
> -
> -      mark_time64_unsupported ();
> +      else
> +	__set_errno (EOVERFLOW);
> +      return r;
>      }
>  
> -#ifndef __ASSUME_TIME64_SYSCALLS
> -  struct timespec ts32, *pts32 = NULL;
> -  if (pts64 != NULL)
> -    {
> -      if (! in_time_t_range (pts64->tv_sec))
> -	{
> -	  __set_errno (EINVAL);
> -	  return -1;
> -	}
> -      ts32.tv_sec = s;
> -      ts32.tv_nsec = ns;
> -      pts32 = &ts32;
> -    }
> -# ifndef __ASSUME_PSELECT
> -#  ifdef __NR__newselect
> -#   undef __NR_select
> -#   define __NR_select __NR__newselect
> -#  endif
> -  r = SYSCALL_CANCEL (select, nfds, readfds, writefds, exceptfds,
> pts32); -# else
> -  r = SYSCALL_CANCEL (pselect6, nfds, readfds, writefds, exceptfds,
> pts32,
> -		      NULL);
> -# endif
> -  if (timeout != NULL)
> -    *timeout = valid_timespec_to_timeval64 (ts32);
> +  return __select32 (nfds, readfds, writefds, exceptfds, pts64,
> timeout); #endif
> -
> -  return r;
>  }
>  
>  #if __TIMESIZE != 64
> diff --git a/sysdeps/unix/sysv/linux/select32.c
> b/sysdeps/unix/sysv/linux/select32.c new file mode 100644
> index 0000000000..b7e122fe2c
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/select32.c
> @@ -0,0 +1,58 @@
> +/* Synchronous I/O multiplexing.  Linux 32-bit time fallback.
> +   Copyright (C) 2020-2021 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 <sys/select.h>
> +#include <sysdep-cancel.h>
> +
> +#ifndef __ASSUME_TIME64_SYSCALLS
> +
> +int
> +__select32 (int nfds, fd_set *readfds, fd_set *writefds,
> +	    fd_set *exceptfds, const struct __timespec64 *ts64,
> +	    struct __timeval64 *timeout)
> +{
> +#ifdef __ASSUME_PSELECT
> +  struct timespec ts32, *pts32 = NULL;
> +  if (ts64 != NULL)
> +    {
> +      ts32.tv_sec = ts64->tv_sec;
> +      ts32.tv_nsec = ts64->tv_nsec;
> +      pts32 = &ts32;
> +    }
> +
> +  int r = SYSCALL_CANCEL (pselect6, nfds, readfds, writefds,
> exceptfds, pts32,
> +			  NULL);
> +  if (timeout != NULL)
> +    TIMESPEC_TO_TIMEVAL (timeout, pts32);
> +  return r;
> +#else
> +  struct timeval tv32, *ptv32 = NULL;
> +  if (ts64 != NULL)
> +    {
> +      tv32 = valid_timespec64_to_timeval (*ts64);
> +      ptv32 = &tv32;
> +    }
> +
> +  int r = SYSCALL_CANCEL (select, nfds, readfds, writefds,
> exceptfds, ptv32);
> +  if (timeout != NULL)
> +    *timeout = valid_timeval_to_timeval64 (tv32);
> +  return r;
> +#endif /* __ASSUME_PSELECT  */
> +}
> +
> +#endif

Reviewed-by: Lukasz Majewski <lukma@denx.de>


Best regards,

Lukasz Majewski

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