Re: [PATCH v4] arc4random: simplify design for better safety

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

On 26/07/22 10:30, Jason A. Donenfeld wrote:

> +      l = __getrandom_nocancel (p, n, 0);
> +      if (l > 0)
> +	{
> +	  if ((size_t) l == n)
> +	    return; /* Done reading, success.  */
> +	  p = (uint8_t *) p + l;
> +	  n -= l;
> +	  continue; /* Interrupted by a signal; keep going.  */
> +	}
> +      else if (l == 0)
> +	arc4random_getrandom_failure (); /* Weird, should never happen.  */
> +      else if (l == -EINTR)
> +	continue; /* Interrupted by a signal; keep going.  */
> +      else if (!__ASSUME_GETRANDOM && l == -ENOSYS)
> +	{
> +	  atomic_store_relaxed (&have_getrandom, false);

I still think there is no much gain in this optimization, the syscall will
most likely be present and it is one less static data.  Also, we avoid to
use __ASSUME_GETRANDOM on generic code (all __ASSUME usage within
sysdeps and/or nptl).

> diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
> index 2ccc92b6b8..2f4f9784ee 100644
> --- a/sysdeps/unix/sysv/linux/Makefile
> +++ b/sysdeps/unix/sysv/linux/Makefile
> @@ -380,7 +380,8 @@ sysdep_routines += xstatconv internal_statvfs \
>  		   open_nocancel open64_nocancel \
>  		   openat_nocancel openat64_nocancel \
>  		   read_nocancel pread64_nocancel \
> -		   write_nocancel statx_cp stat_t64_cp
> +		   write_nocancel statx_cp stat_t64_cp \
> +		   ppoll_nocancel
>  
>  sysdep_headers += bits/fcntl-linux.h
>  
> diff --git a/sysdeps/unix/sysv/linux/Versions b/sysdeps/unix/sysv/linux/Versions
> index 65d2ceda2c..febe1ad421 100644
> --- a/sysdeps/unix/sysv/linux/Versions
> +++ b/sysdeps/unix/sysv/linux/Versions
> @@ -320,6 +320,7 @@ libc {
>      __read_nocancel;
>      __pread64_nocancel;
>      __close_nocancel;
> +    __ppoll_infinity_nocancel;
>      __sigtimedwait;
>      # functions used by nscd
>      __netlink_assert_response;

There is no need to export on GLIBC_PRIVATE, since it is not currently usage
libc.so.  Just define is a hidden (attribute_hidden).

> diff --git a/sysdeps/unix/sysv/linux/kernel-features.h b/sysdeps/unix/sysv/linux/kernel-features.h
> index 74adc3956b..75d5f953d4 100644
> --- a/sysdeps/unix/sysv/linux/kernel-features.h
> +++ b/sysdeps/unix/sysv/linux/kernel-features.h
> @@ -236,4 +236,11 @@
>  # define __ASSUME_FUTEX_LOCK_PI2 0
>  #endif
>  
> +/* The getrandom() syscall was added in 3.17.  */
> +#if __LINUX_KERNEL_VERSION >= 0x031100
> +# define __ASSUME_GETRANDOM 1
> +#else
> +# define __ASSUME_GETRANDOM 0
> +#endif
> +
>  #endif /* kernel-features.h */
> diff --git a/sysdeps/unix/sysv/linux/not-cancel.h b/sysdeps/unix/sysv/linux/not-cancel.h
> index 2c58d5ae2f..d3df8fa79e 100644
> --- a/sysdeps/unix/sysv/linux/not-cancel.h
> +++ b/sysdeps/unix/sysv/linux/not-cancel.h
> @@ -23,6 +23,7 @@
>  #include <sysdep.h>
>  #include <errno.h>
>  #include <unistd.h>
> +#include <sys/poll.h>
>  #include <sys/syscall.h>
>  #include <sys/wait.h>
>  #include <time.h>
> @@ -77,6 +78,10 @@ __getrandom_nocancel (void *buf, size_t buflen, unsigned int flags)
>  /* Uncancelable fcntl.  */
>  __typeof (__fcntl) __fcntl64_nocancel;
>  
> +/* Uncancelable ppoll.  */
> +int
> +__ppoll_infinity_nocancel (struct pollfd *fds, nfds_t nfds);

Use attribute_hidden here and remove it from sysdeps/unix/sysv/linux/Versions.

> +
>  #if IS_IN (libc) || IS_IN (rtld)
>  hidden_proto (__open_nocancel)
>  hidden_proto (__open64_nocancel)
> @@ -87,6 +92,7 @@ hidden_proto (__pread64_nocancel)
>  hidden_proto (__write_nocancel)
>  hidden_proto (__close_nocancel)
>  hidden_proto (__fcntl64_nocancel)
> +hidden_proto (__ppoll_infinity_nocancel)
>  #endif
>  
>  #endif /* NOT_CANCEL_H  */

Also update the hurd sysdeps/mach/hurd/not-cancel.h with a wrapper to 
__poll (since it does not really support pthread cancellation).


> diff --git a/sysdeps/generic/chacha20_arch.h b/sysdeps/unix/sysv/linux/ppoll_nocancel.c
> similarity index 62%
> rename from sysdeps/generic/chacha20_arch.h
> rename to sysdeps/unix/sysv/linux/ppoll_nocancel.c
> index 1b4559ccbc..28c8761566 100644
> --- a/sysdeps/generic/chacha20_arch.h
> +++ b/sysdeps/unix/sysv/linux/ppoll_nocancel.c
> @@ -1,5 +1,5 @@
> -/* Chacha20 implementation, generic interface for encrypt.
> -   Copyright (C) 2022 Free Software Foundation, Inc.
> +/* Linux ppoll syscall implementation -- non-cancellable.
> +   Copyright (C) 2018-2022 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
> @@ -16,9 +16,16 @@
>     License along with the GNU C Library; if not, see
>     <https://www.gnu.org/licenses/>.  */
>  
> -static inline void
> -chacha20_crypt (uint32_t *state, uint8_t *dst, const uint8_t *src,
> -		size_t bytes)
> +#include <unistd.h>
> +#include <sysdep-cancel.h>
> +#include <not-cancel.h>
> +
> +int
> +__ppoll_infinity_nocancel (struct pollfd *fds, nfds_t nfds)
>  {
> -  chacha20_crypt_generic (state, dst, src, bytes);
> +#ifndef __NR_ppoll_time64
> +# define __NR_ppoll_time64 __NR_ppoll
> +#endif
> +  return INLINE_SYSCALL_CALL (ppoll_time64, fds, nfds, NULL, NULL, 0);
>  }
> +hidden_def (__ppoll_infinity_nocancel)

Maybe just add an inline wrapper on sysdeps/unix/sysv/linux/not-cancel.h, 
as for __getrandom_nocancel:

  static inline int
  __ppoll_infinity_nocancel (struct pollfd *fds, nfds_t nfds)
  {
  #ifndef __NR_ppoll_time64
  # define __NR_ppoll_time64 __NR_ppoll
  #endif
    return INLINE_SYSCALL_CALL (ppoll_time64, fds, nfds, NULL, NULL, 0);
  }

It avoids a lot of boilerplate code to add the internal symbol.