faccessat2 workaround for old systemd

The faccessat2 system call is denied by old systemd container, this is a workaround to make it works again and without re-build from source code.

faccessat function is a known way to use the faccessat2 system call in glibc, editing libc.so binary to fallback to the user-space implementation.

00000000000de3e0 <faccessat>:
   de448:	7100989f 	cmp	w4, #0x26
   de44c:	540005e1 	b.ne	de508 <faccessat+0x128>

   de448:	7100989f 	cmp	w4, #0x26
   de44c:	d503201f 	nop

Equivalent to the C source code:

__faccessat (int fd, const char *file, int mode, int flag)
  int ret = INLINE_SYSCALL_CALL (faccessat2, fd, file, mode, flag);
  return ret;
  if (ret == 0 /* || errno != ENOSYS */)
    return ret;


  if ((flag == 0 || ((flag & ~AT_EACCESS) == 0 && ! __libc_enable_secure)))
    return INLINE_SYSCALL (faccessat, 3, fd, file, mode);

  struct stat64 stats;
  if (__fstatat64 (fd, file, &stats, flag & AT_SYMLINK_NOFOLLOW))
    return -1;

  mode &= (X_OK | W_OK | R_OK);	/* Clear any bogus bits. */
# if R_OK != S_IROTH || W_OK != S_IWOTH || X_OK != S_IXOTH
#  error Oops, portability assumptions incorrect.
# endif

  if (mode == F_OK)
    return 0;			/* The file exists. */

  uid_t uid = (flag & AT_EACCESS) ? __geteuid () : __getuid ();

  /* The super-user can read and write any file, and execute any file
     that anyone can execute. */
  if (uid == 0 && ((mode & X_OK) == 0
		   || (stats.st_mode & (S_IXUSR | S_IXGRP | S_IXOTH))))
    return 0;

  int granted = (uid == stats.st_uid
		 ? (unsigned int) (stats.st_mode & (mode << 6)) >> 6
		 : (stats.st_gid == ((flag & AT_EACCESS)
				     ? __getegid () : __getgid ())
		    || __group_member (stats.st_gid))
		 ? (unsigned int) (stats.st_mode & (mode << 3)) >> 3
		 : (stats.st_mode & mode));

  if (granted == mode)
    return 0;

#endif /* !__ASSUME_FACCESSAT2 */

Linux socket bind IPv6 only

Socket options

IPv6 support some protocol-specific socket options that can be set with setsockopt and read with getsockopt. The socket option level for IPv6 is IPPROTO_IPV6. A boolean integer flag is zero with it is false, otherwise true.


If this flag is set to true (nonzero), then the socket is restricted to sending and receiving IPv6 packets only. In this case, an IPv4 and an IPv6 application can bind to a single port at the same time.

If this flag is set to false (zero), then the socket can be used to send and receive packets to and from an IPv6 address or an IPv4-mapped IPv6 address.

The argument is a pointer to a boolean value in an integer.

The default value for this flag is defined by the contents of the file /proc/sys/net/ipv6/bindv6only. The default value for that file is 0 (false).


int one = 1;
setsockopt (fd, IPPROTO_IPV6, IPV6_V6ONLY, &one, sizeof (one));

Refer to: https://man7.org/linux/man-pages/man7/ipv6.7.html

Float-point fused Multiply-Add AArch64 vs LoongArch64

AA64: fmadd  d, n, m, a: d = a + n * m
LA64: fmadd  d, j, k, a: d = a + j * k

AA64: fmsub  d, n, m, a: d = a + (-n) * m
                         d = a - n * m
                         d = - (n * m - a)
LA64: fnmsub d, j, k, a: d = - (j * k - a)
                         d = a - j * k
                         d = a + (-j) * k

AA64: fnmadd d, n, m, a: d = (-a) + (-n) * m
                         d = - (n * m + a)
LA64: fnmadd d, j, k, a: d = - (j * k + a)
                         d = (-a) + (-n * m)

AA64: fnmsub d, n, m, a: d = (-a) + n * m
                         d = n * m - a
LA64: fmsub  d, j, k, a: d = j * k - a
                         d = (-a) + j * k

IP addr-label persistent by systemd

This is an example shows how to make IP address label persistent by systemd-networkd.

/etc/systemd/network/eth0.network :





It’s successful if you can see labels that you configured :

$ ip addrl
prefix ::1/128 label 0 
prefix ::/96 label 3 
prefix ::ffff: label 4 
prefix 2001::/32 label 6 
prefix 2001:10::/28 label 7 
prefix 2606::/16 dev br0 label 100 
prefix 2409::/16 dev br0 label 100 
prefix 3ffe::/16 label 12 
prefix 2002::/16 label 2 
prefix fec0::/10 label 11 
prefix fc00::/7 label 5 
prefix ::/0 label 1

Hotspot stress diagnostic options

SafepointALot: Generate a lot of safepoints. This works with GuaranteedSafepointInterval.

HandshakeALot: Generate a lot of handshakes. This works with GuaranteedSafepointInterval.

DeoptimizeALot: Deoptimize at every exit from the runtime system.

ZombieALot: Create zombies (non-entrant) at exit from the runtime system.

WalkStackALot: Trace stack (no print) at every exit from the runtime system.

DeoptimizeObjectsALot: For testing purposes concurrent threads revert optimizations based on escape analysis at intervals given with DeoptimizeObjectsALotInterval=n. The thread count is given with DeoptimizeObjectsALotThreadCountSingle and DeoptimizeObjectsALotThreadCountAll.

DontYieldALot: Throw away obvious excess yield calls.

StringDeduplicationResizeALot: Force more frequent table resizing.

DeoptimizeNMethodBarriersALot: Make nmethod barriers deoptimise a lot.