string_utils: provide a version of strchrnul() in case it's not available

configure.ac

@@ -640,7 +640,7 @@ AC_CHECK_HEADER([ifaddrs.h],
 AC_HEADER_MAJOR
 
 # Check for some syscalls functions
-AC_CHECK_FUNCS([setns pivot_root sethostname unshare rand_r confstr faccessat gettid memfd_create move_mount open_tree execveat clone3 fsopen fspick fsconfig fsmount, openat2, close_range, statvfs])
+AC_CHECK_FUNCS([setns pivot_root sethostname unshare rand_r confstr faccessat gettid memfd_create move_mount open_tree execveat clone3 fsopen fspick fsconfig fsmount, openat2, close_range, statvfs, strchrnul])
 AC_CHECK_TYPES([__aligned_u64], [], [], [[#include <linux/types.h>]])
 AC_CHECK_TYPES([struct open_how], [], [], [[#include <linux/openat2.h>]])
 AC_CHECK_TYPES([struct clone_args], [], [], [[#include <linux/sched.h>]])

src/include/stchrnul.c

+/* Copyright (C) 1991-2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
+   with help from Dan Sahlin (dan@sics.se) and
+   bug fix and commentary by Jim Blandy (jimb@ai.mit.edu);
+   adaptation to strchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
+   and implemented by Roland McGrath (roland@ai.mit.edu).
+
+   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 <string.h>
+#include <stdlib.h>
+
+/* Find the first occurrence of C in S or the final NUL byte.  */
+char *strchrnul(const char *s, int c_in)
+{
+	const unsigned char *char_ptr;
+	const unsigned long int *longword_ptr;
+	unsigned long int longword, magic_bits, charmask;
+	unsigned char c;
+
+	c = (unsigned char)c_in;
+
+	/* Handle the first few characters by reading one character at a time.
+	   Do this until CHAR_PTR is aligned on a longword boundary.  */
+	for (char_ptr = (const unsigned char *)s;
+	     ((unsigned long int)char_ptr & (sizeof(longword) - 1)) != 0;
+	     ++char_ptr)
+		if (*char_ptr == c || *char_ptr == '\0')
+			return (void *)char_ptr;
+
+	/* All these elucidatory comments refer to 4-byte longwords,
+	   but the theory applies equally well to 8-byte longwords.  */
+
+	longword_ptr = (unsigned long int *)char_ptr;
+
+	/* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
+	   the "holes."  Note that there is a hole just to the left of
+	   each byte, with an extra at the end:
+
+	   bits:  01111110 11111110 11111110 11111111
+	   bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
+
+	   The 1-bits make sure that carries propagate to the next 0-bit.
+	   The 0-bits provide holes for carries to fall into.  */
+	magic_bits = -1;
+	magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
+
+	/* Set up a longword, each of whose bytes is C.  */
+	charmask = c | (c << 8);
+	charmask |= charmask << 16;
+	if (sizeof(longword) > 4)
+		/* Do the shift in two steps to avoid a warning if long has 32 bits.  */
+		charmask |= (charmask << 16) << 16;
+	if (sizeof(longword) > 8)
+		abort();
+
+	/* Instead of the traditional loop which tests each character,
+	   we will test a longword at a time.  The tricky part is testing
+	   if *any of the four* bytes in the longword in question are zero.  */
+	for (;;) {
+		/* We tentatively exit the loop if adding MAGIC_BITS to
+		   LONGWORD fails to change any of the hole bits of LONGWORD.
+
+		   1) Is this safe?  Will it catch all the zero bytes?
+		   Suppose there is a byte with all zeros.  Any carry bits
+		   propagating from its left will fall into the hole at its
+		   least significant bit and stop.  Since there will be no
+		   carry from its most significant bit, the LSB of the
+		   byte to the left will be unchanged, and the zero will be
+		   detected.
+
+		   2) Is this worthwhile?  Will it ignore everything except
+		   zero bytes?  Suppose every byte of LONGWORD has a bit set
+		   somewhere.  There will be a carry into bit 8.  If bit 8
+		   is set, this will carry into bit 16.  If bit 8 is clear,
+		   one of bits 9-15 must be set, so there will be a carry
+		   into bit 16.  Similarly, there will be a carry into bit
+		   24.  If one of bits 24-30 is set, there will be a carry
+		   into bit 31, so all of the hole bits will be changed.
+
+		   The one misfire occurs when bits 24-30 are clear and bit
+		   31 is set; in this case, the hole at bit 31 is not
+		   changed.  If we had access to the processor carry flag,
+		   we could close this loophole by putting the fourth hole
+		   at bit 32!
+
+		   So it ignores everything except 128's, when they're aligned
+		   properly.
+
+		   3) But wait!  Aren't we looking for C as well as zero?
+		   Good point.  So what we do is XOR LONGWORD with a longword,
+		   each of whose bytes is C.  This turns each byte that is C
+		   into a zero.  */
+
+		longword = *longword_ptr++;
+
+		/* Add MAGIC_BITS to LONGWORD.  */
+		if ((((longword + magic_bits)
+
+		      /* Set those bits that were unchanged by the addition.  */
+		      ^ ~longword)
+
+		     /* Look at only the hole bits.  If any of the hole bits
+			are unchanged, most likely one of the bytes was a
+			zero.  */
+		     & ~magic_bits) != 0
+
+		    /* That caught zeroes.  Now test for C.  */
+		    || ((((longword ^ charmask) + magic_bits) ^
+			 ~(longword ^ charmask)) &
+			~magic_bits) != 0) {
+			/* Which of the bytes was C or zero?
+			   If none of them were, it was a misfire; continue the search.  */
+
+			const unsigned char *cp =
+			    (const unsigned char *)(longword_ptr - 1);
+
+			if (*cp == c || *cp == '\0')
+				return (char *)cp;
+			if (*++cp == c || *cp == '\0')
+				return (char *)cp;
+			if (*++cp == c || *cp == '\0')
+				return (char *)cp;
+			if (*++cp == c || *cp == '\0')
+				return (char *)cp;
+			if (sizeof(longword) > 4) {
+				if (*++cp == c || *cp == '\0')
+					return (char *)cp;
+				if (*++cp == c || *cp == '\0')
+					return (char *)cp;
+				if (*++cp == c || *cp == '\0')
+					return (char *)cp;
+				if (*++cp == c || *cp == '\0')
+					return (char *)cp;
+			}
+		}
+	}
+
+	/* This should never happen.  */
+	return NULL;
+}

src/include/stchrnul.h

+/* Copyright (C) 1991-2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
+   with help from Dan Sahlin (dan@sics.se) and
+   bug fix and commentary by Jim Blandy (jimb@ai.mit.edu);
+   adaptation to strchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
+   and implemented by Roland McGrath (roland@ai.mit.edu).
+
+   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 "compiler.h"
+
+__hidden extern char *strchrnul(const char *s, int c_in);

src/lxc/string_utils.h

@@ -14,6 +14,10 @@
 #include "include/strlcat.h"
 #endif
 
+#ifndef HAVE_STRCHRNUL
+#include "include/strchrnul.h"
+#endif
+
 /* convert variadic argument lists to arrays (for execl type argument lists) */
 __hidden extern char **lxc_va_arg_list_to_argv(va_list ap, size_t skip, int do_strdup);
 __hidden extern const char **lxc_va_arg_list_to_argv_const(va_list ap, size_t skip);