libsbutil: add sb_exists function

libsandbox/pre_check_openat.c

@@ -19,7 +19,7 @@ bool sb_openat_pre_check(const char *func, const char *pathname, int dirfd, int
 	save_errno();
 
 	/* Doesn't exist -> skip permission checks */
-	if (faccessat(dirfd, pathname, F_OK, (flags & O_NOFOLLOW) ? AT_SYMLINK_NOFOLLOW : 0) == -1) {
+	if (sb_exists(dirfd, pathname, (flags & O_NOFOLLOW) ? AT_SYMLINK_NOFOLLOW : 0) == -1) {
 		sb_debug_dyn("EARLY FAIL: %s(%s): %s\n", func, pathname, strerror(errno));
 		return false;
 	}

libsandbox/wrapper-funcs/fopen_pre_check.c

@@ -11,7 +11,7 @@ bool sb_fopen_pre_check(const char *func, const char *pathname, const char *mode
 		save_errno();
 
 		/* If we're trying to read, fail normally if file does not stat */
-		if (faccessat(AT_FDCWD, pathname, F_OK, 0) == -1) {
+		if (sb_exists(AT_FDCWD, pathname, 0) == -1) {
 			sb_debug_dyn("EARLY FAIL: %s(%s): %s\n",
 				func, pathname, strerror(errno));
 			return false;

libsbutil/local.mk

@@ -16,6 +16,7 @@ noinst_LTLIBRARIES += %D%/libsbutil.la
 	%D%/environment.c                         \
 	%D%/sb_backtrace.c                        \
 	%D%/sb_efuncs.c                           \
+	%D%/sb_exists.c                           \
 	%D%/sb_gdb.c                              \
 	%D%/sb_method.c                           \
 	%D%/sb_open.c                             \

libsbutil/sb_exists.c

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2023 Gentoo Authors
+ * Distributed under the terms of the GNU General Public License v2
+ */
+
+#include "headers.h"
+#include "sbutil.h"
+
+/* Wrapper for faccessat to work around buggy behavior on musl */
+int sb_exists(int dirfd, const char *pathname, int flags)
+{
+	struct stat64 buf;
+
+	if (faccessat(dirfd, pathname, F_OK, flags) == 0)
+		return 0;
+
+	/* musl's faccessat gives EINVAL when the kernel does not support
+	 * faccessat2 and AT_SYMLINK_NOFOLLOW is set.
+	 * https://www.openwall.com/lists/musl/2023/06/19/1 */
+	if (errno != EINVAL)
+		return -1;
+
+	return fstatat64(dirfd, pathname, &buf, flags);
+}

libsbutil/sbutil.h

@@ -111,6 +111,7 @@ size_t sb_write(int fd, const void *buf, size_t count);
 int sb_close(int fd);
 void sb_close_all_fds(void);
 int sb_copy_file_to_fd(const char *file, int ofd);
+int sb_exists(int dirfd, const char *pathname, int flags);
 
 /* Reliable output */
 __printf(1, 2) void sb_printf(const char *format, ...);

libsbutil/src/file.c

@@ -15,7 +15,7 @@
 bool
 rc_file_exists (const char *pathname)
 {
-  return faccessat(AT_FDCWD, pathname, F_OK, AT_SYMLINK_NOFOLLOW) == 0;
+  return sb_exists(AT_FDCWD, pathname, AT_SYMLINK_NOFOLLOW) == 0;
 }
 
 bool