From 365be71dfcc394fa5a3749834174c02f76afa7d1 Mon Sep 17 00:00:00 2001
From: Jan Polensky <japo@linux.ibm.com>
Date: Mon, 29 Sep 2025 17:17:26 +0200
Subject: [PATCH] util_libc: Add util_readlink() and util_readlinkat() helpers
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Introduce util_readlinkat() to read symbolic links relative to a
directory file descriptor, and util_readlink() as a convenience wrapper
using AT_FDCWD.

util_readlink() delegates to util_readlinkat() instead of duplicating
logic, ensuring a single implementation for both interfaces.

Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Jan Polensky <japo@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>
---
 include/lib/util_libc.h | 24 ++++++++++++++++++++++++
 libutil/util_libc.c     | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/include/lib/util_libc.h b/include/lib/util_libc.h
index 8aaf1199..6c4a080b 100644
--- a/include/lib/util_libc.h
+++ b/include/lib/util_libc.h
@@ -12,6 +12,7 @@
 #ifndef LIB_UTIL_LIBC_H
 #define LIB_UTIL_LIBC_H
 
+#include <fcntl.h>
 #include <stdio.h>
 
 #ifdef __cplusplus
@@ -124,6 +125,29 @@ do {							\
 	va_end(ap);					\
 } while (0)
 
+/**
+ * Reads the target of a symbolic link at the given path.
+ *
+ * @param[in] path   Path to the symbolic link
+ * @return           Newly allocated string with the link target, or NULL on error
+ */
+#define util_readlink(path) __util_readlinkat(__func__, __FILE__, __LINE__, AT_FDCWD, path)
+
+/**
+ * Reads the target of a symbolic link relative to a directory file descriptor.
+ *
+ * Semantics:
+ * - If path is absolute, dirfd is ignored, per readlinkat semantics.
+ * - If path is relative, it is resolved relative to dirfd.
+ *
+ * @param[in] dirfd  Directory file descriptor or AT_FDCWD
+ * @param[in] path   Path to the symbolic link
+ * @return           Newly allocated string with the link target, or NULL on error
+ */
+#define util_readlinkat(dirfd, path) __util_readlinkat(__func__, __FILE__, __LINE__, dirfd, path)
+
+char *__util_readlinkat(const char *func, const char *file, int line, int dirfd, const char *path);
+
 int __util_vsprintf(const char *func, const char *file, int line,
 		    char *str, const char *fmt, va_list ap);
 char *util_strcat_realloc(char *str1, const char *str2);
diff --git a/libutil/util_libc.c b/libutil/util_libc.c
index d038a7dc..ca8965ff 100644
--- a/libutil/util_libc.c
+++ b/libutil/util_libc.c
@@ -10,11 +10,15 @@
  */
 
 #include <ctype.h>
+#include <err.h>
 #include <errno.h>
+#include <limits.h>
 #include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/stat.h>
+#include <unistd.h>
 
 #include "lib/util_base.h"
 #include "lib/util_libc.h"
@@ -295,3 +299,31 @@ size_t util_strlcpy(char *dest, const char *src, size_t size)
 
 	return str_len;
 }
+
+char *__util_readlinkat(const char *func, const char *file, int line, int dirfd, const char *path)
+{
+	ssize_t link_len = PATH_MAX;
+	struct stat st;
+	char *linkdir;
+	ssize_t len;
+
+	if (fstatat(dirfd, path, &st, AT_SYMLINK_NOFOLLOW) == 0 && st.st_size > 0)
+		link_len = st.st_size + 1;
+
+	linkdir = __util_malloc(func, file, line, link_len);
+
+	len = readlinkat(dirfd, path, linkdir, link_len);
+	if (len == -1) {
+		free(linkdir);
+		return NULL;
+	}
+
+	if (len >= link_len) {
+		warnx("%s: Link target too long", path);
+		free(linkdir);
+		return NULL;
+	}
+
+	linkdir[len] = '\0';
+	return __util_realloc(func, file, line, linkdir, (size_t)len + 1);
+}