[klibc] Implement realpath()

This is needed as the basis for the readlink -f option.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

3 files changed, 52 insertions, 1 deletions

diff --git a/usr/include/stdlib.h b/usr/include/stdlib.h
index 406f44600f28dd..856c64721976e0 100644
--- a/usr/include/stdlib.h
+++ b/usr/include/stdlib.h
@@ -92,4 +92,6 @@ static __inline__ int grantpt(int __fd)
 	return 0;		/* devpts does this all for us! */
 }
 
+__extern char *realpath(const char *, char *);
+
 #endif				/* _STDLIB_H */
diff --git a/usr/klibc/Kbuild b/usr/klibc/Kbuild
index d3e2b9fbc99b15..7d95e8755286c1 100644
--- a/usr/klibc/Kbuild
+++ b/usr/klibc/Kbuild
@@ -61,7 +61,7 @@ klib-y += vsnprintf.o snprintf.o vsprintf.o sprintf.o \
 	  send.o recv.o \
 	  access.o chmod.o chown.o dup2.o mknod.o poll.o rename.o stat.o \
 	  lchown.o link.o rmdir.o unlink.o utimes.o lstat.o mkdir.o \
-	  readlink.o select.o symlink.o pipe.o \
+	  readlink.o realpath.o select.o symlink.o pipe.o \
 	  ctype/isalnum.o ctype/isalpha.o ctype/isascii.o \
 	  ctype/isblank.o ctype/iscntrl.o ctype/isdigit.o \
 	  ctype/isgraph.o ctype/islower.o ctype/isprint.o \
diff --git a/usr/klibc/realpath.c b/usr/klibc/realpath.c
new file mode 100644
index 00000000000000..1474b1e501c159
--- /dev/null
+++ b/usr/klibc/realpath.c
@@ -0,0 +1,49 @@
+#include <fcntl.h>
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+/*
+ * Note that this requires name to refer to an existing file.  This is
+ * correct according to POSIX.  However, BSD and GNU implementations
+ * also allow name to refer to a non-existing file in an existing
+ * directory.
+ */
+
+char *realpath(const char *name, char *resolved_name)
+{
+	static const char proc_fd_prefix[] = "/proc/self/fd/";
+	char proc_fd_name[sizeof(proc_fd_prefix) + sizeof(int) * 3];
+	int allocated = 0;
+	int fd;
+	ssize_t len;
+
+	/* Open for path lookup only */
+	fd = open(name, O_PATH);
+	if (fd < 0)
+		return NULL;
+
+	if (!resolved_name) {
+		resolved_name = malloc(PATH_MAX);
+		if (!resolved_name)
+			goto out_close;
+		allocated = 1;
+	}
+
+	/* Use procfs to read back the resolved name */
+	sprintf(proc_fd_name, "%s%d", proc_fd_prefix, fd);
+	len = readlink(proc_fd_name, resolved_name, PATH_MAX - 1);
+	if (len < 0) {
+		if (allocated)
+			free(resolved_name);
+		resolved_name = NULL;
+	} else {
+		resolved_name[len] = 0;
+	}
+
+out_close:
+	close(fd);
+	return resolved_name;
+}