win32: better handling of nested symlinks

Our realpath(3) implementation uses xmalloc_follow_symlinks() to
expand symlinks.  This detects when symlinks are too deeply nested
but didn't set errno, so anything calling realpath(3) was unable to
say what had gone wrong.  (For example, 'ls -L' or 'stat -L'.)
Set errno to ELOOP.

This then leads to the problem that Windows doesn't know about
ELOOP so reports 'Unknown error'.  Add a replacement for strerror(3)
which returns a sensible message.

Costs 96 bytes.

3 files changed, 14 insertions, 0 deletions

diff --git a/include/mingw.h b/include/mingw.h
index c4849215b..03ef89029 100644
--- a/include/mingw.h
+++ b/include/mingw.h
@@ -209,6 +209,9 @@ int unsetenv(const char *env);
  * string.h
  */
 char *strndup(char const *s, size_t n);
+char *mingw_strerror(int errnum);
+
+#define strerror mingw_strerror
 
 /*
  * strings.h
diff --git a/libbb/xreadlink.c b/libbb/xreadlink.c
index f0a63fd9b..b64654b33 100644
--- a/libbb/xreadlink.c
+++ b/libbb/xreadlink.c
@@ -70,6 +70,9 @@ char* FAST_FUNC xmalloc_follow_symlinks(const char *path)
                 }
 
                 if (!--looping) {
+#if ENABLE_PLATFORM_MINGW32
+                        errno = ELOOP;
+#endif
                         free(linkpath);
  free_buf_ret_null:
                         free(buf);
diff --git a/win32/mingw.c b/win32/mingw.c
index 49dcd89d8..c592b8d2c 100644
--- a/win32/mingw.c
+++ b/win32/mingw.c
@@ -161,6 +161,14 @@ int err_win_to_posix(void)
         return error;
 }
 
+#undef strerror
+char *mingw_strerror(int errnum)
+{
+        if (errnum == ELOOP)
+                return (char *)"Too many levels of symbolic links";
+        return strerror(errnum);
+}
+
 static int zero_fd = -1;
 static int rand_fd = -1;