[PATCH] hw/9pfs: Follow native symlinks when security-model=mapped

[Andrey Erokhin <language.lawyer@gmail.com>]

A directory mounted via virtfs with security-model=mapped[-xattr|-file] can contain "native" symlinks

This can happen e.g. when booting from a rootfs directory tree (usually with writable overlay set up on the host side)

Currently, with security-model=mapped, QEMU expects that all host "symlinks" are in "mapped" format, i.e. are files containing the linked path, so it tries to open with O_NOFOLLOW and fails with ELOOP in case of a native symlink

This patch gives such cases a second chance: trying to open as a native symlink, by reusing security-model=[none|passthrough] else if branch

QEMU issues:
https://gitlab.com/qemu-project/qemu/-/issues/173 (from https://bugs.launchpad.net/qemu/+bug/1831354)
https://gitlab.com/qemu-project/qemu/-/issues/3088 (dup of the first one)


Signed-off-by: Andrey Erokhin <language.lawyer@gmail.com>


diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
index 31e216227c..b4f8be2c81 100644
--- a/hw/9pfs/9p-local.c
+++ b/hw/9pfs/9p-local.c
@@ -468,12 +468,14 @@ static ssize_t local_readlink(FsContext *fs_ctx, V9fsPath *fs_path,
  
          fd = local_open_nofollow(fs_ctx, fs_path->data, O_RDONLY, 0);
          if (fd == -1) {
+            if (errno == ELOOP) goto native_symlink;
              return -1;
          }
          tsize = RETRY_ON_EINTR(read(fd, (void *)buf, bufsz));
          close_preserve_errno(fd);
      } else if ((fs_ctx->export_flags & V9FS_SM_PASSTHROUGH) ||
                 (fs_ctx->export_flags & V9FS_SM_NONE)) {
+native_symlink:;
          char *dirpath = g_path_get_dirname(fs_path->data);
          char *name = g_path_get_basename(fs_path->data);
          int dirfd;