Re: [PATCH v7 2/7] virtiofsd: Fix xattr operations overwriting errno

[Dr. David Alan Gilbert]

* Vivek Goyal (vgoyal@redhat.com) wrote:
> getxattr/setxattr/removexattr/listxattr operations handle regualar
> and non-regular files differently. For the case of non-regular files
> we do fchdir(/proc/self/fd) and the xattr operation and then revert
> back to original working directory. After this we are saving errno
> and that's buggy because fchdir() will overwrite the errno.
> 
> FCHDIR_NOFAIL(lo->proc_self_fd);
> ret = getxattr(procname, name, value, size);
> FCHDIR_NOFAIL(lo->root.fd);
> 
> if (ret == -1)
>     saverr = errno
> 
> In above example, if getxattr() failed, we will still return 0 to caller
> as errno must have been written by FCHDIR_NOFAIL(lo->root.fd) call.
> Fix all such instances and capture "errno" early and save in "saverr"
> variable.
> 
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

I think the existing code is actually safe; I don't think fchdir
will/should set errno unless there's an error, and we're explictly
asserting there isn't one.

However, I do prefer doing this save since we already have the save
variables and it makes the chance of screwing it up from any other
forgotten syscall less likely, so


Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  tools/virtiofsd/passthrough_ll.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/tools/virtiofsd/passthrough_ll.c 
> b/tools/virtiofsd/passthrough_ll.c
> index 49c21fd855..ec91b3c133 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c
> @@ -2791,15 +2791,17 @@ static void lo_getxattr(fuse_req_t req, fuse_ino_t 
> ino, const char *in_name,
>              goto out_err;
>          }
>          ret = fgetxattr(fd, name, value, size);
> +        saverr = ret == -1 ? errno : 0;
>      } else {
>          /* fchdir should not fail here */
>          FCHDIR_NOFAIL(lo->proc_self_fd);
>          ret = getxattr(procname, name, value, size);
> +        saverr = ret == -1 ? errno : 0;
>          FCHDIR_NOFAIL(lo->root.fd);
>      }
>  
>      if (ret == -1) {
> -        goto out_err;
> +        goto out;
>      }
>      if (size) {
>          saverr = 0;
> @@ -2864,15 +2866,17 @@ static void lo_listxattr(fuse_req_t req, fuse_ino_t 
> ino, size_t size)
>              goto out_err;
>          }
>          ret = flistxattr(fd, value, size);
> +        saverr = ret == -1 ? errno : 0;
>      } else {
>          /* fchdir should not fail here */
>          FCHDIR_NOFAIL(lo->proc_self_fd);
>          ret = listxattr(procname, value, size);
> +        saverr = ret == -1 ? errno : 0;
>          FCHDIR_NOFAIL(lo->root.fd);
>      }
>  
>      if (ret == -1) {
> -        goto out_err;
> +        goto out;
>      }
>      if (size) {
>          saverr = 0;
> @@ -2998,15 +3002,15 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t 
> ino, const char *in_name,
>              goto out;
>          }
>          ret = fsetxattr(fd, name, value, size, flags);
> +        saverr = ret == -1 ? errno : 0;
>      } else {
>          /* fchdir should not fail here */
>          FCHDIR_NOFAIL(lo->proc_self_fd);
>          ret = setxattr(procname, name, value, size, flags);
> +        saverr = ret == -1 ? errno : 0;
>          FCHDIR_NOFAIL(lo->root.fd);
>      }
>  
> -    saverr = ret == -1 ? errno : 0;
> -
>  out:
>      if (fd >= 0) {
>          close(fd);
> @@ -3064,15 +3068,15 @@ static void lo_removexattr(fuse_req_t req, fuse_ino_t 
> ino, const char *in_name)
>              goto out;
>          }
>          ret = fremovexattr(fd, name);
> +        saverr = ret == -1 ? errno : 0;
>      } else {
>          /* fchdir should not fail here */
>          FCHDIR_NOFAIL(lo->proc_self_fd);
>          ret = removexattr(procname, name);
> +        saverr = ret == -1 ? errno : 0;
>          FCHDIR_NOFAIL(lo->root.fd);
>      }
>  
> -    saverr = ret == -1 ? errno : 0;
> -
>  out:
>      if (fd >= 0) {
>          close(fd);
> -- 
> 2.25.4
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK