Re: [PATCH 037/104] virtiofsd: passthrough_ll: add fallback for racy ops

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Masayoshi Mizuma (msys.mizuma@gmail.com) wrote:
> On Thu, Dec 12, 2019 at 04:37:57PM +0000, Dr. David Alan Gilbert (git) wrote:
> > From: Miklos Szeredi <mszeredi@redhat.com>
> > 
> > We have two operations that cannot be done race-free on a symlink in
> > certain cases: utimes and link.
> > 
> > Add racy fallback for these if the race-free method doesn't work.  We do
> > our best to avoid races even in this case:
> > 
> >   - get absolute path by reading /proc/self/fd/NN symlink
> > 
> >   - lookup parent directory: after this we are safe against renames in
> >     ancestors
> > 
> >   - lookup name in parent directory, and verify that we got to the original
> >     inode,  if not retry the whole thing
> > 
> > Both utimes(2) and link(2) hold i_lock on the inode across the operation,
> > so a racing rename/delete by this fuse instance is not possible, only from
> > other entities changing the filesystem.
> > 
> > If the "norace" option is given, then disable the racy fallbacks.
> > 
> > Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
> > ---
> >  tools/virtiofsd/passthrough_ll.c | 159 +++++++++++++++++++++++++++----
> >  1 file changed, 142 insertions(+), 17 deletions(-)
> > 
> > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> > index 93e74cce21..1faae2753f 100644
> > --- a/tools/virtiofsd/passthrough_ll.c
> > +++ b/tools/virtiofsd/passthrough_ll.c
> > @@ -98,6 +98,7 @@ enum {
> >  struct lo_data {
> >      pthread_mutex_t mutex;
> >      int debug;
> > +    int norace;
> >      int writeback;
> >      int flock;
> >      int xattr;
> > @@ -124,10 +125,15 @@ static const struct fuse_opt lo_opts[] = {
> >      { "cache=never", offsetof(struct lo_data, cache), CACHE_NEVER },
> >      { "cache=auto", offsetof(struct lo_data, cache), CACHE_NORMAL },
> >      { "cache=always", offsetof(struct lo_data, cache), CACHE_ALWAYS },
> > -
> > +    { "norace", offsetof(struct lo_data, norace), 1 },
> >      FUSE_OPT_END
> >  };
> >  
> > +static void unref_inode(struct lo_data *lo, struct lo_inode *inode, uint64_t n);
> > +
> > +static struct lo_inode *lo_find(struct lo_data *lo, struct stat *st);
> > +
> > +
> >  static struct lo_data *lo_data(fuse_req_t req)
> >  {
> >      return (struct lo_data *)fuse_req_userdata(req);
> > @@ -347,23 +353,127 @@ static void lo_getattr(fuse_req_t req, fuse_ino_t ino,
> >      fuse_reply_attr(req, &buf, lo->timeout);
> >  }
> >  > -static int utimensat_empty_nofollow(struct lo_inode *inode,
> > -                                    const struct timespec *tv)
> > +static int lo_parent_and_name(struct lo_data *lo, struct lo_inode *inode,
> > +                              char path[PATH_MAX], struct lo_inode **parent)
> >  {
> > -    int res;
> >      char procname[64];
> > +    char *last;
> > +    struct stat stat;
> > +    struct lo_inode *p;
> > +    int retries = 2;
> > +    int res;
> > +
> > +retry:
> > +    sprintf(procname, "/proc/self/fd/%i", inode->fd);
> > +
> > +    res = readlink(procname, path, PATH_MAX);
> > +    if (res < 0) {
> 
> > +        fuse_log(FUSE_LOG_WARNING, "lo_parent_and_name: readlink failed: %m\n");
> 
> I think it's better to use __func__ macro in case the function name is
> changed in the future.

Yes, agreed - I've changed them all

>            fuse_log(FUSE_LOG_WARNING, "%s: readlink failed: %m\n", __func__);
> 
> > +        goto fail_noretry;
> > +    }
> > +
> > +    if (res >= PATH_MAX) {
> 
> > +        fuse_log(FUSE_LOG_WARNING, "lo_parent_and_name: readlink overflowed\n");
> 
>            fuse_log(FUSE_LOG_WARNING, "%s: readlink overflowed\n", __func__);
> 
> > +        goto fail_noretry;
> > +    }
> > +    path[res] = '\0';
> > +
> > +    last = strrchr(path, '/');
> > +    if (last == NULL) {
> > +        /* Shouldn't happen */
> 
> > +        fuse_log(
> > +            FUSE_LOG_WARNING,
> > +            "lo_parent_and_name: INTERNAL ERROR: bad path read from proc\n");
> 
>            fuse_log(
>                FUSE_LOG_WARNING,
>             "%s: INTERNAL ERROR: bad path read from proc\n", __func__);
> 
> > +        goto fail_noretry;
> > +    }
> > +    if (last == path) {
> > +        p = &lo->root;
> > +        pthread_mutex_lock(&lo->mutex);
> > +        p->refcount++;
> > +        pthread_mutex_unlock(&lo->mutex);
> > +    } else {
> > +        *last = '\0';
> > +        res = fstatat(AT_FDCWD, last == path ? "/" : path, &stat, 0);
> > +        if (res == -1) {
> > +            if (!retries) {
> 
> > +                fuse_log(FUSE_LOG_WARNING,
> > +                         "lo_parent_and_name: failed to stat parent: %m\n");
> 
>                    fuse_log(FUSE_LOG_WARNING,
>                             "%s: failed to stat parent: %m\n", __func__);
> 
> > +            }
> > +            goto fail;
> > +        }
> > +        p = lo_find(lo, &stat);
> > +        if (p == NULL) {
> > +            if (!retries) {
> 
> > +                fuse_log(FUSE_LOG_WARNING,
> > +                         "lo_parent_and_name: failed to find parent\n");
> 
>                    fuse_log(FUSE_LOG_WARNING,
>                          "%s: failed to find parent\n", __func__);
> 
> > +            }
> > +            goto fail;
> > +        }
> > +    }
> > +    last++;
> > +    res = fstatat(p->fd, last, &stat, AT_SYMLINK_NOFOLLOW);
> > +    if (res == -1) {
> > +        if (!retries) {
> 
> > +            fuse_log(FUSE_LOG_WARNING,
> > +                     "lo_parent_and_name: failed to stat last\n");
> 
>                fuse_log(FUSE_LOG_WARNING,
>                      "%s: failed to stat last\n", __func__);
> 
> > +        }
> > +        goto fail_unref;
> > +    }
> > +    if (stat.st_dev != inode->dev || stat.st_ino != inode->ino) {
> > +        if (!retries) {
> 
> > +            fuse_log(FUSE_LOG_WARNING,
> > +                     "lo_parent_and_name: failed to match last\n");
> 
>                fuse_log(FUSE_LOG_WARNING,
>                         "%s: failed to match last\n", __func__);
> 
> > +        }
> > +        goto fail_unref;
> > +    }
> > +    *parent = p;
> > +    memmove(path, last, strlen(last) + 1);
> > +
> > +    return 0;
> > +
> > +fail_unref:
> > +    unref_inode(lo, p, 1);
> > +fail:
> > +    if (retries) {
> > +        retries--;
> > +        goto retry;
> > +    }
> > +fail_noretry:
> > +    errno = EIO;
> > +    return -1;
> > +}
> > +
> > +static int utimensat_empty(struct lo_data *lo, struct lo_inode *inode,
> > +                           const struct timespec *tv)
> > +{
> > +    int res;
> > +    struct lo_inode *parent;
> > +    char path[PATH_MAX];
> >  
> >      if (inode->is_symlink) {
> > -        res = utimensat(inode->fd, "", tv, AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW);
> > +        res = utimensat(inode->fd, "", tv, AT_EMPTY_PATH);
> >          if (res == -1 && errno == EINVAL) {
> >              /* Sorry, no race free way to set times on symlink. */
> > -            errno = EPERM;
> > +            if (lo->norace) {
> > +                errno = EPERM;
> > +            } else {
> > +                goto fallback;
> > +            }
> >          }
> >          return res;
> >      }
> > -    sprintf(procname, "/proc/self/fd/%i", inode->fd);
> > +    sprintf(path, "/proc/self/fd/%i", inode->fd);
> > +
> > +    return utimensat(AT_FDCWD, path, tv, 0);
> >  
> > -    return utimensat(AT_FDCWD, procname, tv, 0);
> > +fallback:
> > +    res = lo_parent_and_name(lo, inode, path, &parent);
> > +    if (res != -1) {
> > +        res = utimensat(parent->fd, path, tv, AT_SYMLINK_NOFOLLOW);
> > +        unref_inode(lo, parent, 1);
> > +    }
> > +
> > +    return res;
> >  }
> >  
> >  static int lo_fi_fd(fuse_req_t req, struct fuse_file_info *fi)
> > @@ -387,6 +497,7 @@ static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr,
> >  {
> >      int saverr;
> >      char procname[64];
> > +    struct lo_data *lo = lo_data(req);
> >      struct lo_inode *inode;
> >      int ifd;
> >      int res;
> > @@ -459,7 +570,7 @@ static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr,
> >          if (fi) {
> >              res = futimens(fd, tv);
> >          } else {
> > -            res = utimensat_empty_nofollow(inode, tv);
> > +            res = utimensat_empty(lo, inode, tv);
> >          }
> >          if (res == -1) {
> >              goto out_err;
> > @@ -692,24 +803,38 @@ static void lo_symlink(fuse_req_t req, const char *link, fuse_ino_t parent,
> >      lo_mknod_symlink(req, parent, name, S_IFLNK, 0, link);
> >  }
> >  
> > -static int linkat_empty_nofollow(struct lo_inode *inode, int dfd,
> > -                                 const char *name)
> > +static int linkat_empty_nofollow(struct lo_data *lo, struct lo_inode *inode,
> > +                                 int dfd, const char *name)
> >  {
> >      int res;
> > -    char procname[64];
> > +    struct lo_inode *parent;
> > +    char path[PATH_MAX];
> >  
> >      if (inode->is_symlink) {
> >          res = linkat(inode->fd, "", dfd, name, AT_EMPTY_PATH);
> >          if (res == -1 && (errno == ENOENT || errno == EINVAL)) {
> >              /* Sorry, no race free way to hard-link a symlink. */
> > -            errno = EPERM;
> > +            if (lo->norace) {
> > +                errno = EPERM;
> > +            } else {
> > +                goto fallback;
> > +            }
> >          }
> >          return res;
> >      }
> >  
> > -    sprintf(procname, "/proc/self/fd/%i", inode->fd);
> > +    sprintf(path, "/proc/self/fd/%i", inode->fd);
> > +
> > +    return linkat(AT_FDCWD, path, dfd, name, AT_SYMLINK_FOLLOW);
> > +
> > +fallback:
> > +    res = lo_parent_and_name(lo, inode, path, &parent);
> > +    if (res != -1) {
> > +        res = linkat(parent->fd, path, dfd, name, 0);
> > +        unref_inode(lo, parent, 1);
> > +    }
> >  
> > -    return linkat(AT_FDCWD, procname, dfd, name, AT_SYMLINK_FOLLOW);
> > +    return res;
> >  }
> >  
> >  static void lo_link(fuse_req_t req, fuse_ino_t ino, fuse_ino_t parent,
> > @@ -731,7 +856,7 @@ static void lo_link(fuse_req_t req, fuse_ino_t ino, fuse_ino_t parent,
> >      e.attr_timeout = lo->timeout;
> >      e.entry_timeout = lo->timeout;
> >  
> > -    res = linkat_empty_nofollow(inode, lo_fd(req, parent), name);
> > +    res = linkat_empty_nofollow(lo, inode, lo_fd(req, parent), name);
> >      if (res == -1) {
> >          goto out_err;
> >      }
> > @@ -1544,7 +1669,7 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
> >      }
> >  
> >      if (inode->is_symlink) {
> > -        /* Sorry, no race free way to setxattr on symlink. */
> > +        /* Sorry, no race free way to removexattr on symlink. */

I've undone that change; since it looks bogus.

> >          saverr = EPERM;
> >          goto out;
> >      }
> 
> Reviewed-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>

Thanks.

--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK