On Thu, May 14, 2020 at 04:07:36PM +0200, Miklos Szeredi wrote:
> Path lookup in the kernel has special rules for looking up magic symlinks
> under /proc. If a filesystem operation is instructed to follow symlinks
> (e.g. via AT_SYMLINK_FOLLOW or lack of AT_SYMLINK_NOFOLLOW), and the final
> component is such a proc symlink, then the target of the magic symlink is
> used for the operation, even if the target itself is a symlink. I.e. path
> lookup is always terminated after following a final magic symlink.
>
Hi Miklos,
Are these mentioned special rules supported by a recent kernel version
or are they there from day one linux?
thanks,
liubo
> I was erronously assuming that in the above case the target symlink would
> also be followed, and so workarounds were added for a couple of operations
> to handle the symlink case. Since the symlink can be handled simply by
> following the proc symlink, these workardouds are not needed.
>
> Also remove the "norace" option, which disabled the workarounds.
>
> Commit bdfd66788349 ("virtiofsd: Fix xattr operations") already dealt with
> the same issue for xattr operations.
>
> Signed-off-by: Miklos Szeredi <mszer...@redhat.com>
> ---
> tools/virtiofsd/passthrough_ll.c | 175 ++-----------------------------
> 1 file changed, 6 insertions(+), 169 deletions(-)
>
> diff --git a/tools/virtiofsd/passthrough_ll.c
> b/tools/virtiofsd/passthrough_ll.c
> index 3ba1d9098460..2ce7c96085bf 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c
> @@ -140,7 +140,6 @@ enum {
> struct lo_data {
> pthread_mutex_t mutex;
> int debug;
> - int norace;
> int writeback;
> int flock;
> int posix_lock;
> @@ -176,7 +175,6 @@ static const struct fuse_opt lo_opts[] = {
> { "cache=none", offsetof(struct lo_data, cache), CACHE_NONE },
> { "cache=auto", offsetof(struct lo_data, cache), CACHE_AUTO },
> { "cache=always", offsetof(struct lo_data, cache), CACHE_ALWAYS },
> - { "norace", offsetof(struct lo_data, norace), 1 },
> { "readdirplus", offsetof(struct lo_data, readdirplus_set), 1 },
> { "no_readdirplus", offsetof(struct lo_data, readdirplus_clear), 1 },
> FUSE_OPT_END
> @@ -592,136 +590,6 @@ static void lo_getattr(fuse_req_t req, fuse_ino_t ino,
> fuse_reply_attr(req, &buf, lo->timeout);
> }
>
> -/*
> - * Increments parent->nlookup and caller must release refcount using
> - * lo_inode_put(&parent).
> - */
> -static int lo_parent_and_name(struct lo_data *lo, struct lo_inode *inode,
> - char path[PATH_MAX], struct lo_inode **parent)
> -{
> - char procname[64];
> - char *last;
> - struct stat stat;
> - struct lo_inode *p;
> - int retries = 2;
> - int res;
> -
> -retry:
> - sprintf(procname, "%i", inode->fd);
> -
> - res = readlinkat(lo->proc_self_fd, procname, path, PATH_MAX);
> - if (res < 0) {
> - fuse_log(FUSE_LOG_WARNING, "%s: readlink failed: %m\n", __func__);
> - goto fail_noretry;
> - }
> -
> - if (res >= PATH_MAX) {
> - 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,
> - "%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->nlookup++;
> - g_atomic_int_inc(&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,
> - "%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,
> - "%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,
> - "%s: failed to stat last\n", __func__);
> - }
> - goto fail_unref;
> - }
> - if (stat.st_dev != inode->key.dev || stat.st_ino != inode->key.ino) {
> - if (!retries) {
> - 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_lolocked(lo, p, 1);
> - lo_inode_put(lo, &p);
> -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 (S_ISLNK(inode->filetype)) {
> - res = utimensat(inode->fd, "", tv, AT_EMPTY_PATH);
> - if (res == -1 && errno == EINVAL) {
> - /* Sorry, no race free way to set times on symlink. */
> - if (lo->norace) {
> - errno = EPERM;
> - } else {
> - goto fallback;
> - }
> - }
> - return res;
> - }
> - sprintf(path, "%i", inode->fd);
> -
> - return utimensat(lo->proc_self_fd, path, 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_lolocked(lo, parent, 1);
> - lo_inode_put(lo, &parent);
> - }
> -
> - return res;
> -}
> -
> static int lo_fi_fd(fuse_req_t req, struct fuse_file_info *fi)
> {
> struct lo_data *lo = lo_data(req);
> @@ -828,7 +696,8 @@ 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(lo, inode, tv);
> + sprintf(procname, "%i", inode->fd);
> + res = utimensat(lo->proc_self_fd, procname, tv, 0);
> }
> if (res == -1) {
> goto out_err;
> @@ -1129,41 +998,6 @@ 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_data *lo, struct lo_inode *inode,
> - int dfd, const char *name)
> -{
> - int res;
> - struct lo_inode *parent;
> - char path[PATH_MAX];
> -
> - if (S_ISLNK(inode->filetype)) {
> - 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. */
> - if (lo->norace) {
> - errno = EPERM;
> - } else {
> - goto fallback;
> - }
> - }
> - return res;
> - }
> -
> - sprintf(path, "%i", inode->fd);
> -
> - return linkat(lo->proc_self_fd, 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_lolocked(lo, parent, 1);
> - lo_inode_put(lo, &parent);
> - }
> -
> - return res;
> -}
> -
> static void lo_link(fuse_req_t req, fuse_ino_t ino, fuse_ino_t parent,
> const char *name)
> {
> @@ -1172,6 +1006,7 @@ static void lo_link(fuse_req_t req, fuse_ino_t ino,
> fuse_ino_t parent,
> struct lo_inode *parent_inode;
> struct lo_inode *inode;
> struct fuse_entry_param e;
> + char procname[64];
> int saverr;
>
> if (!is_safe_path_component(name)) {
> @@ -1190,7 +1025,9 @@ 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(lo, inode, parent_inode->fd, name);
> + sprintf(procname, "%i", inode->fd);
> + res = linkat(lo->proc_self_fd, procname, parent_inode->fd, name,
> + AT_SYMLINK_FOLLOW);
> if (res == -1) {
> goto out_err;
> }
> --
> 2.21.1
>
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