Re: [Qemu-devel] [PATCH v2 04/28] 9pfs: introduce openat_nofollow() helper

[Stefan Hajnoczi]

On Sun, Feb 26, 2017 at 11:42:03PM +0100, Greg Kurz wrote:
> +int openat_nofollow(int dirfd, const char *path, int flags, mode_t mode)
> +{
> +    int fd;
> +
> +    fd = dup(dirfd);
> +    if (fd == -1) {
> +        return -1;
> +    }
> +
> +    while (*path) {
> +        const char *c;
> +        int next_fd;
> +        char *head;
> +
> +        head = g_strdup(path);
> +        c = strchr(path, '/');
> +        if (c) {
> +            head[c - path] = 0;
> +            next_fd = openat_dir(fd, head);
> +        } else {
> +            next_fd = openat_file(fd, head, flags, mode);
> +        }
> +        g_free(head);
> +        if (next_fd == -1) {
> +            close_preserve_errno(fd);
> +            return -1;
> +        }
> +        close(fd);
> +        fd = next_fd;
> +
> +        if (!c) {
> +            break;
> +        }
> +        path = c + 1;
> +    }
> +
> +    return fd;
> +}

If I understand the Linux openat(2) implementation correctly this
function fails with ENOENT if:

1. An absolute path is given
2. A path contains consecutive slashes ("a///b")

Both of these behaviors are problematic.  If the function doesn't
support absolute paths it should be called relative_openat_nofollow()
and have an error if path[0] == '/'.

I believe guests can pass in paths with consecutive slashes, so the
function must cope with them.

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