[FFmpeg-devel] [PATCH v3 1/2] avformat/url: fix logic for removing ".." path components

[Steven Liu]

Josef Zlomek <josef at pex.com> 于2020年7月28日周二 下午12:11写道：
>
> Fixes: 8814
>
> The logic for removing ".." path components and their corresponding
> upper directories was reworked.
>
> Now, the function trim_double_dot_url splits the path by "/" into
> components, and processes the components one ny one:
> - if the component is "..", the last path component in output buffer is removed
> - if the component is not empty, it is added to the output buffer
> No temporary buffers are used anymore.
>
> Also the path containing no '/' after '://' is returned as it is.
>
> The duplicate logic was removed from ff_make_absolute_url.
>
> Signed-off-by: Josef Zlomek <josef at pex.com>
> ---
>  libavformat/url.c | 101 +++++++++++++++++++++++-----------------------
>  1 file changed, 50 insertions(+), 51 deletions(-)
>
> diff --git a/libavformat/url.c b/libavformat/url.c
> index 20463a6674..46a08e88e3 100644
> --- a/libavformat/url.c
> +++ b/libavformat/url.c
> @@ -82,41 +82,62 @@ static void trim_double_dot_url(char *buf, const char *rel, int size)
>  {
>      const char *p = rel;
>      const char *root = rel;
> -    char tmp_path[MAX_URL_SIZE] = {0, };
> -    char *sep;
> -    char *node;
> +    char *dst;
> +    int dst_len = 0;
> +    const char *sep;
> +    const char *next;
> +    int last_is_dir;
>
>      /* Get the path root of the url which start by "://" */
>      if (p && (sep = strstr(p, "://"))) {
>          sep += 3;
>          root = strchr(sep, '/');
> -        if (!root)
> +        if (!root) {
> +            av_strlcpy(buf, rel, size);
>              return;
> +        }
>      }
> +    if (*root == '/')
> +        ++root;
>
> -    /* set new current position if the root node is changed */
> -    p = root;
> -    while (p && (node = strstr(p, ".."))) {
> -        av_strlcat(tmp_path, p, node - p + strlen(tmp_path));
> -        p = node + 3;
> -        sep = strrchr(tmp_path, '/');
> -        if (sep)
> -            sep[0] = '\0';
> -        else
> -            tmp_path[0] = '\0';
> -    }
> -
> -    if (!av_stristart(p, "/", NULL) && root != rel)
> -        av_strlcat(tmp_path, "/", size);
> -
> -    av_strlcat(tmp_path, p, size);
> -    /* start set buf after temp path process. */
> +    /* Copy the root to the output buffer */
>      av_strlcpy(buf, rel, root - rel + 1);
> +    size -= root - rel;
> +    dst = &buf[root - rel];
> +
> +    /* Split the path by "/" and remove ".." and its corresponding directory. */
> +    last_is_dir = 1;
> +    for (p = root; *p; p = next) {
> +        next = strchr(p, '/');
> +        if (!next) {
> +            next = p + strlen(p);
> +            last_is_dir = 0;
> +        }
>
> -    if (!av_stristart(tmp_path, "/", NULL) && root != rel)
> -        av_strlcat(buf, "/", size);
> +        if (next - p == 2 && !strncmp(p, "..", 2)) {
> +            /* remove the last directory from dst */
> +            while (dst_len > 0 && dst[--dst_len] != '/')
> +                ;
> +            dst[dst_len] = '\0';
> +            last_is_dir = 1;
> +        } else if (next > p) {
> +            int len_including_zero;
> +            /* copy the current path component to dst (including '/') */
> +            if (dst_len) {
> +                av_strlcpy(dst + dst_len, "/", size - dst_len);
> +                ++dst_len;
> +            }
> +            len_including_zero = FFMIN(size - dst_len, next - p + 1);
> +            av_strlcpy(dst + dst_len, p, len_including_zero);
> +            dst_len += len_including_zero - 1;
> +        }
>
> -    av_strlcat(buf, tmp_path, size);
> +        /* skip "/" */
> +        while (*next == '/')
> +            ++next;
> +    }
> +    if (last_is_dir && dst_len)
> +        av_strlcpy(dst + dst_len, "/", size - dst_len);
>  }
>
>  void ff_make_absolute_url(char *buf, int size, const char *base,
> @@ -175,14 +196,11 @@ void ff_make_absolute_url(char *buf, int size, const char *base,
>
>      root = p = buf;
>      /* Get the path root of the url which start by "://" */
> -    if (p && strstr(p, "://")) {
> -        sep = strstr(p, "://");
> -        if (sep) {
> -            sep += 3;
> -            root = strchr(sep, '/');
> -            if (!root)
> -                return;
> -        }
> +    if (p && (sep = strstr(p, "://"))) {
> +        sep += 3;
> +        root = strchr(sep, '/');
> +        if (!root)
> +            return;
>      }
>
>      /* Remove the file name from the base url */
> @@ -194,26 +212,7 @@ void ff_make_absolute_url(char *buf, int size, const char *base,
>          sep[1] = '\0';
>      else
>          buf[0] = '\0';
> -    while (av_strstart(rel, "..", NULL) && sep) {
> -        /* Remove the path delimiter at the end */
> -        if (sep > root) {
> -            sep[0] = '\0';
> -            sep = strrchr(buf, '/');
> -        }
>
> -        /* If the next directory name to pop off is "..", break here */
> -        if (!strcmp(sep ? &sep[1] : buf, "..")) {
> -            /* Readd the slash we just removed */
> -            av_strlcat(buf, "/", size);
> -            break;
> -        }
> -        /* Cut off the directory name */
> -        if (sep)
> -            sep[1] = '\0';
> -        else
> -            buf[0] = '\0';
> -        rel += 3;
> -    }
>      av_strlcat(buf, rel, size);
>      trim_double_dot_url(tmp_path, buf, size);
As Marton said, tmp_path should remove too.

>      memset(buf, 0, size);
> --
> 2.17.1
>
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