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

[Josef Zlomek]

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