diff --git a/libavcodec/h264_mp4toannexb_bsf.c b/libavcodec/h264_mp4toannexb_bsf.c
index c9b13d43b4..a28859c9f5 100644
--- a/libavcodec/h264_mp4toannexb_bsf.c
+++ b/libavcodec/h264_mp4toannexb_bsf.c
@@ -58,6 +58,72 @@ static int alloc_and_copy(uint8_t **poutbuf, int *poutbuf_size,
     return 0;
 }
 
+static int h264_extradata_to_annexb(AVCodecContext *avctx, const int padding)
+{
+    uint16_t unit_size;
+    uint64_t total_size                 = 0;
+    uint8_t *out                        = NULL, unit_nb, sps_done = 0,
+             sps_seen                   = 0, pps_seen = 0;
+    const uint8_t *extradata            = avctx->extradata + 4;
+    static const uint8_t nalu_header[4] = { 0, 0, 0, 1 };
+    int length_size = (*extradata++ & 0x3) + 1; // retrieve length coded size
+
+    /* retrieve sps and pps unit(s) */
+    unit_nb = *extradata++ & 0x1f; /* number of sps unit(s) */
+    if (!unit_nb) {
+        goto pps;
+    } else {
+        sps_seen = 1;
+    }
+
+    while (unit_nb--) {
+        void *tmp;
+
+        unit_size   = AV_RB16(extradata);
+        total_size += unit_size + 4;
+        if (total_size > INT_MAX - padding ||
+            extradata + 2 + unit_size > avctx->extradata +
+            avctx->extradata_size) {
+            av_free(out);
+            return AVERROR(EINVAL);
+        }
+        tmp = av_realloc(out, total_size + padding);
+        if (!tmp) {
+            av_free(out);
+            return AVERROR(ENOMEM);
+        }
+        out = tmp;
+        memcpy(out + total_size - unit_size - 4, nalu_header, 4);
+        memcpy(out + total_size - unit_size, extradata + 2, unit_size);
+        extradata += 2 + unit_size;
+pps:
+        if (!unit_nb && !sps_done++) {
+            unit_nb = *extradata++; /* number of pps unit(s) */
+            if (unit_nb)
+                pps_seen = 1;
+        }
+    }
+
+    if (out)
+        memset(out + total_size, 0, FF_INPUT_BUFFER_PADDING_SIZE);
+
+    if (!sps_seen)
+        av_log(avctx, AV_LOG_WARNING,
+               "Warning: SPS NALU missing or invalid. "
+               "The resulting stream may not play.\n");
+
+    if (!pps_seen)
+        av_log(avctx, AV_LOG_WARNING,
+               "Warning: PPS NALU missing or invalid. "
+               "The resulting stream may not play.\n");
+
+    av_free(avctx->extradata);
+    avctx->extradata      = out;
+    avctx->extradata_size = total_size;
+
+    return length_size;
+}
+
 static int h264_mp4toannexb_filter(AVBitStreamFilterContext *bsfc,
                                    AVCodecContext *avctx, const char *args,
                                    uint8_t **poutbuf, int *poutbuf_size,
@@ -70,7 +136,7 @@ static int h264_mp4toannexb_filter(AVBitStreamFilterContext *bsfc,
     int32_t nal_size;
     uint32_t cumul_size    = 0;
     const uint8_t *buf_end = buf + buf_size;
-    int ret = AVERROR(EINVAL);
+    int ret = 0;
 
     /* nothing to filter */
     if (!avctx->extradata || avctx->extradata_size < 6) {
@@ -81,68 +147,10 @@ static int h264_mp4toannexb_filter(AVBitStreamFilterContext *bsfc,
 
     /* retrieve sps and pps NAL units from extradata */
     if (!ctx->extradata_parsed) {
-        uint16_t unit_size;
-        uint64_t total_size                 = 0;
-        uint8_t *out                        = NULL, unit_nb, sps_done = 0,
-                 sps_seen                   = 0, pps_seen = 0;
-        const uint8_t *extradata            = avctx->extradata + 4;
-        static const uint8_t nalu_header[4] = { 0, 0, 0, 1 };
-
-        /* retrieve length coded size */
-        ctx->length_size = (*extradata++ & 0x3) + 1;
-
-        /* retrieve sps and pps unit(s) */
-        unit_nb = *extradata++ & 0x1f; /* number of sps unit(s) */
-        if (!unit_nb) {
-            goto pps;
-        } else {
-            sps_seen = 1;
-        }
-
-        while (unit_nb--) {
-            void *tmp;
-
-            unit_size   = AV_RB16(extradata);
-            total_size += unit_size + 4;
-            if (total_size > INT_MAX - FF_INPUT_BUFFER_PADDING_SIZE ||
-                extradata + 2 + unit_size > avctx->extradata +
-                avctx->extradata_size) {
-                av_free(out);
-                return AVERROR(EINVAL);
-            }
-            tmp = av_realloc(out, total_size + FF_INPUT_BUFFER_PADDING_SIZE);
-            if (!tmp) {
-                av_free(out);
-                return AVERROR(ENOMEM);
-            }
-            out = tmp;
-            memcpy(out + total_size - unit_size - 4, nalu_header, 4);
-            memcpy(out + total_size - unit_size, extradata + 2, unit_size);
-            extradata += 2 + unit_size;
-pps:
-            if (!unit_nb && !sps_done++) {
-                unit_nb = *extradata++; /* number of pps unit(s) */
-                if (unit_nb)
-                    pps_seen = 1;
-            }
-        }
-
-        if (out)
-            memset(out + total_size, 0, FF_INPUT_BUFFER_PADDING_SIZE);
-
-        if (!sps_seen)
-            av_log(avctx, AV_LOG_WARNING,
-                   "Warning: SPS NALU missing or invalid. "
-                   "The resulting stream may not play.\n");
-
-        if (!pps_seen)
-            av_log(avctx, AV_LOG_WARNING,
-                   "Warning: PPS NALU missing or invalid. "
-                   "The resulting stream may not play.\n");
-
-        av_free(avctx->extradata);
-        avctx->extradata      = out;
-        avctx->extradata_size = total_size;
+        ret = h264_extradata_to_annexb(avctx, FF_INPUT_BUFFER_PADDING_SIZE);
+        if (ret < 0)
+            return ret;
+        ctx->length_size      = ret;
         ctx->first_idr        = 1;
         ctx->extradata_parsed = 1;
     }