[FFmpeg-devel] [PATCH] avutil/bprint: fix av_bprint_strftime with %p format string reporting truncated output (PR #20330)

[Marton Balint via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

PR #20330 opened by Marton Balint (cus)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20330
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20330.patch

And a minor factorization.


From f8e83bce6269c95fbad90f34434ceb641bf753d5 Mon Sep 17 00:00:00 2001
From: Marton Balint <cus@passwd.hu>
Date: Sun, 24 Aug 2025 21:42:54 +0200
Subject: [PATCH 1/2] avutil/bprint: make av_bprintf use av_vbprintf

No reason to duplicate the code.

Signed-off-by: Marton Balint <cus@passwd.hu>
---
 libavutil/bprint.c | 33 +++++++++------------------------
 1 file changed, 9 insertions(+), 24 deletions(-)

diff --git a/libavutil/bprint.c b/libavutil/bprint.c
index 4e9571715c..932c03ce50 100644
--- a/libavutil/bprint.c
+++ b/libavutil/bprint.c
@@ -96,35 +96,12 @@ void av_bprint_init_for_buffer(AVBPrint *buf, char *buffer, unsigned size)
     *buf->str = 0;
 }
 
-void av_bprintf(AVBPrint *buf, const char *fmt, ...)
-{
-    unsigned room;
-    char *dst;
-    va_list vl;
-    int extra_len;
-
-    while (1) {
-        room = av_bprint_room(buf);
-        dst = room ? buf->str + buf->len : NULL;
-        va_start(vl, fmt);
-        extra_len = vsnprintf(dst, room, fmt, vl);
-        va_end(vl);
-        if (extra_len <= 0)
-            return;
-        if (extra_len < room)
-            break;
-        if (av_bprint_alloc(buf, extra_len))
-            break;
-    }
-    av_bprint_grow(buf, extra_len);
-}
-
 void av_vbprintf(AVBPrint *buf, const char *fmt, va_list vl_arg)
 {
     unsigned room;
     char *dst;
-    int extra_len;
     va_list vl;
+    int extra_len;
 
     while (1) {
         room = av_bprint_room(buf);
@@ -142,6 +119,14 @@ void av_vbprintf(AVBPrint *buf, const char *fmt, va_list vl_arg)
     av_bprint_grow(buf, extra_len);
 }
 
+void av_bprintf(AVBPrint *buf, const char *fmt, ...)
+{
+    va_list vl;
+    va_start(vl, fmt);
+    av_vbprintf(buf, fmt, vl);
+    va_end(vl);
+}
+
 void av_bprint_chars(AVBPrint *buf, char c, unsigned n)
 {
     unsigned room, real_n;
-- 
2.49.1


From 4532caf820dcb2bc2cbac3d4e782ad27cf9fd76b Mon Sep 17 00:00:00 2001
From: Marton Balint <cus@passwd.hu>
Date: Sun, 24 Aug 2025 21:35:41 +0200
Subject: [PATCH 2/2] avutil/bprint: fix av_bprint_strftime with %p format
 string reporting truncated output

strftime returns 0 in case of an empty output (e.g. %p format string with some
locales), there is no way to distinguish this from a buffer-too-small error
condition. So we must use some heuristics to handle this case, and not consume
INT_MAX RAM and falsely report a truncated output.

Signed-off-by: Marton Balint <cus@passwd.hu>
---
 libavutil/bprint.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/libavutil/bprint.c b/libavutil/bprint.c
index 932c03ce50..fa244b2e04 100644
--- a/libavutil/bprint.c
+++ b/libavutil/bprint.c
@@ -167,6 +167,7 @@ void av_bprint_strftime(AVBPrint *buf, const char *fmt, const struct tm *tm)
 {
     unsigned room;
     size_t l;
+    size_t fmt_len = 0;
 
     if (!*fmt)
         return;
@@ -174,9 +175,16 @@ void av_bprint_strftime(AVBPrint *buf, const char *fmt, const struct tm *tm)
         room = av_bprint_room(buf);
         if (room && (l = strftime(buf->str + buf->len, room, fmt, tm)))
             break;
+
+        if (!fmt_len)
+            fmt_len = strlen(fmt);
+        /* A 256x space requirement for output is unlikely, so it is likely empty */
+        if (room >> 8 > fmt_len)
+            break;
+
         /* strftime does not tell us how much room it would need: let us
            retry with twice as much until the buffer is large enough */
-        room = !room ? strlen(fmt) + 1 :
+        room = !room ? fmt_len + 1 :
                room <= INT_MAX / 2 ? room * 2 : INT_MAX;
         if (av_bprint_alloc(buf, room)) {
             /* impossible to grow, try to manage something useful anyway */
-- 
2.49.1

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