[FFmpeg-devel] [PATCH] avformat/dump: Avoid unnecessary implicit calculation of strlen

[FFmpeg-devel] [PATCH] avformat/dump: Avoid unnecessary implicit calculation of strlen

[Andreas Rheinhardt]

av_strlcpy() returns the length of the src string to enable
the caller to check for truncation. It is currently used in
the following way in dump_metadata(): Every metadata value
is searched for \b, \n, \v, \f, \r and then the data up to
the first of these characters found is copied to a small
temporary buffer via av_strlcpy() (but of course not more
than fits into said buffer) and then printed; all characters up
to the character found earlier are then treated as consumed.

But this is bad performance-wise if the while string is big
and contains many of these characters, because av_strlcpy()
will unnecessarily calculate the length of the whole remaining string.
(dump_metadata() actually ignored the return value of av_strlcpy().)

Fix this by just not copying the data in a temporary buffer at all.
Instead just use the %.*s to bound the number of characters output.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
---
 libavformat/dump.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/libavformat/dump.c b/libavformat/dump.c
index e3f0056c20..cafcef36c6 100644
--- a/libavformat/dump.c
+++ b/libavformat/dump.c
@@ -145,10 +145,8 @@ static void dump_metadata(void *ctx, const AVDictionary *m, const char *indent)
                 av_log(ctx, AV_LOG_INFO,
                        "%s  %-16s: ", indent, tag->key);
                 while (*p) {
-                    char tmp[256];
                     size_t len = strcspn(p, "\x8\xa\xb\xc\xd");
-                    av_strlcpy(tmp, p, FFMIN(sizeof(tmp), len+1));
-                    av_log(ctx, AV_LOG_INFO, "%s", tmp);
+                    av_log(ctx, AV_LOG_INFO, "%.*s", (int)(FFMIN(255, len)), p);
                     p += len;
                     if (*p == 0xd) av_log(ctx, AV_LOG_INFO, " ");
                     if (*p == 0xa) av_log(ctx, AV_LOG_INFO, "\n%s  %-16s: ", indent, "");
-- 
2.34.1

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Re: [FFmpeg-devel] [PATCH] avformat/dump: Avoid unnecessary implicit calculation of strlen

[Paul B Mahol]

On 9/14/22, Andreas Rheinhardt <andreas.rheinhardt@outlook.com> wrote:
> av_strlcpy() returns the length of the src string to enable
> the caller to check for truncation. It is currently used in
> the following way in dump_metadata(): Every metadata value
> is searched for \b, \n, \v, \f, \r and then the data up to
> the first of these characters found is copied to a small
> temporary buffer via av_strlcpy() (but of course not more
> than fits into said buffer) and then printed; all characters up
> to the character found earlier are then treated as consumed.
>
> But this is bad performance-wise if the while string is big
> and contains many of these characters, because av_strlcpy()
> will unnecessarily calculate the length of the whole remaining string.
> (dump_metadata() actually ignored the return value of av_strlcpy().)
>
> Fix this by just not copying the data in a temporary buffer at all.
> Instead just use the %.*s to bound the number of characters output.
>
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> ---
>  libavformat/dump.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/libavformat/dump.c b/libavformat/dump.c
> index e3f0056c20..cafcef36c6 100644
> --- a/libavformat/dump.c
> +++ b/libavformat/dump.c
> @@ -145,10 +145,8 @@ static void dump_metadata(void *ctx, const AVDictionary
> *m, const char *indent)
>                  av_log(ctx, AV_LOG_INFO,
>                         "%s  %-16s: ", indent, tag->key);
>                  while (*p) {
> -                    char tmp[256];
>                      size_t len = strcspn(p, "\x8\xa\xb\xc\xd");
> -                    av_strlcpy(tmp, p, FFMIN(sizeof(tmp), len+1));
> -                    av_log(ctx, AV_LOG_INFO, "%s", tmp);
> +                    av_log(ctx, AV_LOG_INFO, "%.*s", (int)(FFMIN(255,
> len)), p);
>                      p += len;
>                      if (*p == 0xd) av_log(ctx, AV_LOG_INFO, " ");
>                      if (*p == 0xa) av_log(ctx, AV_LOG_INFO, "\n%s  %-16s:
> ", indent, "");
> --
> 2.34.1
>
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> ffmpeg-devel@ffmpeg.org
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>
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> ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".
>


LGTM
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