`libjpeg` replaces non-EXIF APP1 sections with empty EXIF ones

[glebignatieff]

Description

APP1 sections are not EXIF only, but also XMP. Currently, libjpeg leaves an empty EXIF section instead of copy-pasting XMP section.

Before copy:

JPEG APP1 (8188 bytes):
  + [XMP directory, 8159 bytes]
  | About = DJI Meta Data
  | ModifyDate = 2022-04-22
  | CreateDate = 2022-04-22
...

After copy:

JPEG APP1 (20 bytes):
  ExifByteOrder = MM
  + [IFD0 directory with 0 entries]

MWE

extern "C"
{
#include <libjpeg/jpeg-data.h>
}

auto main() -> int
{
    auto jpeg_data = ::jpeg_data_new_from_file("source.JPG");
    auto exif_data = ::jpeg_data_get_exif_data(jpeg_data);
    ::jpeg_data_set_exif_data(jpeg_data, exif_data);
    ::jpeg_data_save_file(jpeg_data, "dest.JPG");
    return 0;
}

Suggested solution

1. In jpeg_data_load_data() check if EXIF buffer is NULL and if it is, then fall back to default, i.e. generic instead of app1. https://github.com/libexif/exif/blob/4cd915d1119d7dfd33ac7b9148c46afe6f92fbc2/libjpeg/jpeg-data.c#L241-L242
2. In jpeg_data_save_data() check if generic buffer size is 0, if it is, then handle as EXIF, otherwise fallback to default.

I can't upload a file with the patch for some reason, so here it is.

```diff diff --git a/libjpeg/jpeg-data.c b/libjpeg/jpeg-data.c index a64d362..a2528d4 100644 --- a/libjpeg/jpeg-data.c +++ b/libjpeg/jpeg-data.c @@ -25,6 +25,7 @@ #include #include #include +#include /* This refers to the exif-i18n.h file from the "exif" package and is * NOT to be confused with the libexif/i18n.h file. @@ -45,6 +46,11 @@ struct _JPEGDataPrivate ExifLog *log; }; +static bool is_exif_section(JPEGSection * section) +{ + return section->marker == JPEG_MARKER_APP1 && section->content.generic.size == 0; +} + JPEGData * jpeg_data_new (void) { @@ -78,7 +84,7 @@ jpeg_data_append_section (JPEGData *data) s = realloc (data->sections, sizeof (JPEGSection) * (data->count + 1)); if (!s) { - EXIF_LOG_NO_MEMORY (data->priv->log, "jpeg-data", + EXIF_LOG_NO_MEMORY (data->priv->log, "jpeg-data", sizeof (JPEGSection) * (data->count + 1)); return; } @@ -142,19 +148,23 @@ jpeg_data_save_data (JPEGData *data, unsigned char **d, unsigned int *ds) case JPEG_MARKER_EOI: break; case JPEG_MARKER_APP1: - ed = NULL; - exif_data_save_data (s.content.app1, &ed, &eds); - if (!ed) break; - CLEANUP_REALLOC (*d, sizeof (char) * (*ds + 2)); - (*d)[*ds + 0] = (eds + 2) >> 8; - (*d)[*ds + 1] = (eds + 2) >> 0; - *ds += 2; - CLEANUP_REALLOC (*d, sizeof (char) * (*ds + eds)); - memcpy (*d + *ds, ed, eds); - *ds += eds; - free (ed); - break; - default: + if (is_exif_section(&s)) { + ed = NULL; + exif_data_save_data(s.content.app1, &ed, &eds); + if (!ed) { + break; + } + CLEANUP_REALLOC(*d, sizeof(char) * (*ds + 2)); + (*d)[*ds + 0] = (eds + 2) >> 8; + (*d)[*ds + 1] = (eds + 2) >> 0; + *ds += 2; + CLEANUP_REALLOC(*d, sizeof(char) * (*ds + eds)); + memcpy(*d + *ds, ed, eds); + *ds += eds; + free(ed); + break; + } + default: CLEANUP_REALLOC (*d, sizeof (char) * (*ds + s.content.generic.size + 2)); (*d)[*ds + 0] = (s.content.generic.size + 2) >> 8; @@ -238,10 +248,12 @@ jpeg_data_load_data (JPEGData *data, const unsigned char *d, switch (s->marker) { case JPEG_MARKER_APP1: - s->content.app1 = exif_data_new_from_data ( - d + o - 4, len + 4); - break; - default: + s->content.app1 = exif_data_new_from_data(d + o - 4, len + 4); + if (s->content.app1->data != NULL) { + break; + } + exif_data_unref(s->content.app1); + default: s->content.generic.data = malloc (sizeof (char) * len); if (!s->content.generic.data) { @@ -393,9 +405,11 @@ jpeg_data_free (JPEGData *data) case JPEG_MARKER_EOI: break; case JPEG_MARKER_APP1: - exif_data_unref (s.content.app1); - break; - default: + if (is_exif_section(&s)) { + exif_data_unref(s.content.app1); + break; + } + default: free (s.content.generic.data); break; } ```