Since revision 1.78 of libexif/exif-entry.c, it is no longer possible for the caller of exif_entry_get_value to determine whether or not the call succeeded, and therefore whether the contents of the buffer is valid.
Known impact: This bug causes swiggle to be unable to correctly read exif data using libexif. There is a workaround for swiggle, but it has highlighted this issue as the root cause.
The patch which caused this issue was bug #1051994. The problem which the patch aimed to fix is only hidden by this patch, and the correct fix is to revert the patch, and ensure all callers of exif_entry_get_value handle failure correctly.
Within libexif, this means two calls in exif-data.c, both in exif_data_get_type_maker_note:
/* Canon */
if (!strcmp (exif_entry_get_value (em, value, sizeof (value)), "Canon"))
if (exif_entry_get_value(em, value, sizeof(value)) && !strcmp(value, "Canon"))
would be more appropriate.
Similarly for the Nikon case immediately below.
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