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When a read filter is flushed (e.g. via stream_filter_remove()) and the flushed data exceeds the remaining read buffer capacity, _php_stream_filter_flush() reallocates stream->readbuf but leaves stream->readbuflen at its old value. Once writepos exceeds the stale readbuflen, the next refill in _php_stream_fill_read_buffer() computes readbuflen - writepos as a wrapped size_t and passes it to the read op, causing EFAULT on plain files and out-of-bounds writes on memory streams. Keep readbuflen in sync with the reallocation, as streams.c does.
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When a read filter is flushed (e.g. via stream_filter_remove()) and the flushed data exceeds the remaining read buffer capacity, _php_stream_filter_flush() reallocates stream->readbuf but leaves stream->readbuflen at its old value. Once writepos exceeds the stale readbuflen, the next refill in _php_stream_fill_read_buffer() computes readbuflen - writepos as a wrapped size_t and passes it to the read op, causing EFAULT on plain files and out-of-bounds writes on memory streams.
Keep readbuflen in sync with the reallocation, as streams.c does.