Fix stream filter flush corrupting a partially-read buffer#22424
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When a read filter is flushed (for example via stream_filter_remove()) while the stream has already been partially read, _php_stream_filter_flush() backs the unconsumed tail of the read buffer up to offset 0. Two defects in that block diverged from the equivalent code in streams.c. The copy used memcpy() on source and destination ranges that overlap whenever writepos - readpos exceeds readpos, which is undefined behavior (ASAN reports memcpy-param-overlap). And writepos was shrunk with writepos -= readpos after readpos had already been zeroed, making the subtraction a no-op, so writepos stayed inflated by readpos bytes: the stale tail was kept as live data and later flushed buckets were appended past the real end, duplicating bytes and risking an out-of-bounds write. Use memmove() and subtract before zeroing, matching streams.c.
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When a read filter is flushed (for example via stream_filter_remove()) while the stream has already been partially read, _php_stream_filter_flush() backs the unconsumed tail of the read buffer up to offset 0. Two defects in that block diverged from the equivalent code in streams.c. The copy used memcpy() on source/destination ranges that overlap whenever writepos - readpos exceeds readpos, which is undefined behavior (ASAN reports memcpy-param-overlap). And writepos was shrunk with writepos -= readpos after readpos had already been zeroed, making the subtraction a no-op, so writepos stayed inflated by readpos bytes: the stale tail was kept live and later flushed buckets were appended past the real end, duplicating bytes and risking an out-of-bounds write. Use memmove() and subtract before zeroing, matching streams.c. The block dates to the original stream_filter_remove() commit (3455038) in 2004.