Read-only /proc/<pid>/mem inspection of the local laboratories. No gdb and no agent inside the target: a sidecar container shares the PID namespace and reads the Apache worker's memory.

Extracted from each image's libphp.so with nm -D:

The 7.4 image ships libphp7.so; the oracle now matches any libphp* mapping.

docker run --rm --pid=container:p2s-lab-8335 --cap-add=SYS_PTRACE \
  -v "$PWD/docs/instrumentation/mem_dump.php:/m.php:ro" \
  php:8.2-fpm php /m.php 0x101ca90

mem_dump.php prints PS(http_session_vars) (R0), the reference at R0 and the session array at val.value (R1). id_dump.php prints PS(id).

After the stage-3 "A" request:

R0+0x00  02000000 1a000000   refcount=2, type=0x1a (IS_REFERENCE|flags)
R0+0x08  e087c584 ffff0000   val.value = chunk+0x587e0
R0+0x10  07030000 00000000   val.type  = 0x307 (IS_ARRAY)

R1+0x00  01000000 07000000   refcount=1, type=7 (IS_ARRAY)
R1+0x08  00000000 f0ffffff   flags=0, nTableMask=-16
R1+0x10  805bc684 ffff0000   arData = chunk+0x65b80
R1+0x30  c0fc1287 ffff0000   pDestructor = libphp+...

Both objects are valid and referenced. zval_ptr_dtor on the session zval only decrements the reference (2 -> 1); it does not destroy it. The 8.4/8.5 stage-3 carrier assumes it can overwrite or destroy the reference; on 8.3 the object is alive and cannot be reclaimed by heap reuse. This is the concrete blocker.

PS(id) is non-NULL after A (0xffff984a7000), so the stale pointer exists. B requests with values of 16, 24, 32, 40, 48, 56, 64, 80, 96, 112 and 128 bytes never reclaim it: at 56/64/80 the response carries a new 32-character id, at the other sizes no cookie is emitted.

The working 8.4.26 image is amd64 under emulation and its Zend MM chunk is not exposed as a 2 MB rw-p mapping, so mem_dump.php finds no chunk. The leak still reports chunk=0x7ffffc400000; the oracle needs to accept that mapping shape before the differential comparison can be completed.

1. Adapt the oracle to the 8.4.26 mapping and dump the same fields; if its reference is freed/dangling, that is the condition 8.3 lacks.
2. For 8.3, target the live array at chunk+0x587e0 instead of R0, or force the OOM inside php_session_initialize so the reference is left half-built.
3. For 8.2 and the EOL branches, repeat the id/value-size sweep and inspect the B session lifecycle before the cookie is built.