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_modbus_receive_msg() passes the same timeval to every select() call and relies on select() to decrement it. Only Linux does that. On Windows, macOS and the BSDs the full timeout starts again after each received byte, so when the byte timeout is disabled a peer sending one byte just before each expiry keeps the receive going for as long as it wants (CVE-2026-51539). The response or indication timeout now sets an absolute deadline on a monotonic clock, and each select() gets the time left. When the byte timeout is enabled nothing changes: it takes over after the first byte as before. Fixes stephane#843
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_modbus_receive_msg() passes the same timeval to every select() call and relies on select() to decrement it. Only Linux does that. On Windows, macOS and the BSDs the full timeout starts again after each received byte, so when the byte timeout is disabled a peer sending one byte just before each expiry keeps the receive going for as long as it wants (CVE-2026-51539).
The response or indication timeout now sets an absolute deadline on a monotonic clock, and each select() gets the time left. When the byte timeout is enabled nothing changes: it takes over after the first byte as before.
Fixes #843