GUACAMOLE-1625: Close sockets inherited by connection processes. - #698
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I'm concerned about the potential performance impact of looping through all file descriptors at connection start. Would |
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Yeah, fair point. I'll push an update soon. |
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Switched to
Benchmarked against the previous per-descriptor approach, 25 runs per point:
The RHEL 8 fallback matters too: with the default We've deployed a build of this to our own production, since the issue had become critical for us. |
added 2 commits
August 6, 2026 10:48
Connection processes inherit every descriptor open in the parent, as guacd_create_proc() closes only its own end of the socketpair it shares with the new child. For a busy parent, this means the parent's end of every other connection's user socketpair, the parent's end of every other connection process' socketpair, and the socket guacd listens on. FD_CLOEXEC cannot help, as connection processes are forked but never exec'd. Those inherited descriptors keep unrelated sockets referenced after the parent has closed them, so their peers never observe EOF. A write which should fail with EPIPE instead blocks forever, and that blocked write is performed while holding a read lock on the client's list of users, so guac_client_remove_user() can never acquire the corresponding write lock. The count of connected users therefore never reaches zero, guacd_proc_stop() is never called, and the process never exits. The effect compounds, as the more connections are open, the more descriptors each new process inherits. A parent with 469 such processes was observed holding tens of gigabytes, with descriptor counts rising from 98 in the oldest process to 756 in the youngest, and a single socket held by 40 of 40 sampled processes. Close inherited sockets within the newly-forked process. Only sockets are closed, as descriptors for anything else may be held by libraries unaware that they are being closed, and reusing such a number for a later socket would silently redirect their reads and writes.
Testing each descriptor individually costs a syscall per descriptor, which close_range() avoids entirely. Where it is unavailable, the cost is bounded by closing only those descriptors which are actually open, as the limit on the number of descriptors may be orders of magnitude greater than the number in use. A C library may lack a wrapper for close_range() while the kernel still provides the system call, in which case it is invoked directly. That is the case for musl, and thus for the guacd Docker image. Descriptors are no longer tested to determine whether they refer to sockets. Every descriptor inherited here is a socket, and neither libuuid nor OpenSSL retains one across the fork, both obtaining randomness through getrandom().
mike-jumper
approved these changes
Aug 6, 2026
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Connection processes inherit every descriptor open in the parent, as guacd_create_proc() closes only its own end of the socketpair it shares with the new child. For a busy parent, this means the parent's end of every other connection's user socketpair, the parent's end of every other connection process' socketpair, and the socket guacd listens on. FD_CLOEXEC cannot help, as connection processes are forked but never exec'd.
Those inherited descriptors keep unrelated sockets referenced after the parent has closed them, so their peers never observe EOF. A write which should fail with EPIPE instead blocks forever, and that blocked write is performed while holding a read lock on the client's list of users, so guac_client_remove_user() can never acquire the corresponding write lock. The count of connected users therefore never reaches zero, guacd_proc_stop() is never called, and the process never exits.
The effect compounds, as the more connections are open, the more descriptors each new process inherits. A parent with 469 such processes was observed holding tens of gigabytes, with descriptor counts rising from 98 in the oldest process to 756 in the youngest, and a single socket held by 40 of 40 sampled processes.
Close inherited sockets within the newly-forked process. Only sockets are closed, as descriptors for anything else may be held by libraries unaware that they are being closed, and reusing such a number for a later socket would silently redirect their reads and writes.