wolfsshd: stop polling when the SFTP channel has nothing buffered - #1207
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Pull request overview
This PR fixes a busy-polling condition in wolfsshd’s SFTP subsystem loop by treating a wolfSSH_stream_peek() return of 0 as an idle/no-buffered-data state, allowing select() to block for a normal timeout instead of repeatedly hitting its 100µs floor. It also adds a regression test that verifies an idle SFTP session does not consume significant CPU time on Linux.
Changes:
- Update the SFTP subsystem loop to set a 1-second timeout and continue when
wolfSSH_stream_peek()returns0(idle channel with empty buffer). - Add a Linux-only test script that measures the connection process CPU ticks via
/proc/<pid>/statduring an idle SFTP session. - Register the new test in
run_all_sshd_tests.sh.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.
| File | Description |
|---|---|
| apps/wolfsshd/wolfsshd.c | Avoids tight polling by treating stream_peek()==0 as idle and increasing the next select() timeout. |
| apps/wolfsshd/test/sshd_sftp_idle_cpu_test.sh | Adds a regression test to detect excessive CPU usage during idle SFTP sessions. |
| apps/wolfsshd/test/run_all_sshd_tests.sh | Includes the new idle-CPU regression test in the sshd test suite. |
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wolfSSH_stream_peek() returns 0 for a live channel with an empty buffer, which is the ordinary idle case. None of the arms after the peek match that, so the loop falls through with the timeout still at TEST_SFTP_TIMEOUT_NONE and tcp_select() returns on its 100 us floor. An idle SFTP session keeps a core busy for as long as it stays connected. - take the peek's zero return as "nothing to do" and let the next select wait a second, the same value the want-read paths already use - sshd_sftp_idle_cpu_test.sh parks an idle SFTP session on the daemon and reads the connection process's CPU time out of /proc, failing if it spends 5 ticks or more over ten seconds The measurement the test automates: 21 ticks per 10 seconds before, 0 after. It skips where there is no /proc or no local daemon to measure.
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Fenrir Automated Review — PR #1207
Scan targets checked: wolfssh-bugs, wolfssh-src
Fenrir result: Approved ✅
No new issues found in the changed files.
Advisory only — this automated result does not count as a GitHub approval.
wolfSSH_stream_peek()returns 0 for a live channel with an empty buffer, which is the ordinary idle case. None of the arms after the peek match that, so the loop falls through with the timeout still atTEST_SFTP_TIMEOUT_NONEandtcp_select()returns on its 100 us floor. An idle SFTP session keeps a core busy for as long as it stays connected.sshd_sftp_idle_cpu_test.shparks an idle SFTP session on the daemon and reads the connection process's CPU time out of/proc, failing if it spends 5 ticks or more over ten secondsThe measurement the test automates: 21 ticks per 10 seconds before, 0 after. It skips where there is no
/procor no local daemon to measure,so it is a no-op outside Linux.