dtrace: could not enable tracing: BPF program load for '...' failed: No space left on device #6
Comments
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The BPF verifier walks code paths to ensure that BPF code that is run is safe. An action like quantize() must quantize values into one of 127 different bins, representing numerous possible code paths. This taxes the BPF verifier. While that is in itself a challenge, the problem in this bug is unnecessary. A 16-Mbyte buffer is passed to the kernel, and the error is simply that the buffer we pass is too small. |
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One can imagine several fixes. One is not to pass a buffer! The script in question would pass! Never passing a buffer, however, would mean that BPF verifier problems would never be reported. Another is to pass a larger buffer, perhaps under user control. A hybrid solution seems to make most sense. Try to load the BPF program without specifying a log buffer. If the load is successful, one is done. If there is a problem, retry the load, this time with a log buffer. If the error is ENOSPC, warn the user that a larger buffer is needed to capture the problem. Otherwise, simply just report the buffer. |
A D script that produces BPF code with many code paths can result in 16 Mbytes of BPF log file, ending with the above error message. The log file really says nothing about why the BPF load failed. The actual problem is that we supplied a 16-Mbyte log buffer that is too small. If no log buffer is supplied, this problem is not encountered. Change DTrace's BPF program load to use no log buffer at first. If the load fails, then retry with a log buffer. The load should again fail, but if the failure is not ENOSPC, we can simply report the log and be done. If the failure becomes ENOSPC, inform the user of the problem and what action can be taken to increase the buffer size. Provide a new DTrace option to control the log buffer size. Add tests for this fix. Specifically, the aggregation function quantize() can be used, since it must quantize a value into one of 127 different bins. The algorithm used has many code paths and thereby exercises the BPF verifier well. #6 Signed-off-by: Eugene Loh <eugene.loh@oracle.com> Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>
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Verified as fixed. |
A script like
x = 1;
@ = quantize(x);
@ = quantize(x);
[....97 similar lines...]
@ = quantize(x);
fails, printing about 16 Mbyte of BPF verifier log and ending in:
dtrace: could not enable tracing: BPF program load for '...' failed:
No space left on device
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