On 21/08/2020 13.37, Petr Mladek wrote: > On Mon 2020-08-17 09:06:32, Rasmus Villemoes wrote: >> On 17/08/2020 06.30, Arpitha Raghunandan wrote: >>> Converts test lib/test_printf.c to KUnit. >>> More information about KUnit can be found at >>> https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html. >>> KUnit provides a common framework for unit tests in the kernel. >> >> So I can continue to build a kernel with some appropriate CONFIG set to >> y, boot it under virt-me, run dmesg and see if I broke printf? That's >> what I do now, and I don't want to have to start using some enterprisy >> framework. > > I had the same concern. I have tried it. Thanks for doing that and reporting the results. > #> modprobe printf_kunit > > produced the following in dmesg: > > [ 60.931175] printf_kunit: module verification failed: signature and/or required key missing - tainting kernel > [ 60.942209] TAP version 14 > [ 60.945197] # Subtest: printf-kunit-test > [ 60.945200] 1..1 > [ 60.951092] ok 1 - selftest > [ 60.953414] ok 1 - printf-kunit-test > > I could live with the above. Then I tried to break a test by the following change: > > > diff --git a/lib/printf_kunit.c b/lib/printf_kunit.c > index 68ac5f9b8d28..1689dadd70a3 100644 > --- a/lib/printf_kunit.c > +++ b/lib/printf_kunit.c > @@ -395,7 +395,7 @@ ip4(struct kunit *kunittest) > sa.sin_port = cpu_to_be16(12345); > sa.sin_addr.s_addr = cpu_to_be32(0x7f000001); > > - test(kunittest, "127.000.000.001|127.0.0.1", "%pi4|%pI4", &sa.sin_addr, &sa.sin_addr); > + test(kunittest, "127-000.000.001|127.0.0.1", "%pi4|%pI4", &sa.sin_addr, &sa.sin_addr); > test(kunittest, "127.000.000.001|127.0.0.1", "%piS|%pIS", &sa, &sa); > sa.sin_addr.s_addr = cpu_to_be32(0x01020304); > test(kunittest, "001.002.003.004:12345|1.2.3.4:12345", "%piSp|%pISp", &sa, &sa); > > > It produced:: > > [ 56.786858] TAP version 14 > [ 56.787493] # Subtest: printf-kunit-test > [ 56.787494] 1..1 > [ 56.788612] # selftest: EXPECTATION FAILED at lib/printf_kunit.c:76 > Expected memcmp(test_buffer, expect, written) == 0, but > memcmp(test_buffer, expect, written) == 1 > 0 == 0 > vsnprintf(buf, 256, "%pi4|%pI4", ...) wrote '127.000.000.001|127.0.0.1', expected '127-000.000.001|127.0.0.1' > [ 56.795433] # selftest: EXPECTATION FAILED at lib/printf_kunit.c:76 > Expected memcmp(test_buffer, expect, written) == 0, but > memcmp(test_buffer, expect, written) == 1 > 0 == 0 > vsnprintf(buf, 20, "%pi4|%pI4", ...) wrote '127.000.000.001|127', expected '127-000.000.001|127' > [ 56.800909] # selftest: EXPECTATION FAILED at lib/printf_kunit.c:108 > Expected memcmp(p, expect, elen+1) == 0, but > memcmp(p, expect, elen+1) == 1 > 0 == 0 > kvasprintf(..., "%pi4|%pI4", ...) returned '127.000.000.001|127.0.0.1', expected '127-000.000.001|127.0.0.1' > [ 56.806497] not ok 1 - selftest > [ 56.806497] not ok 1 - printf-kunit-test > > while the original code would have written the following error messages: > > [ 95.859225] test_printf: loaded. > [ 95.860031] test_printf: vsnprintf(buf, 256, "%pi4|%pI4", ...) wrote '127.000.000.001|127.0.0.1', expected '127-000.000.001|127.0.0.1' > [ 95.862630] test_printf: vsnprintf(buf, 6, "%pi4|%pI4", ...) wrote '127.0', expected '127-0' > [ 95.864118] test_printf: kvasprintf(..., "%pi4|%pI4", ...) returned '127.000.000.001|127.0.0.1', expected '127-000.000.001|127.0.0.1' > [ 95.866589] test_printf: failed 3 out of 388 tests > > > Even the error output is acceptable for me. Urgh. Yeah, perhaps, but the original is much more readable; it really doesn't matter that a memcmp() fails to compare equal to 0, that's merely how we figured out that the output was wrong. I am just curious why > the 2nd failure is different: > > + original code: vsnprintf(buf, 6, "%pi4|%pI4", ...) wrote '127.0', expected '127-0' > + kunit code: vsnprintf(buf, 20, "%pi4|%pI4", ...) wrote '127.000.000.001|127', expected '127-000.000.001|127' That's by design. If you read the code, there's a comment that says we do every test case four times: With a buffer that is large enough to do the whole output, with a 0 size buffer (that's essential to allowing kasprintf to know how much to allocate), with kvasprintf - but also with a buffer size that's guaranteed to ensure the output gets truncated somewhere. And that size is chosen randomly - I guess one could test every single buffer size between 0 and elen+1, but that's overkill. Now I should probably have made the tests deterministic in the sense of getting a random seed for a PRNG, printing that seed and allowing a module parameter to set the seed in order to repeat the exact same tests. But so far I haven't really seen any bugs caught by test_printf which would have been easier to fix with that. The reason I added that "chop it off somewhere randomly" was, IIRC, due to some %p extensions that behaved rather weirdly depending on whether there was enough room left or not, but I fixed those bugs before creating test_printf (and they were in turn the reason for creating test_printf). See for example 41416f2330, where %pE at the beginning of the format string would work ok, but if anything preceded it and the buffer was too small we'd crash. > > I am also a bit scared by the following note at > https://www.kernel.org/doc/html/latest/dev-tools/kunit/start.html#running-tests-without-the-kunit-wrapper > > "KUnit is not designed for use in a production system, and it’s > possible that tests may reduce the stability or security of the > system." > > What does it mean thay it might reduce stability or security? > Is it because the tests might cause problems? > Or because the kunit framework modifies functionality of the running > system all the time? Hm, yeah, that sounds a little frightening. Rasmus
