On Sun 2020-10-25 22:48:42, Rasmus Villemoes wrote: > The printf test suite does each test with a few different buffer sizes > to ensure vsnprintf() behaves correctly with respect to truncation and > size reporting. It calls vsnprintf() with a buffer size that is > guaranteed to be big enough, a buffer size of 0 to ensure that nothing > gets written to the buffer, but it also calls vsnprintf() with a > buffer size chosen to guarantee the output gets truncated somewhere in > the middle. > > That buffer size is chosen randomly to increase the chance of finding > some corner case bug (for example, there used to be some %p<foo> > extension that would fail to produce any output if there wasn't room > enough for it all, despite the requirement of producing as much as > there's room for). I'm not aware of that having found anything yet, > but should it happen, it's annoying not to be able to repeat the > test with the same sequence of truncated lengths. > > For demonstration purposes, if we break one of the test cases > deliberately, we still get different buffer sizes if we don't pass the > seed parameter: > > root@(none):/# modprobe test_printf > [ 15.317783] test_printf: vsnprintf(buf, 18, "%piS|%pIS", ...) wrote '127.000.000.001|1', expected '127-000.000.001|1' > [ 15.323182] test_printf: failed 3 out of 388 tests > [ 15.324034] test_printf: random seed used was 0x278bb9311979cc91 > modprobe: ERROR: could not insert 'test_printf': Invalid argument > > root@(none):/# modprobe test_printf > [ 13.940909] test_printf: vsnprintf(buf, 22, "%piS|%pIS", ...) wrote '127.000.000.001|127.0', expected '127-000.000.001|127.0' > [ 13.944744] test_printf: failed 3 out of 388 tests > [ 13.945607] test_printf: random seed used was 0x9f72eee1c9dc02e5 > modprobe: ERROR: could not insert 'test_printf': Invalid argument > > but to repeat a specific sequence of tests, we can do > > root@(none):/# modprobe test_printf seed=0x9f72eee1c9dc02e5 > [ 448.328685] test_printf: vsnprintf(buf, 22, "%piS|%pIS", ...) wrote '127.000.000.001|127.0', expected '127-000.000.001|127.0' > [ 448.331650] test_printf: failed 3 out of 388 tests > [ 448.332295] test_printf: random seed used was 0x9f72eee1c9dc02e5 > modprobe: ERROR: could not insert 'test_printf': Invalid argument > > Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> Great feature! Reviewed-by: Petr Mladek <pmladek@xxxxxxxx> Best Regards, Petr
