On 2019-10-11, Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote: > On a machine with a 64K PAGE_SIZE, the nested for loops in > test_check_nonzero_user() can lead to soft lockups, eg: > > watchdog: BUG: soft lockup - CPU#4 stuck for 22s! [modprobe:611] > Modules linked in: test_user_copy(+) vmx_crypto gf128mul crc32c_vpmsum virtio_balloon ip_tables x_tables autofs4 > CPU: 4 PID: 611 Comm: modprobe Tainted: G L 5.4.0-rc1-gcc-8.2.0-00001-gf5a1a536fa14-dirty #1151 > ... > NIP __might_sleep+0x20/0xc0 > LR __might_fault+0x40/0x60 > Call Trace: > check_zeroed_user+0x12c/0x200 > test_user_copy_init+0x67c/0x1210 [test_user_copy] > do_one_initcall+0x60/0x340 > do_init_module+0x7c/0x2f0 > load_module+0x2d94/0x30e0 > __do_sys_finit_module+0xc8/0x150 > system_call+0x5c/0x68 > > Even with a 4K PAGE_SIZE the test takes multiple seconds. Instead > tweak it to only scan a 1024 byte region, but make it cross the > page boundary. > > Fixes: f5a1a536fa14 ("lib: introduce copy_struct_from_user() helper") > Suggested-by: Aleksa Sarai <cyphar@xxxxxxxxxx> > Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> > --- > lib/test_user_copy.c | 23 ++++++++++++++++++++--- > 1 file changed, 20 insertions(+), 3 deletions(-) > > How does this look? It runs in < 1s on my machine here. > > cheers > > diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c > index 950ee88cd6ac..9fb6bc609d4c 100644 > --- a/lib/test_user_copy.c > +++ b/lib/test_user_copy.c > @@ -47,9 +47,26 @@ static bool is_zeroed(void *from, size_t size) > static int test_check_nonzero_user(char *kmem, char __user *umem, size_t size) > { > int ret = 0; > - size_t start, end, i; > - size_t zero_start = size / 4; > - size_t zero_end = size - zero_start; > + size_t start, end, i, zero_start, zero_end; > + > + if (test(size < 1024, "buffer too small")) > + return -EINVAL; > + > + /* > + * We want to cross a page boundary to exercise the code more > + * effectively. We assume the buffer we're passed has a page boundary at > + * size / 2. We also don't want to make the size we scan too large, > + * otherwise the test can take a long time and cause soft lockups. So > + * scan a 1024 byte region across the page boundary. > + */ > + start = size / 2 - 512; > + size = 1024; I don't think it's necessary to do "size / 2" here -- you can just use PAGE_SIZE directly and check above that "size == 2*PAGE_SIZE" (not that this check is exceptionally necessary -- since there's only one caller of this function and it's in the same file). > + > + kmem += start; > + umem += start; > + > + zero_start = size / 4; > + zero_end = size - zero_start; > > /* > * We conduct a series of check_nonzero_user() tests on a block of memory -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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