Re: splice: infinite busy loop lockup bug

Al Viro
2020-09-01T04:08:52Z

On Mon, Aug 31, 2020 at 11:32:28PM -0400, Qian Cai wrote:

> I used a new debug patch but not sure how to capture without
> printk_ratelimited() because the call sites are large,

if (!strcmp(current->comm, "bugger"))
printk(KERN_ERR....
and call the binary you are running ./bugger... And I'd slap such
printk into the beginning of iterate_iovec() as well, if not into
the entry of iov_iter_copy_from_user_atomic(). That BS value of
n must've come from somewhere; it should expand to 'bytes'.
What we have in the beginning is

const struct iovec *iov;
struct iovec v;
size_t skip = i->iov_offset;
size_t left;
size_t wanted = bytes;

iov = i->iov;

__v.iov_len = min(bytes, iov->iov_len - skip);
if (likely(__v.iov_len)) {
__v.iov_base = iov->iov_base + skip;
left = copyin((p += v.iov_len) - v.iov_len, v.iov_base, v.iov_len);
__v.iov_len -= left;
skip += __v.iov_len;
bytes -= __v.iov_len;
} else {
left = 0;
}

and something leaves you with bytes bumped to 22476968. What was in that first
iovec? Incidentally, what's in 'wanted'? And... Timestamps don't look like
that crap has come from generic_perform_write() - it's about 4 seconds later.

While we are at it, there are other users of iterate_all_kinds(), and some of them
can very well get large sizes; they are not copying anything (iov_iter_advance(),
for starters). There that kind of values would be just fine; are you sure those
printks came from iov_iter_copy_from_user_atomic()?