From: Dominique Martinet asmadeus@codewreck.org
It's apparently possible to get an iov forwarded all the way up to the end of the current page we're looking at, e.g.
(gdb) p *iter $24 = {iter_type = 4 '\004', nofault = false, data_source = false, iov_offset = 4096, {__ubuf_iovec = { iov_base = 0xffff88800f5bc000, iov_len = 655}, {{__iov = 0xffff88800f5bc000, kvec = 0xffff88800f5bc000, bvec = 0xffff88800f5bc000, folioq = 0xffff88800f5bc000, xarray = 0xffff88800f5bc000, ubuf = 0xffff88800f5bc000}, count = 655}}, {nr_segs = 2, folioq_slot = 2 '\002', xarray_start = 2}}
Where iov_offset is 4k with 4k-sized folios
This should have been because we're only in the 2nd slot and there's another one after this, but iterate_folioq should not try to map a folio that skips the whole size, and more importantly part here does not end up zero (because 'PAGE_SIZE - skip % PAGE_SIZE' ends up PAGE_SIZE and not zero..), so skip forward to the "advance to next folio" code.
Reported-by: Maximilian Bosch maximilian@mbosch.me Reported-by: Ryan Lahfa ryan@lahfa.xyz Reported-by: Christian Theune ct@flyingcircus.io Reported-by: Arnout Engelen arnout@bzzt.net Link: https://lkml.kernel.org/r/D4LHHUNLG79Y.12PI0X6BEHRHW@mbosch.me/ Fixes: db0aa2e9566f ("mm: Define struct folio_queue and ITER_FOLIOQ to handle a sequence of folios") Cc: stable@vger.kernel.org # v6.12+ Signed-off-by: Dominique Martinet asmadeus@codewreck.org --- include/linux/iov_iter.h | 3 +++ 1 file changed, 3 insertions(+)
diff --git a/include/linux/iov_iter.h b/include/linux/iov_iter.h index c4aa58032faf874ee5b29bd37f9e23c479741bef..7988a0fc94ad0525b475196035dc5d754fd3d117 100644 --- a/include/linux/iov_iter.h +++ b/include/linux/iov_iter.h @@ -168,6 +168,8 @@ size_t iterate_folioq(struct iov_iter *iter, size_t len, void *priv, void *priv2 break;
fsize = folioq_folio_size(folioq, slot); + if (skip >= fsize) + goto next; base = kmap_local_folio(folio, skip); part = umin(len, PAGE_SIZE - skip % PAGE_SIZE); remain = step(base, progress, part, priv, priv2); @@ -177,6 +179,7 @@ size_t iterate_folioq(struct iov_iter *iter, size_t len, void *priv, void *priv2 progress += consumed; skip += consumed; if (skip >= fsize) { +next: skip = 0; slot++; if (slot == folioq_nr_slots(folioq) && folioq->next) {