Re: [PATCH 1/6] lib/scatterlist: Tidy types and fix overflow checking in sgl_alloc_order
From: Tvrtko Ursulin
Date: Fri Mar 09 2018 - 09:05:09 EST
On 07/03/18 17:06, Tvrtko Ursulin wrote:
On 07/03/18 16:10, Bart Van Assche wrote:
On Wed, 2018-03-07 at 12:47 +0000, Tvrtko Ursulin wrote:
sgl_alloc_order explicitly takes a 64-bit length (unsigned long long)
but
then rejects it in overflow checking if greater than 4GiB allocation was
requested. This is a consequence of using unsigned int for the right
hand
side condition which then natuarally overflows when shifted left,
earlier
than nent otherwise would.
Fix is to promote the right hand side of the conditional to unsigned
long.
Agreed.
It is also not useful to allow for 64-bit lenght on 32-bit platforms so
I have changed this type to a natural unsigned long. Like this it
changes
size naturally depending on the architecture.
I do not agree. Although uncommon, it is possible that e.g. a SCSI
initiator
sends a transfer of more than 4 GB to a target system and that that
transfer
must not be split. Since this code is used by the SCSI target, I think
that's
an example of an application where it is useful to allow allocations
of more
than 4 GB at once on a 32-bit system.
If it can work on 32-bit (it can DMA from highmem or what?) and
allocation can realistically succeed then I'm happy to defer to storage
experts on this one.
Furthermore on this specific point, the only caller of sgl_alloc_order
in the tree passes in u32 for length.
So even if there will be some realistic use case for >4GiB allocations
on 32-bit systems (where unsigned long is 32-bit, to be more precise) I
don't see a problem with my patch right now.
First five patches from the series are all IMO fixes and cleanup of
unused parts of the API.
Regards,
Tvrtko
2.
elem_len should not be explicitly sized u32 but unsigned int, to match
the underlying struct scatterlist nents type. Same for the nent_p output
parameter type.
Are you sure it is useful to support allocations with an order that
exceeds
(31 - PAGE_SHIFT)? Since memory gets fragmented easily in the Linux
kernel I
think that it's unlikely that such allocations will succeed.
Not sure what you are getting at here.
There are not explicit width types anywhere in the SGL API apart this
u32 elem_lem.
So I changed it to unsigned int not to confuse. It gets passed in to
sg_set_page which takes unsigned int. So no reason for it to be u32.
I renamed this to chunk_len and consolidated its use throughout the
function.
Please undo this change such that the diff remains as short as possible.
Name change only? Yeah can do that. Even though chunk as a term is
somewhat established elsewhere in lib/scatterlist.c.
-void sgl_free_n_order(struct scatterlist *sgl, int nents, int order)
+void sgl_free_n_order(struct scatterlist *sgl, unsigned int nents,
+ÂÂÂÂÂÂÂÂÂÂÂÂÂ unsigned int order)
 {
ÂÂÂÂÂ struct scatterlist *sg;
ÂÂÂÂÂ struct page *page;
-ÂÂÂ int i;
+ÂÂÂ unsigned int i;
ÂÂÂÂÂ for_each_sg(sgl, sg, nents, i) {
ÂÂÂÂÂÂÂÂÂ if (!sg)
@@ -583,9 +587,9 @@ EXPORT_SYMBOL(sgl_free_n_order);
ÂÂ * @sgl: Scatterlist with one or more elements
ÂÂ * @order: Second argument for __free_pages()
ÂÂ */
-void sgl_free_order(struct scatterlist *sgl, int order)
+void sgl_free_order(struct scatterlist *sgl, unsigned int order)
 {
-ÂÂÂ sgl_free_n_order(sgl, INT_MAX, order);
+ÂÂÂ sgl_free_n_order(sgl, UINT_MAX, order);
 }
 EXPORT_SYMBOL(sgl_free_order);
Do you have an application that calls these functions to allocate more
than
INT_MAX * PAGE_SIZE bytes at once? If not, please leave these changes
out.
There is no reason to used signed int here and it is even inconsistent
with itself because sgl_alloc_order returns you nents in an unsigned
int. And sg_init_table takes unsigned int for nents. So really I see no
reason to have signed types for nents on sgl_free side of the API.
Regards,
Tvrtko