On 2020-11-03 7:54 a.m., Bodo Stroesser wrote:
Am 19.10.20 um 21:19 schrieb Douglas Gilbert:
This patch removes a check done by sgl_alloc_order() before it starts
any allocations. The comment before the removed code says: "Check for
integer overflow" arguably gives a false sense of security. The right
hand side of the expression in the condition is resolved as u32 so
cannot exceed UINT32_MAX (4 GiB) which means 'length' cannot exceed
that amount. If that was the intention then the comment above it
could be dropped and the condition rewritten more clearly as:
if (length > UINT32_MAX) <<failure path >>;
I think the intention of the check is to reject calls, where length is so high, that calculation of nent overflows unsigned int nent/nalloc.
Consistently a similar check is done few lines later before incrementing nalloc due to chainable = true.
So I think the code tries to allow length values up to 4G << (PAGE_SHIFT + order).
That said I think instead of removing the check it better should be fixed, e.g. by adding an unsigned long long cast before nent
BTW: I don't know why there are two checks. I think one check after conditionally incrementing nalloc would be enough.
Okay, I'm working on a "v4" patchset. Apart from the above, my plan is
to extend sgl_compare_sgl() with a helper that additionally yields
the byte index of the first miscompare.
Doug Gilbert
The author's intention is to use sgl_alloc_order() to replace
vmalloc(unsigned long) for a large allocation (debug ramdisk).
vmalloc has no limit at 4 GiB so its seems unreasonable that:
sgl_alloc_order(unsigned long long length, ....)
does. sgl_s made with sgl_alloc_order(chainable=false) have equally
sized segments placed in a scatter gather array. That allows O(1)
navigation around a big sgl using some simple integer maths.
Having previously sent a patch to fix a memory leak in
sg_alloc_order() take the opportunity to put a one line comment above
sgl_free()'s declaration that it is not suitable when order > 0 . The
mis-use of sgl_free() when order > 0 was the reason for the memory
leak. The other users of sgl_alloc_order() in the kernel where
checked and found to handle free-ing properly.
Signed-off-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>
---
include/linux/scatterlist.h | 1 +
lib/scatterlist.c | 3 ---
2 files changed, 1 insertion(+), 3 deletions(-)
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 45cf7b69d852..80178afc2a4a 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -302,6 +302,7 @@ struct scatterlist *sgl_alloc(unsigned long long length, gfp_t gfp,
unsigned int *nent_p);
void sgl_free_n_order(struct scatterlist *sgl, int nents, int order);
void sgl_free_order(struct scatterlist *sgl, int order);
+/* Only use sgl_free() when order is 0 */
void sgl_free(struct scatterlist *sgl);
#endif /* CONFIG_SGL_ALLOC */
diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index c448642e0f78..d5770e7f1030 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -493,9 +493,6 @@ struct scatterlist *sgl_alloc_order(unsigned long long length,
u32 elem_len;
nent = round_up(length, PAGE_SIZE << order) >> (PAGE_SHIFT + order);
- /* Check for integer overflow */
- if (length > (nent << (PAGE_SHIFT + order)))
- return NULL;
nalloc = nent;
if (chainable) {
/* Check for integer overflow */
