linux-kernel - [PATCH v3 1/4] sgl_alloc_order: remove 4 GiB limit, sgl

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Date:   Mon, 19 Oct 2020 15:19:25 -0400
From:   Douglas Gilbert <dgilbert@...erlog.com>
To:     linux-scsi@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     martin.petersen@...cle.com, axboe@...nel.dk, bvanassche@....org,
        bostroesser@...il.com
Subject: [PATCH v3 1/4] sgl_alloc_order: remove 4 GiB limit, sgl_free() warning

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 >>;

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@...erlog.com>
---
 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 */
-- 
2.25.1