Re: [PATCH v3 1/4] sgl_alloc_order: remove 4 GiB limit, sgl_free() warning

[Bodo Stroesser <bostroesser@gmail.com>]

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.

> 
> 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@interlog.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 */
>