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

[Douglas Gilbert]

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 */