[PATCH 5/9 next] scsi: Use iovec_import() instead of import_iovec().

[PATCH 5/9 next] scsi: Use iovec_import() instead of import_iovec().

[David Laight]

iovec_import() has a safer calling convention than import_iovec().

Signed-off-by: David Laight <david.laight@aculab.com>
---
 block/scsi_ioctl.c | 14 ++++++++------
 drivers/scsi/sg.c  | 14 +++++++-------
 2 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/block/scsi_ioctl.c b/block/scsi_ioctl.c
index ef722f04f88a..0343918a84d3 100644
--- a/block/scsi_ioctl.c
+++ b/block/scsi_ioctl.c
@@ -331,20 +331,22 @@ static int sg_io(struct request_queue *q, struct gendisk *bd_disk,
 	ret = 0;
 	if (hdr->iovec_count) {
 		struct iov_iter i;
-		struct iovec *iov = NULL;
+		struct iovec *iov;
 
 #ifdef CONFIG_COMPAT
 		if (in_compat_syscall())
-			ret = compat_import_iovec(rq_data_dir(rq),
+			iov = compat_iovec_import(rq_data_dir(rq),
 				   hdr->dxferp, hdr->iovec_count,
-				   0, &iov, &i);
+				   NULL, &i);
 		else
 #endif
-			ret = import_iovec(rq_data_dir(rq),
+			iov = iovec_import(rq_data_dir(rq),
 				   hdr->dxferp, hdr->iovec_count,
-				   0, &iov, &i);
-		if (ret < 0)
+				   NULL, &i);
+		if (IS_ERR(iov)) {
+			ret = PTR_ERR(iov);
 			goto out_free_cdb;
+		}
 
 		/* SG_IO howto says that the shorter of the two wins */
 		iov_iter_truncate(&i, hdr->dxfer_len);
diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 20472aaaf630..1dbc0a74add5 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -1817,19 +1817,19 @@ sg_start_req(Sg_request *srp, unsigned char *cmd)
 	}
 
 	if (iov_count) {
-		struct iovec *iov = NULL;
+		struct iovec *iov;
 		struct iov_iter i;
 
 #ifdef CONFIG_COMPAT
 		if (in_compat_syscall())
-			res = compat_import_iovec(rw, hp->dxferp, iov_count,
-						  0, &iov, &i);
+			iov = compat_iovec_import(rw, hp->dxferp, iov_count,
+						  NULL, &i);
 		else
 #endif
-			res = import_iovec(rw, hp->dxferp, iov_count,
-					   0, &iov, &i);
-		if (res < 0)
-			return res;
+			iov = iovec_import(rw, hp->dxferp, iov_count,
+					   NULL, &i);
+		if (IS_ERR(iov))
+			return PTR_ERR(iov);
 
 		iov_iter_truncate(&i, hp->dxfer_len);
 		if (!iov_iter_count(&i)) {
-- 
2.25.1

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Re: [PATCH 5/9 next] scsi: Use iovec_import() instead of import_iovec().

[Christoph Hellwig]

So looking at the various callers I'm not sure this API is the
best.  If we want to do something fancy I'd hide the struct iovec
instances entirely with something like:

struct iov_storage {
	struct iovec stack[UIO_FASTIOV], *vec;
}

int iov_iter_import_iovec(struct iov_iter *iter, struct iov_storage *s,
		const struct iovec __user *vec, unsigned long nr_segs,
		int type);

and then add a new helper to free the thing if needed:

void iov_iter_release_iovec(struct iov_storage *s)
{
	if (s->vec != s->stack)
		kfree(s->vec);
}

RE: [PATCH 5/9 next] scsi: Use iovec_import() instead of import_iovec().

[David Laight]

From: Christoph Hellwig
> Sent: 21 September 2020 15:22
> 
> So looking at the various callers I'm not sure this API is the
> best.  If we want to do something fancy I'd hide the struct iovec
> instances entirely with something like:
> 
> struct iov_storage {
> 	struct iovec stack[UIO_FASTIOV], *vec;
> }
> 
> int iov_iter_import_iovec(struct iov_iter *iter, struct iov_storage *s,
> 		const struct iovec __user *vec, unsigned long nr_segs,
> 		int type);
> 
> and then add a new helper to free the thing if needed:
> 
> void iov_iter_release_iovec(struct iov_storage *s)
> {
> 	if (s->vec != s->stack)
> 		kfree(s->vec);
> }

I didn't think of going that far.
There are 2 call sites (in scsi) that don't pass the cache.

Given that the 'buffer to free' address probably needs to
be spilled to stack forcing in into an on-stack structure
that is already passed by address is probably a good idea.

The iov_iter_release_iovec() should be static inline and just:
	if (s->vec)
		kfree(s->vec);
You want the test because 99.99% of the time it will be NULL.
The kernel iov[] to use is iter.iov not part of the cache.

That will be a bigger change on the io_uring code.
(The patch I didn't write.)

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

RE: [PATCH 5/9 next] scsi: Use iovec_import() instead of import_iovec().

[David Laight]

From: Christoph Hellwig
> Sent: 21 September 2020 15:22
> 
> So looking at the various callers I'm not sure this API is the
> best.  If we want to do something fancy I'd hide the struct iovec
> instances entirely with something like:
> 
> struct iov_storage {
> 	struct iovec stack[UIO_FASTIOV], *vec;
> }
> 
> int iov_iter_import_iovec(struct iov_iter *iter, struct iov_storage *s,
> 		const struct iovec __user *vec, unsigned long nr_segs,
> 		int type);
> 
> and then add a new helper to free the thing if needed:
> 
> void iov_iter_release_iovec(struct iov_storage *s)
> {
> 	if (s->vec != s->stack)
> 		kfree(s->vec);
> }

I've been looking at this code again now most of the pending changes
are in Linus's tree.

I was actually looking at going one stage further.
The 'iov_iter' is always allocated with the 'iov_storage' *above).
Usually both are on the callers stack - possibly in different functions.

So add:
struct iovec_iter {
	struct iov_iter iter;
	struct iovec to_free;
	struct iovec stack[UIO_FASTIOV];
};

int __iovec_import(struct iovec_iter *, const struct iovec __user *vec,
	unsigned long nr_segs, int type, bool compat);

And a 'clean' function to do kfree(iovec->to_free);

This reduces the complexity of most of the callers.

I started doing the changes, but got in a mess in io_uring.c (as usual).
I think I've got a patch pending (in my brain) to simplify the io_uring code.

The plan is to add:
	if (iter->iov != xxx->to_free)
		iter->iov = xxx->stack;
Prior to every use of the iter.
This fixes up anything that got broken by a memcpy() of the fields.
The tidyup code is then always kfree(xxx->to_free).

	David

	

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)