Date:   Mon, 04 Jun 2018 15:39:40 +0530
From:   Chandan Rajendra <chandan@...ux.vnet.ibm.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Eric Biggers <ebiggers3@...il.com>, linux-fscrypt@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH V3 07/12] mpage_readpage[s]: Introduce post process callback parameters

On Wednesday, May 30, 2018 10:36:42 AM IST Theodore Y. Ts'o wrote:
> Note: we may want to move this thread so that it's on linux-fscrypt
> exclusively.  I'm thinking that we should consider using linux-fscrypt
> for fscrypt and fsverity discussions.  That way we can avoid adding
> extra noise to the linux-fsdevel and linux-ext4 lists.  Comments?
> 
> On Wed, May 30, 2018 at 08:39:11AM +0530, Chandan Rajendra wrote:
> > 
> > I had misunderstood the requirement. Sorry about that. I had written the
> > patchset in its current form with the understanding that fs/buffer.c and
> > fs/ext4/*.c would need to get compiled even when fscrypt code isn't compiled
> > at all. When the fscrypt module isn't selected for build in the kernel config,
> > calls to fscrypt_*() functions would end up calling the equivalent nop
> > functions in fscrypt_notsupp.h file.
> > 
> > For the generic code to be completely unaware of several stages of "post
> > processinhg" functionality, I would most likely have to add more callback
> > pointers into the newly introduced "struct post_process_read" structure. 
> 
> It's still a work in progress, but I have an initial integration of
> ext4 with fsverity.  See the fsverity branch on ext4.git, and in
> particular, the changes made to fs/ext4/readpage.c.
> 
> Let's be clear that neither Eric and I are completely happy with how
> the fsveirty post-read processing is being done.  What's there right
> now is a place-holder so we can continue to development/debug the
> other aspects of fsverity.  In particular, we're aware that there is
> code duplication between code in fs/f2fs/data.c and fs/ext4/readpage.c
> 
> One of the things which is a bit tricky right now is that fscrypt and
> fsverity can be enabled on a per-file system basis.  That is, there are
> separate CONFIG options:
> 
>    * CONFIG_EXT4_FS_ENCRYPTION
>    * CONFIG_F2FS_FS_ENCRYPTION
>    * CONFIG_EXT4_FS_VERITY
>    * CONFIG_F2FS_FS_VERITY
> 
> And in each file system, you have to do this before including the header files:
> 
> #define __FS_HAS_ENCRYPTION IS_ENABLED(CONFIG_EXT4_FS_ENCRYPTION)
> #include <linux/fscrypt.h>
> 
> #define __FS_HAS_VERITY IS_ENABLED(CONFIG_EXT4_FS_VERITY)
> #include <linux/fsverity.ha>
> 
> That's because whether you get the function prototype or an inline
> function which returns EOhPNOTSUPP for functions such as
> fscrypt_decrypt_bio() depends on how __FS_HAS_ENCRYPTION is defined
> before including linux/fscrypt.h.
> 
> I now think this was a mistake, and that we should handle this the
> same way we handle CONFIG_QUOTA.  If we enable fscrypt or fsverity, it
> should be enabled for all file sytems which support that feature.
> Otherwise it becomes too hard to try to support this in a file system
> independent way --- and we end up having code which is cut-and-pasted
> for each file system (e.g., in fs/f2fs/data.c and fs/ext4/readpage.c)
> but which may compile to something quite different thanks to the C
> preprocessor magic which is going on at the moment.
> 
> > I will work on this and post the results in the next version of the patchset.
> 
> So in order to avoid your wasting time and energy, and perhaps getting
> unnecessarily frustrated, I'd recommend that before you immediately
> start diving into implementation, that we have a design discussion
> about the best way to proceed.  And then when we have a common
> agreement about how to proceed, let's get something upstream first
> which changes the infrastructure used by the file systems and by
> fscrypt first.  And let's get that working, *before* we start
> integrating the changes for supporting fscrypt for 4k blocks for
> systems with 32k pagesize, or before we start making final (e.g.,
> non-prototype) changes to integrate fsverity.
> 
> I'll include my first attempt that I played around over the weekend
> with in terms of generic infrastructure, before I realized that we
> need to decide whether the current way we configure fscrypt and
> fsverity makes sense or not.  It's an example of why we should have
> design discussions first, and not just immediately start diving into
> code.
> 
> Fortunately (perhaps because I've some experience with these sorts of
> things) I only spent about an hour or so working on the prototype, and
> none trying to integrate it and doing all of the testing and
> debugging, before recognizing that we needed to have a meeting of the
> minds about design and requirements first.  :-)
> 
> Cheers,
> 
> 						- Ted
> 
> ------------- include/linux/bio_post_read.h
> 
> #ifndef _LINUX_BIO_POST_READ_H
> #define _LINUX_BIO_POST_READ_H
> 
> #include <linux/bio.h>
> 
> enum bio_post_read_step {
> 	STEP_INITIAL = 0,
> 	STEP_DECRYPT,
> 	STEP_VERITY,
> };
> 
> struct bio_post_read_ctx {
> 	struct bio *bio;
> 	struct work_struct work;
> 	unsigned int cur_step;
> 	unsigned int enabled_steps;
> };
> 
> static inline bool bpr_required(struct bio *bio)
> {
> 	return bio->bi_private && !bio->bi_status;
> }
> 
> extern void __bpr_read_end_io(struct bio *bio);
> void bpr_do_processing(struct bio_post_read_ctx *ctx);
> 
> #endif /* _LINUX_BIO_POST_READ_H */
> 
> ------------- fs/bio_post_read.c
> 
> // SPDX-License-Identifier: GPL-2.0
> /*
>  * fs/bio_post_read.c
>  *
>  * Copyright (C) 2018, Google LLC
>  *
>  * Contains helper functions used by file systems which use fscrypt
>  * and/or fsverity
>  */
> 
> #include <linux/mempool.h>
> #include <linux/module.h>
> #include <linux/slab.h>
> #include <linux/bio_post_read.h>
> #include <linux/pagemap.h>
> #include <linux/fscrypt.h>
> #include <linux/fsverity.h>
> 
> static unsigned int num_prealloc_post_read_ctxs = 128;
> 
> module_param(num_prealloc_post_read_ctxs, uint, 0444);
> MODULE_PARM_DESC(num_prealloc_post_read_ctxs,
> 		"Number of bio_post_read contexts to preallocate");
> 
> static struct kmem_cache *bpr_ctx_cache;
> static mempool_t *bpr_ctx_pool;
> 
> void __bpr_read_end_io(struct bio *bio)
> {
> 	struct page *page;
> 	struct bio_vec *bv;
> 	int i;
> 
> 	bio_for_each_segment_all(bv, bio, i) {
> 		page = bv->bv_page;
> 
> 		/* PG_error was set if any post_read step failed */
> 		if (bio->bi_status || PageError(page)) {
> 			ClearPageUptodate(page);
> 			SetPageError(page);
> 		} else {
> 			SetPageUptodate(page);
> 		}
> 		unlock_page(page);
> 	}
> 	if (bio->bi_private)
> 		mempool_free(bio->bi_private, bpr_ctx_pool);
> 	bio_put(bio);
> }
> 
> static void decrypt_work(struct work_struct *work)
> {
> 	struct bio_post_read_ctx *ctx =
> 		container_of(work, struct bio_post_read_ctx, work);
> 
> 	fscrypt_decrypt_bio(ctx->bio);
> 
> 	bpr_do_processing(ctx);
> }
> 
> static void verity_work(struct work_struct *work)
> {
> 	struct bio_post_read_ctx *ctx =
> 		container_of(work, struct bio_post_read_ctx, work);
> 
> 	fsverity_verify_bio(ctx->bio);
> 
> 	bpr_do_processing(ctx);
> }
> 
> void bpr_do_processing(struct bio_post_read_ctx *ctx)
> {
> 	/*
> 	 * We use different work queues for decryption and for verity because
> 	 * verity may require reading metadata pages that need decryption, and
> 	 * we shouldn't recurse to the same workqueue.
> 	 */
> 	switch (++ctx->cur_step) {
> 	case STEP_DECRYPT:
> 		if (ctx->enabled_steps & (1 << STEP_DECRYPT)) {
> 			INIT_WORK(&ctx->work, decrypt_work);
> 			fscrypt_enqueue_decrypt_work(&ctx->work);
> 			return;
> 		}
> 		ctx->cur_step++;
> 		/* fall-through */
> 	case STEP_VERITY:
> 		if (ctx->enabled_steps & (1 << STEP_VERITY)) {
> 			INIT_WORK(&ctx->work, verity_work);
> 			fsverity_enqueue_verify_work(&ctx->work);
> 			return;
> 		}
> 		ctx->cur_step++;
> 		/* fall-through */
> 	default:
> 		__bpr_read_end_io(ctx->bio);
> 	}
> }

decrypt_work() and verity_work() can be defined in fscrypt and fsverity
modules. inode->[i_crypt_info|i_verity_info] can hold the pointers to
decrypt_work() and verity_work() functions. The newly introduced function
pointers in inode->[i_crypt_info|i_verity_info] can be initialized in
fscrypt_get_encryption_info() and create_fsverity_info() respectively.

'struct bio_post_read_ctx' should have a new member to store a pointer to the
bpr_do_processing() function. Once *_work() functions complete their job, they
could invoke ctx->do_processing() to continue with the next stage of bio
processing.


> 
> int __init bpr_init(void)
> {
> 	bpr_ctx_cache = KMEM_CACHE(bio_post_read_ctx, 0);
> 	if (!bpr_ctx_cache)
> 		goto fail;
> 	bpr_ctx_pool = mempool_create_slab_pool(num_prealloc_post_read_ctxs,
> 						bpr_ctx_cache);
> 	if (!bpr_ctx_pool)
> 		goto fail_free_cache;
> 	return 0;
> 
> fail_free_cache:
> 	kmem_cache_destroy(bpr_ctx_cache);
> fail:
> 	return -ENOMEM;
> }
> 
> void __exit bpr_exit(void)
> {
> 	mempool_destroy(bpr_ctx_pool);
> 	kmem_cache_destroy(bpr_ctx_cache);
> }
> 
> module_init(bpr_init);
> module_exit(bpr_exit);
> MODULE_LICENSE("GPL");
> 
> 


-- 
chandan

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