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([2a02:8071:5055:3f20:7ad9:a400:6d51:83e6]) by smtp.gmail.com with ESMTPSA id q4sm16466877wrs.56.2021.11.08.01.58.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 08 Nov 2021 01:58:07 -0800 (PST) Message-ID: Date: Mon, 8 Nov 2021 10:58:06 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: [PATCH 5/7] block: Pass BdrvChild ** to replace_child_noperm To: Kevin Wolf References: <20211104103849.46855-1-hreitz@redhat.com> <20211104103849.46855-6-hreitz@redhat.com> From: Hanna Reitz In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=hreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=hreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -65 X-Spam_score: -6.6 X-Spam_bar: ------ X-Spam_report: (-6.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.7, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-3.06, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 05.11.21 16:15, Kevin Wolf wrote: > Am 04.11.2021 um 11:38 hat Hanna Reitz geschrieben: >> bdrv_replace_child_noperm() modifies BdrvChild.bs, and can potentially >> set it to NULL. That is dangerous, because BDS parents generally assume >> that their children's .bs pointer is never NULL. We therefore want to >> let bdrv_replace_child_noperm() set the corresponding BdrvChild pointer >> to NULL, too. >> >> This patch lays the foundation for it by passing a BdrvChild ** pointer >> to bdrv_replace_child_noperm() so that it can later use it to NULL the >> BdrvChild pointer immediately after setting BdrvChild.bs to NULL. >> >> Signed-off-by: Hanna Reitz >> --- >> block.c | 59 ++++++++++++++++++++++++++++++++------------------------- >> 1 file changed, 33 insertions(+), 26 deletions(-) >> >> diff --git a/block.c b/block.c >> index 6d230ad3d1..ff45447686 100644 >> --- a/block.c >> +++ b/block.c >> @@ -87,7 +87,7 @@ static BlockDriverState *bdrv_open_inherit(const char *filename, >> static bool bdrv_recurse_has_child(BlockDriverState *bs, >> BlockDriverState *child); >> >> -static void bdrv_replace_child_noperm(BdrvChild *child, >> +static void bdrv_replace_child_noperm(BdrvChild **child, >> BlockDriverState *new_bs); >> static void bdrv_remove_file_or_backing_child(BlockDriverState *bs, >> BdrvChild *child, >> @@ -2254,6 +2254,7 @@ static int bdrv_drv_set_perm(BlockDriverState *bs, uint64_t perm, >> >> typedef struct BdrvReplaceChildState { >> BdrvChild *child; >> + BdrvChild **childp; > Would it be clearer to rename child to old_child now that it can be > changed? > > I would also consider having childp first because this is really the > object that we're modifying and potentially rolling back now. > >> BlockDriverState *old_bs; >> } BdrvReplaceChildState; >> >> @@ -2269,8 +2270,8 @@ static void bdrv_replace_child_abort(void *opaque) >> BdrvReplaceChildState *s = opaque; >> BlockDriverState *new_bs = s->child->bs; >> >> - /* old_bs reference is transparently moved from @s to @s->child */ >> - bdrv_replace_child_noperm(s->child, s->old_bs); >> + /* old_bs reference is transparently moved from @s to *s->childp */ >> + bdrv_replace_child_noperm(s->childp, s->old_bs); >> bdrv_unref(new_bs); >> } >> >> @@ -2287,21 +2288,23 @@ static TransactionActionDrv bdrv_replace_child_drv = { >> * >> * The function doesn't update permissions, caller is responsible for this. >> */ >> -static void bdrv_replace_child_tran(BdrvChild *child, BlockDriverState *new_bs, >> +static void bdrv_replace_child_tran(BdrvChild **childp, >> + BlockDriverState *new_bs, >> Transaction *tran) >> { >> BdrvReplaceChildState *s = g_new(BdrvReplaceChildState, 1); >> *s = (BdrvReplaceChildState) { >> - .child = child, >> - .old_bs = child->bs, >> + .child = *childp, >> + .childp = childp, >> + .old_bs = (*childp)->bs, >> }; >> tran_add(tran, &bdrv_replace_child_drv, s); >> >> if (new_bs) { >> bdrv_ref(new_bs); >> } >> - bdrv_replace_child_noperm(child, new_bs); >> - /* old_bs reference is transparently moved from @child to @s */ >> + bdrv_replace_child_noperm(childp, new_bs); >> + /* old_bs reference is transparently moved from *childp to @s */ >> } >> >> /* >> @@ -2672,9 +2675,10 @@ uint64_t bdrv_qapi_perm_to_blk_perm(BlockPermission qapi_perm) >> return permissions[qapi_perm]; >> } >> >> -static void bdrv_replace_child_noperm(BdrvChild *child, >> +static void bdrv_replace_child_noperm(BdrvChild **childp, >> BlockDriverState *new_bs) >> { >> + BdrvChild *child = *childp; >> BlockDriverState *old_bs = child->bs; >> int new_bs_quiesce_counter; >> int drain_saldo; >> @@ -2767,7 +2771,7 @@ static void bdrv_attach_child_common_abort(void *opaque) >> BdrvChild *child = *s->child; >> BlockDriverState *bs = child->bs; >> >> - bdrv_replace_child_noperm(child, NULL); >> + bdrv_replace_child_noperm(s->child, NULL); >> >> if (bdrv_get_aio_context(bs) != s->old_child_ctx) { >> bdrv_try_set_aio_context(bs, s->old_child_ctx, &error_abort); >> @@ -2867,7 +2871,7 @@ static int bdrv_attach_child_common(BlockDriverState *child_bs, >> } >> >> bdrv_ref(child_bs); >> - bdrv_replace_child_noperm(new_child, child_bs); >> + bdrv_replace_child_noperm(&new_child, child_bs); >> >> *child = new_child; >> >> @@ -2927,12 +2931,12 @@ static int bdrv_attach_child_noperm(BlockDriverState *parent_bs, >> return 0; >> } >> >> -static void bdrv_detach_child(BdrvChild *child) >> +static void bdrv_detach_child(BdrvChild **childp) >> { >> - BlockDriverState *old_bs = child->bs; >> + BlockDriverState *old_bs = (*childp)->bs; >> >> - bdrv_replace_child_noperm(child, NULL); >> - bdrv_child_free(child); >> + bdrv_replace_child_noperm(childp, NULL); >> + bdrv_child_free(*childp); >> >> if (old_bs) { >> /* >> @@ -3038,7 +3042,7 @@ void bdrv_root_unref_child(BdrvChild *child) >> BlockDriverState *child_bs; >> >> child_bs = child->bs; >> - bdrv_detach_child(child); >> + bdrv_detach_child(&child); >> bdrv_unref(child_bs); >> } >> >> @@ -4891,30 +4895,33 @@ static void bdrv_remove_file_or_backing_child(BlockDriverState *bs, >> BdrvChild *child, >> Transaction *tran) >> { >> + BdrvChild **childp; >> BdrvRemoveFilterOrCowChild *s; >> >> - assert(child == bs->backing || child == bs->file); >> + if (child == bs->backing) { >> + childp = &bs->backing; >> + } else if (child == bs->file) { >> + childp = &bs->file; >> + } else { >> + g_assert_not_reached(); >> + } >> >> if (!child) { >> return; >> } >> >> if (child->bs) { >> - bdrv_replace_child_tran(child, NULL, tran); >> + bdrv_replace_child_tran(childp, NULL, tran); >> } >> >> s = g_new(BdrvRemoveFilterOrCowChild, 1); >> *s = (BdrvRemoveFilterOrCowChild) { >> .child = child, >> - .is_backing = (child == bs->backing), >> + .is_backing = (childp == &bs->backing), >> }; >> tran_add(tran, &bdrv_remove_filter_or_cow_child_drv, s); >> >> - if (s->is_backing) { >> - bs->backing = NULL; >> - } else { >> - bs->file = NULL; >> - } >> + *childp = NULL; >> } > Hmm... This looks a bit dangerous. Is it guaranteed that bs lives until > the end of the transaction? I guess it has to because we want to be able > to roll back, so probably this is okay, though I'm not sure if I would > bet money on it. Well, if it should live long enough, a bdrv_ref()+unref() definitely shouldn’t hurt. > But... > >> @@ -4950,7 +4957,7 @@ static int bdrv_replace_node_noperm(BlockDriverState *from, >> c->name, from->node_name); >> return -EPERM; >> } >> - bdrv_replace_child_tran(c, to, tran); >> + bdrv_replace_child_tran(&c, to, tran); >> } > ...here, c is a local stack variable that is even reused in a loop. > bdrv_replace_child_tran() now keeps a pointer to the same variable in > the transaction state for each BdrvChild in the whole parent list, and > by the time the transaction is finalised, I think they are all dangling > pointers anyway because they pointed to stack variables in a function > that has already returned. Oh no.  Yes, that’s a bit of a problem... > bdrv_replace_child_tran() should probably have a comment like we already > have in bdrv_attach_child_common(): > > * @child is saved to a new entry of @tran, so that *@child could be reverted to > * NULL on abort(). So referenced variable must live at least until transaction > * end. Right.  Now just to figure out how to solve the concrete problem... :/ (Yes, a borrow checker would’ve helped :)) Hanna