From: Eric Biggers <email@example.com> To: firstname.lastname@example.org Cc: email@example.com, firstname.lastname@example.org, email@example.com, Satya Tangirala <firstname.lastname@example.org> Subject: [PATCH v2 4/4] blk-crypto: update inline encryption documentation Date: Thu, 16 Sep 2021 10:22:49 -0700 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <firstname.lastname@example.org> From: Eric Biggers <email@example.com> Rework most of inline-encryption.rst to be easier to follow, to correct some information, to add some important details and remove some unimportant details, and to take into account the renaming from blk_keyslot_manager to blk_crypto_profile. Signed-off-by: Eric Biggers <firstname.lastname@example.org> --- Documentation/block/inline-encryption.rst | 439 ++++++++++++---------- 1 file changed, 236 insertions(+), 203 deletions(-) diff --git a/Documentation/block/inline-encryption.rst b/Documentation/block/inline-encryption.rst index 7f9b40d6b416b..2af53915654a9 100644 --- a/Documentation/block/inline-encryption.rst +++ b/Documentation/block/inline-encryption.rst @@ -8,229 +8,262 @@ Background ========== Inline encryption hardware sits logically between memory and the disk, and can -en/decrypt data as it goes in/out of the disk. Inline encryption hardware has a -fixed number of "keyslots" - slots into which encryption contexts (i.e. the -encryption key, encryption algorithm, data unit size) can be programmed by the -kernel at any time. Each request sent to the disk can be tagged with the index -of a keyslot (and also a data unit number to act as an encryption tweak), and -the inline encryption hardware will en/decrypt the data in the request with the -encryption context programmed into that keyslot. This is very different from -full disk encryption solutions like self encrypting drives/TCG OPAL/ATA -Security standards, since with inline encryption, any block on disk could be -encrypted with any encryption context the kernel chooses. - +en/decrypt data as it goes in/out of the disk. For each I/O request, software +can control exactly how the inline encryption hardware will en/decrypt the data +in terms of key, algorithm, data unit size (the granularity of en/decryption), +and data unit number (a value that determines the initialization vector(s)). + +Some inline encryption hardware accepts the key, algorithm, and data unit size +directly in I/O requests. However, most inline encryption hardware instead has +a fixed number of "keyslots" and requires that the key, algorithm, and data unit +size first be programmed into a keyslot. I/O requests then just reference +keyslot indices. + +Note that inline encryption hardware is very different from "self-encrypting +drives", such as those based on the TCG Opal or ATA Security standards. +Self-encrypting drives don't provide fine-grained software control of encryption +and provide no way to verify the correctness of the resulting ciphertext. In +contrast, inline encryption hardware provides fine-grained control of +encryption, including the choice of key and initialization vector for each +sector, and can be tested for correctness. Objective ========= -We want to support inline encryption (IE) in the kernel. -To allow for testing, we also want a crypto API fallback when actual -IE hardware is absent. We also want IE to work with layered devices -like dm and loopback (i.e. we want to be able to use the IE hardware -of the underlying devices if present, or else fall back to crypto API -en/decryption). - +We want to support inline encryption in the kernel. To make testing easier, we +also want support for falling back to the kernel crypto API when actual inline +encryption hardware is absent. We also want inline encryption to work with +layered devices like device-mapper and loopback (i.e. we want to be able to use +the inline encryption hardware of the underlying devices if present, or else +fall back to crypto API en/decryption). Constraints and notes ===================== -- IE hardware has a limited number of "keyslots" that can be programmed - with an encryption context (key, algorithm, data unit size, etc.) at any time. - One can specify a keyslot in a data request made to the device, and the - device will en/decrypt the data using the encryption context programmed into - that specified keyslot. When possible, we want to make multiple requests with - the same encryption context share the same keyslot. - -- We need a way for upper layers like filesystems to specify an encryption - context to use for en/decrypting a struct bio, and a device driver (like UFS) - needs to be able to use that encryption context when it processes the bio. - -- We need a way for device drivers to expose their inline encryption - capabilities in a unified way to the upper layers. - - -Design -====== - -We add a struct bio_crypt_ctx to struct bio that can -represent an encryption context, because we need to be able to pass this -encryption context from the upper layers (like the fs layer) to the -device driver to act upon. - -While IE hardware works on the notion of keyslots, the FS layer has no -knowledge of keyslots - it simply wants to specify an encryption context to -use while en/decrypting a bio. - -We introduce a keyslot manager (KSM) that handles the translation from -encryption contexts specified by the FS to keyslots on the IE hardware. -This KSM also serves as the way IE hardware can expose its capabilities to -upper layers. The generic mode of operation is: each device driver that wants -to support IE will construct a KSM and set it up in its struct request_queue. -Upper layers that want to use IE on this device can then use this KSM in -the device's struct request_queue to translate an encryption context into -a keyslot. The presence of the KSM in the request queue shall be used to mean -that the device supports IE. - -The KSM uses refcounts to track which keyslots are idle (either they have no -encryption context programmed, or there are no in-flight struct bios -referencing that keyslot). When a new encryption context needs a keyslot, it -tries to find a keyslot that has already been programmed with the same -encryption context, and if there is no such keyslot, it evicts the least -recently used idle keyslot and programs the new encryption context into that -one. If no idle keyslots are available, then the caller will sleep until there -is at least one. - - -blk-mq changes, other block layer changes and blk-crypto-fallback -================================================================= - -We add a pointer to a ``bi_crypt_context`` and ``keyslot`` to -struct request. These will be referred to as the ``crypto fields`` -for the request. This ``keyslot`` is the keyslot into which the -``bi_crypt_context`` has been programmed in the KSM of the ``request_queue`` -that this request is being sent to. - -We introduce ``block/blk-crypto-fallback.c``, which allows upper layers to remain -blissfully unaware of whether or not real inline encryption hardware is present -underneath. When a bio is submitted with a target ``request_queue`` that doesn't -support the encryption context specified with the bio, the block layer will -en/decrypt the bio with the blk-crypto-fallback. - -If the bio is a ``WRITE`` bio, a bounce bio is allocated, and the data in the bio -is encrypted stored in the bounce bio - blk-mq will then proceed to process the -bounce bio as if it were not encrypted at all (except when blk-integrity is -concerned). ``blk-crypto-fallback`` sets the bounce bio's ``bi_end_io`` to an -internal function that cleans up the bounce bio and ends the original bio. - -If the bio is a ``READ`` bio, the bio's ``bi_end_io`` (and also ``bi_private``) -is saved and overwritten by ``blk-crypto-fallback`` to -``bio_crypto_fallback_decrypt_bio``. The bio's ``bi_crypt_context`` is also -overwritten with ``NULL``, so that to the rest of the stack, the bio looks -as if it was a regular bio that never had an encryption context specified. -``bio_crypto_fallback_decrypt_bio`` will decrypt the bio, restore the original -``bi_end_io`` (and also ``bi_private``) and end the bio again. - -Regardless of whether real inline encryption hardware is used or the +- We need a way for upper layers (e.g. filesystems) to specify an encryption + context to use for en/decrypting a bio, and device drivers (e.g. UFSHCD) need + to be able to use that encryption context when they process the request. + Encryption contexts also introduce constraints on bio merging; the block layer + needs to be aware of these constraints. + +- Different inline encryption hardware has different supported algorithms, + supported data unit sizes, maximum data unit numbers, etc. We call these + properties the "crypto capabilities". We need a way for device drivers to + advertise crypto capabilities to upper layers in a generic way. + +- Inline encryption hardware usually (but not always) requires that keys be + programmed into keyslots before being used. Since programming keyslots may be + slow and there may not be very many keyslots, we shouldn't just program the + key for every I/O request, but rather keep track of which keys are in the + keyslots and reuse an already-programmed keyslot when possible. + +- Upper layers typically define a specific end-of-life for crypto keys, e.g. + when an encrypted directory is locked or when a crypto mapping is torn down. + At these times, keys are wiped from memory. We must provide a way for upper + layers to also evict keys from any keyslots they are present in. + +- When possible, device-mapper devices must be able to pass through the inline + encryption support of their underlying devices. However, it doesn't make + sense for device-mapper devices to have keyslots themselves. + +Basic design +============ + +We introduce ``struct blk_crypto_key`` to represent an inline encryption key and +how it will be used. This includes the actual bytes of the key; the size of the +key; the algorithm and data unit size the key will be used with; and the number +of bytes needed to represent the maximum data unit number the key will be used +with. + +We introduce ``struct bio_crypt_ctx`` to represent an encryption context. It +contains a data unit number and a pointer to a blk_crypto_key. We add pointers +to a bio_crypt_ctx to ``struct bio`` and ``struct request``; this allows users +of the block layer (e.g. filesystems) to provide an encryption context when +creating a bio and have it be passed down the stack for processing by the block +layer and device drivers. Note that the encryption context doesn't explicitly +say whether to encrypt or decrypt, as that is implicit from the direction of the +bio; WRITE means encrypt, and READ means decrypt. + +We also introduce ``struct blk_crypto_profile`` to contain all generic inline +encryption-related state for a particular inline encryption device. The +blk_crypto_profile serves as the way that drivers for inline encryption hardware +advertise their crypto capabilities and provide certain functions (e.g., +functions to program and evict keys) to upper layers. Each device driver that +wants to support inline encryption will construct a blk_crypto_profile, then +associate it with the disk's request_queue. + +The blk_crypto_profile also manages the hardware's keyslots, when applicable. +This happens in the block layer, so that users of the block layer can just +specify encryption contexts and don't need to know about keyslots at all, nor do +device drivers need to care about most details of keyslot management. + +Specifically, for each keyslot, the block layer (via the blk_crypto_profile) +keeps track of which blk_crypto_key that keyslot contains (if any), and how many +in-flight I/O requests are using it. When the block layer creates a ``struct +request`` for a bio that has an encryption context, it grabs a keyslot that +already contains the key if possible. Otherwise it waits for an idle keyslot (a +keyslot that was either never used or one that isn't in-use by any I/O), then +programs the key into the least-recently-used idle keyslot using the function +the device driver provided. In both cases, the resulting keyslot is stored in +the ``crypt_keyslot`` field of the request, where it is then accessible to +device drivers, and is released after the request completes. + +``struct request`` also contains a pointer to the original bio_crypt_ctx, in +addition to the keyslot. Requests can be built from multiple bios, and the +block layer must take the encryption context into account when trying to merge +bios and requests. For two bios/requests to be merged, they must have +compatible encryption contexts: both unencrypted, or both encrypted with the +same key and contiguous data unit numbers. Only the encryption context for the +first bio in a request is retained, since the remaining bios have been verified +to be merge-compatible with the first bio. + +To make it possible for inline encryption to work with request_queue based +layered devices, when a request is cloned, its encryption context is cloned as +well. When the cloned request is submitted, it is then processed as usual; this +includes getting a keyslot from the clone's target device if needed. + +blk-crypto-fallback +=================== + +It is desirable for the inline encryption support of upper layers (e.g. +filesystems) to be testable without real inline encryption hardware, and +likewise for the block layer's keyslot management logic. It is also desirable +to allow upper layers to just always use inline encryption rather than +implementing encryption in multiple ways. + +Therefore, we also introduce "blk-crypto-fallback", which is an implementation +of inline encryption using the kernel crypto API. blk-crypto-fallback is built +into the block layer, so it works on any block device without any special setup. +Essentially, when a bio with an encryption context is submitted to a +request_queue that doesn't support that encryption context, the block layer will +handle en/decryption of the bio using blk-crypto-fallback. + +For encryption, the data cannot be encrypted in-place, as callers usually rely +on it being unmodified. Instead, blk-crypto-fallback allocates bounce pages, +fills a new bio with those bounce pages, encrypts the data into those bounce +pages, and submits that "bounce" bio. When the bounce bio completes, +blk-crypto-fallback completes the original bio. If the original bio is too +large, multiple bounce bios may be required; see the code for details. + +For decryption, blk-crypto-fallback "wraps" the bio's completion callback +(``bi_complete``) and private data (``bi_private``) with its own, unsets the +bio's encryption context, then submits the bio. If the read completes +successfully, blk-crypto-fallback restores the bio's original completion +callback and private data, then decrypts the bio's data in-place using the +kernel crypto API. Decryption happens from a workqueue, as it may sleep. +Afterwards, blk-crypto-fallback completes the bio. + +In both cases, the bios that blk-crypto-fallback submits no longer have an +encryption context. Therefore, lower layers only see standard unencrypted I/O. + +blk-crypto-fallback also defines its own blk_crypto_profile and has its own +"keyslots"; its keyslots contain ``struct crypto_skcipher`` objects. The reason +for this is twofold. First, it allows the keyslot management logic to be tested +without actual inline encryption hardware. Second, similar to actual inline +encryption hardware, the crypto API doesn't accept keys directly in requests but +rather requires that they be set again of time, and setting keys can be +expensive; moreover, allocating a crypto_skcipher can't happen on the I/O path +at all due to the locks it takes. Therefore, the concept of keyslots still +makes sense for blk-crypto-fallback. + +Note that regardless of whether real inline encryption hardware or blk-crypto-fallback is used, the ciphertext written to disk (and hence the on-disk format of data) will be the same (assuming the hardware's implementation of the algorithm being used adheres to spec and functions correctly). -If a ``request queue``'s inline encryption hardware claimed to support the -encryption context specified with a bio, then it will not be handled by the -``blk-crypto-fallback``. We will eventually reach a point in blk-mq when a -struct request needs to be allocated for that bio. At that point, -blk-mq tries to program the encryption context into the ``request_queue``'s -keyslot_manager, and obtain a keyslot, which it stores in its newly added -``keyslot`` field. This keyslot is released when the request is completed. - -When the first bio is added to a request, ``blk_crypto_rq_bio_prep`` is called, -which sets the request's ``crypt_ctx`` to a copy of the bio's -``bi_crypt_context``. bio_crypt_do_front_merge is called whenever a subsequent -bio is merged to the front of the request, which updates the ``crypt_ctx`` of -the request so that it matches the newly merged bio's ``bi_crypt_context``. In particular, the request keeps a copy of the ``bi_crypt_context`` of the first -bio in its bio-list (blk-mq needs to be careful to maintain this invariant -during bio and request merges). - -To make it possible for inline encryption to work with request queue based -layered devices, when a request is cloned, its ``crypto fields`` are cloned as -well. When the cloned request is submitted, blk-mq programs the -``bi_crypt_context`` of the request into the clone's request_queue's keyslot -manager, and stores the returned keyslot in the clone's ``keyslot``. - +blk-crypto-fallback is optional and is controlled by the +``CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK`` kernel configuration option. API presented to users of the block layer ========================================= -``struct blk_crypto_key`` represents a crypto key (the raw key, size of the -key, the crypto algorithm to use, the data unit size to use, and the number of -bytes required to represent data unit numbers that will be specified with the -``bi_crypt_context``). - -``blk_crypto_init_key`` allows upper layers to initialize such a -``blk_crypto_key``. - -``bio_crypt_set_ctx`` should be called on any bio that a user of -the block layer wants en/decrypted via inline encryption (or the -blk-crypto-fallback, if hardware support isn't available for the desired -crypto configuration). This function takes the ``blk_crypto_key`` and the -data unit number (DUN) to use when en/decrypting the bio. - -``blk_crypto_config_supported`` allows upper layers to query whether or not the -an encryption context passed to request queue can be handled by blk-crypto -(either by real inline encryption hardware, or by the blk-crypto-fallback). -This is useful e.g. when blk-crypto-fallback is disabled, and the upper layer -wants to use an algorithm that may not supported by hardware - this function -lets the upper layer know ahead of time that the algorithm isn't supported, -and the upper layer can fallback to something else if appropriate. - -``blk_crypto_start_using_key`` - Upper layers must call this function on -``blk_crypto_key`` and a ``request_queue`` before using the key with any bio -headed for that ``request_queue``. This function ensures that either the -hardware supports the key's crypto settings, or the crypto API fallback has -transforms for the needed mode allocated and ready to go. Note that this -function may allocate an ``skcipher``, and must not be called from the data -path, since allocating ``skciphers`` from the data path can deadlock. - -``blk_crypto_evict_key`` *must* be called by upper layers before a -``blk_crypto_key`` is freed. Further, it *must* only be called only once -there are no more in-flight requests that use that ``blk_crypto_key``. -``blk_crypto_evict_key`` will ensure that a key is removed from any keyslots in -inline encryption hardware that the key might have been programmed into (or the blk-crypto-fallback). +``blk_crypto_init_key()`` allows users to initialize a blk_crypto_key. + +``blk_crypto_config_supported()`` allows users to check ahead of time whether +inline encryption with particular crypto settings will work on a particular +request_queue -- either via hardware or via blk-crypto-fallback. This function +takes in a ``struct blk_crypto_config`` which is like blk_crypto_key, but omits +the actual bytes of the key and instead just contains the algorithm, data unit +size, etc. This function can be useful if blk-crypto-fallback is disabled. + +Users must call ``blk_crypto_start_using_key()`` before actually starting to use +a blk_crypto_key on a request_queue (even if ``blk_crypto_config_supported()`` +was called earlier). This is needed to initialize blk-crypto-fallback if it +will be needed. This must not be called from the data path, as this may have to +allocate resources, which may deadlock in that case. + +Next, to attach an encryption context to a bio, users should call +``bio_crypt_set_ctx()``. This function allocates a bio_crypt_ctx and attaches +it to a bio, given the blk_crypto_key and the data unit number that will be used +for en/decryption. Users don't need to worry about freeing the bio_crypt_ctx +later, as that happens automatically when the bio is freed or reset. + +Finally, when done using inline encryption with a blk_crypto_key on a +request_queue, users must call ``blk_crypto_evict_key()``. This ensures that +the key is evicted from all keyslots it may be programmed into, and unlinked +from any kernel data structures it may be linked into. + +In summary, for users of the block layer, the lifecycle of a blk_crypto_key is +as follows: + +1. ``blk_crypto_config_supported()`` (optional) +2. ``blk_crypto_init_key()`` +3. ``blk_crypto_start_using_key()`` +4. ``bio_crypt_set_ctx()`` (potentially many times) +5. ``blk_crypto_evict_key()`` (after all I/O has completed) +6. Zeroize the blk_crypto_key (this has no dedicated function) + +If a blk_crypto_key is being used on multiple request_queues, then +``blk_crypto_config_supported()`` (if used), ``blk_crypto_start_using_key()``, +and ``blk_crypto_evict_key()`` must be called on each request_queue. API presented to device drivers =============================== -A :c:type:``struct blk_keyslot_manager`` should be set up by device drivers in -the ``request_queue`` of the device. The device driver needs to call -``blk_ksm_init`` (or its resource-managed variant ``devm_blk_ksm_init``) on the -``blk_keyslot_manager``, while specifying the number of keyslots supported by -the hardware. - -The device driver also needs to tell the KSM how to actually manipulate the -IE hardware in the device to do things like programming the crypto key into -the IE hardware into a particular keyslot. All this is achieved through the -struct blk_ksm_ll_ops field in the KSM that the device driver -must fill up after initing the ``blk_keyslot_manager``. - -The KSM also handles runtime power management for the device when applicable -(e.g. when it wants to program a crypto key into the IE hardware, the device -must be runtime powered on) - so the device driver must also set the ``dev`` -field in the ksm to point to the `struct device` for the KSM to use for runtime -power management. - -``blk_ksm_reprogram_all_keys`` can be called by device drivers if the device -needs each and every of its keyslots to be reprogrammed with the key it -"should have" at the point in time when the function is called. This is useful -e.g. if a device loses all its keys on runtime power down/up. - -If the driver used ``blk_ksm_init`` instead of ``devm_blk_ksm_init``, then -``blk_ksm_destroy`` should be called to free up all resources used by a -``blk_keyslot_manager`` once it is no longer needed. +A device driver that wants to support inline encryption must set up a +blk_crypto_profile in the request_queue of its device. To do this, it first +must call ``blk_crypto_profile_init()`` (or its resource-managed variant +``devm_blk_crypto_profile_init()``), providing the number of keyslots. + +Next, it must advertise its crypto capabilities by setting fields in the +blk_crypto_profile, e.g. ``modes_supported`` and ``max_dun_bytes_supported``. + +It then must set function pointers in the ``ll_ops`` field of the +blk_crypto_profile to tell upper layers how to control the inline encryption +hardware, e.g. how to program and evict keyslots. Most drivers will need to +implement ``keyslot_program`` and ``keyslot_evict``. For details, see the +comments for ``struct blk_crypto_ll_ops``. + +Once the driver registers a blk_crypto_profile with a request_queue, I/O +requests the driver receives via that queue may have an encryption context. All +encryption contexts will be compatible with the crypto capabilities declared in +the blk_crypto_profile, so drivers don't need to worry about handling +unsupported requests. Also, if a nonzero number of keyslots was declared in the +blk_crypto_profile, then all I/O requests that have an encryption context will +also have a keyslot which was already programmed with the appropriate key. + +If the driver implements runtime suspend and its blk_crypto_ll_ops don't work +while the device is runtime-suspended, then the driver must also set the ``dev`` +field of the blk_crypto_profile to point to the ``struct device`` that will be +resumed before any of the low-level operations are called. + +If there are situations where the inline encryption hardware loses the contents +of its keyslots, e.g. device resets, the driver must handle reprogramming the +keyslots. To do this, the driver may call ``blk_crypto_reprogram_all_keys()``. + +Finally, if the driver used ``blk_crypto_profile_init()`` instead of +``devm_blk_crypto_profile_init()``, then it is responsible for calling +``blk_crypto_profile_destroy()`` when the crypto profile is no longer needed. Layered Devices =============== -Request queue based layered devices like dm-rq that wish to support IE need to -create their own keyslot manager for their request queue, and expose whatever -functionality they choose. When a layered device wants to pass a clone of that -request to another ``request_queue``, blk-crypto will initialize and prepare the -clone as necessary - see ``blk_crypto_insert_cloned_request`` in -``blk-crypto.c``. - - -Future Optimizations for layered devices -======================================== - -Creating a keyslot manager for a layered device uses up memory for each -keyslot, and in general, a layered device merely passes the request on to a -"child" device, so the keyslots in the layered device itself are completely -unused, and don't need any refcounting or keyslot programming. We can instead -define a new type of KSM; the "passthrough KSM", that layered devices can use -to advertise an unlimited number of keyslots, and support for any encryption -algorithms they choose, while not actually using any memory for each keyslot. -Another use case for the "passthrough KSM" is for IE devices that do not have a -limited number of keyslots. - +Request queue based layered devices like dm-rq that wish to support inline +encryption need to create their own blk_crypto_profile for their request_queue, +and expose whatever functionality they choose. When a layered device wants to +pass a clone of that request to another request_queue, blk-crypto will +initialize and prepare the clone as necessary; see +``blk_crypto_insert_cloned_request()``. Interaction between inline encryption and blk integrity ======================================================= @@ -257,7 +290,7 @@ Because there isn't any real hardware yet, it seems prudent to assume that hardware implementations might not implement both features together correctly, and disallow the combination for now. Whenever a device supports integrity, the kernel will pretend that the device does not support hardware inline encryption -(by essentially setting the keyslot manager in the request_queue of the device -to NULL). When the crypto API fallback is enabled, this means that all bios with -and encryption context will use the fallback, and IO will complete as usual. -When the fallback is disabled, a bio with an encryption context will be failed. +(by setting the blk_crypto_profile in the request_queue of the device to NULL). +When the crypto API fallback is enabled, this means that all bios with and +encryption context will use the fallback, and IO will complete as usual. When +the fallback is disabled, a bio with an encryption context will be failed. -- 2.33.0
prev parent reply other threads:[~2021-09-16 17:44 UTC|newest] Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-09-16 17:22 [PATCH v2 0/4] blk-crypto cleanups Eric Biggers 2021-09-16 17:22 ` [PATCH v2 1/4] blk-crypto-fallback: properly prefix function and struct names Eric Biggers 2021-09-16 23:07 ` Chaitanya Kulkarni 2021-09-16 17:22 ` [PATCH v2 2/4] blk-crypto: rename keyslot-manager files to blk-crypto-profile Eric Biggers 2021-09-16 17:22 ` [PATCH v2 3/4] blk-crypto: rename blk_keyslot_manager to blk_crypto_profile Eric Biggers 2021-09-16 17:22 ` Eric Biggers [this message]
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