From 2c3c1048746a4622d8c89a29670120dc8fab93c4 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sun, 7 Apr 2024 20:49:45 +0200 Subject: Adding upstream version 6.1.76. Signed-off-by: Daniel Baumann --- Documentation/livepatch/shadow-vars.rst | 226 ++++++++++++++++++++++++++++++++ 1 file changed, 226 insertions(+) create mode 100644 Documentation/livepatch/shadow-vars.rst (limited to 'Documentation/livepatch/shadow-vars.rst') diff --git a/Documentation/livepatch/shadow-vars.rst b/Documentation/livepatch/shadow-vars.rst new file mode 100644 index 000000000..7a7098bfb --- /dev/null +++ b/Documentation/livepatch/shadow-vars.rst @@ -0,0 +1,226 @@ +================ +Shadow Variables +================ + +Shadow variables are a simple way for livepatch modules to associate +additional "shadow" data with existing data structures. Shadow data is +allocated separately from parent data structures, which are left +unmodified. The shadow variable API described in this document is used +to allocate/add and remove/free shadow variables to/from their parents. + +The implementation introduces a global, in-kernel hashtable that +associates pointers to parent objects and a numeric identifier of the +shadow data. The numeric identifier is a simple enumeration that may be +used to describe shadow variable version, class or type, etc. More +specifically, the parent pointer serves as the hashtable key while the +numeric id subsequently filters hashtable queries. Multiple shadow +variables may attach to the same parent object, but their numeric +identifier distinguishes between them. + + +1. Brief API summary +==================== + +(See the full API usage docbook notes in livepatch/shadow.c.) + +A hashtable references all shadow variables. These references are +stored and retrieved through a pair. + +* The klp_shadow variable data structure encapsulates both tracking + meta-data and shadow-data: + + - meta-data + + - obj - pointer to parent object + - id - data identifier + + - data[] - storage for shadow data + +It is important to note that the klp_shadow_alloc() and +klp_shadow_get_or_alloc() are zeroing the variable by default. +They also allow to call a custom constructor function when a non-zero +value is needed. Callers should provide whatever mutual exclusion +is required. + +Note that the constructor is called under klp_shadow_lock spinlock. It allows +to do actions that can be done only once when a new variable is allocated. + +* klp_shadow_get() - retrieve a shadow variable data pointer + - search hashtable for pair + +* klp_shadow_alloc() - allocate and add a new shadow variable + - search hashtable for pair + + - if exists + + - WARN and return NULL + + - if doesn't already exist + + - allocate a new shadow variable + - initialize the variable using a custom constructor and data when provided + - add to the global hashtable + +* klp_shadow_get_or_alloc() - get existing or alloc a new shadow variable + - search hashtable for pair + + - if exists + + - return existing shadow variable + + - if doesn't already exist + + - allocate a new shadow variable + - initialize the variable using a custom constructor and data when provided + - add pair to the global hashtable + +* klp_shadow_free() - detach and free a shadow variable + - find and remove a reference from global hashtable + + - if found + + - call destructor function if defined + - free shadow variable + +* klp_shadow_free_all() - detach and free all <_, id> shadow variables + - find and remove any <_, id> references from global hashtable + + - if found + + - call destructor function if defined + - free shadow variable + + +2. Use cases +============ + +(See the example shadow variable livepatch modules in samples/livepatch/ +for full working demonstrations.) + +For the following use-case examples, consider commit 1d147bfa6429 +("mac80211: fix AP powersave TX vs. wakeup race"), which added a +spinlock to net/mac80211/sta_info.h :: struct sta_info. Each use-case +example can be considered a stand-alone livepatch implementation of this +fix. + + +Matching parent's lifecycle +--------------------------- + +If parent data structures are frequently created and destroyed, it may +be easiest to align their shadow variables lifetimes to the same +allocation and release functions. In this case, the parent data +structure is typically allocated, initialized, then registered in some +manner. Shadow variable allocation and setup can then be considered +part of the parent's initialization and should be completed before the +parent "goes live" (ie, any shadow variable get-API requests are made +for this pair.) + +For commit 1d147bfa6429, when a parent sta_info structure is allocated, +allocate a shadow copy of the ps_lock pointer, then initialize it:: + + #define PS_LOCK 1 + struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata, + const u8 *addr, gfp_t gfp) + { + struct sta_info *sta; + spinlock_t *ps_lock; + + /* Parent structure is created */ + sta = kzalloc(sizeof(*sta) + hw->sta_data_size, gfp); + + /* Attach a corresponding shadow variable, then initialize it */ + ps_lock = klp_shadow_alloc(sta, PS_LOCK, sizeof(*ps_lock), gfp, + NULL, NULL); + if (!ps_lock) + goto shadow_fail; + spin_lock_init(ps_lock); + ... + +When requiring a ps_lock, query the shadow variable API to retrieve one +for a specific struct sta_info::: + + void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta) + { + spinlock_t *ps_lock; + + /* sync with ieee80211_tx_h_unicast_ps_buf */ + ps_lock = klp_shadow_get(sta, PS_LOCK); + if (ps_lock) + spin_lock(ps_lock); + ... + +When the parent sta_info structure is freed, first free the shadow +variable:: + + void sta_info_free(struct ieee80211_local *local, struct sta_info *sta) + { + klp_shadow_free(sta, PS_LOCK, NULL); + kfree(sta); + ... + + +In-flight parent objects +------------------------ + +Sometimes it may not be convenient or possible to allocate shadow +variables alongside their parent objects. Or a livepatch fix may +require shadow variables for only a subset of parent object instances. +In these cases, the klp_shadow_get_or_alloc() call can be used to attach +shadow variables to parents already in-flight. + +For commit 1d147bfa6429, a good spot to allocate a shadow spinlock is +inside ieee80211_sta_ps_deliver_wakeup():: + + int ps_lock_shadow_ctor(void *obj, void *shadow_data, void *ctor_data) + { + spinlock_t *lock = shadow_data; + + spin_lock_init(lock); + return 0; + } + + #define PS_LOCK 1 + void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta) + { + spinlock_t *ps_lock; + + /* sync with ieee80211_tx_h_unicast_ps_buf */ + ps_lock = klp_shadow_get_or_alloc(sta, PS_LOCK, + sizeof(*ps_lock), GFP_ATOMIC, + ps_lock_shadow_ctor, NULL); + + if (ps_lock) + spin_lock(ps_lock); + ... + +This usage will create a shadow variable, only if needed, otherwise it +will use one that was already created for this pair. + +Like the previous use-case, the shadow spinlock needs to be cleaned up. +A shadow variable can be freed just before its parent object is freed, +or even when the shadow variable itself is no longer required. + + +Other use-cases +--------------- + +Shadow variables can also be used as a flag indicating that a data +structure was allocated by new, livepatched code. In this case, it +doesn't matter what data value the shadow variable holds, its existence +suggests how to handle the parent object. + + +3. References +============= + +* https://github.com/dynup/kpatch + + The livepatch implementation is based on the kpatch version of shadow + variables. + +* http://files.mkgnu.net/files/dynamos/doc/papers/dynamos_eurosys_07.pdf + + Dynamic and Adaptive Updates of Non-Quiescent Subsystems in Commodity + Operating System Kernels (Kritis Makris, Kyung Dong Ryu 2007) presented + a datatype update technique called "shadow data structures". -- cgit v1.2.3