| 1 | ================
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| 2 | Shadow Variables
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| 3 | ================
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| 4 |
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| 5 | Shadow variables are a simple way for livepatch modules to associate
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| 6 | additional "shadow" data with existing data structures. Shadow data is
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| 7 | allocated separately from parent data structures, which are left
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| 8 | unmodified. The shadow variable API described in this document is used
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| 9 | to allocate/add and remove/free shadow variables to/from their parents.
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| 10 |
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| 11 | The implementation introduces a global, in-kernel hashtable that
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| 12 | associates pointers to parent objects and a numeric identifier of the
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| 13 | shadow data. The numeric identifier is a simple enumeration that may be
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| 14 | used to describe shadow variable version, class or type, etc. More
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| 15 | specifically, the parent pointer serves as the hashtable key while the
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| 16 | numeric id subsequently filters hashtable queries. Multiple shadow
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| 17 | variables may attach to the same parent object, but their numeric
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| 18 | identifier distinguishes between them.
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| 19 |
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| 20 |
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| 21 | 1. Brief API summary
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| 22 | ====================
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| 23 |
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| 24 | (See the full API usage docbook notes in livepatch/shadow.c.)
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| 25 |
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| 26 | A hashtable references all shadow variables. These references are
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| 27 | stored and retrieved through a <obj, id> pair.
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| 28 |
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| 29 | * The klp_shadow variable data structure encapsulates both tracking
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| 30 | meta-data and shadow-data:
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| 31 |
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| 32 | - meta-data
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| 33 |
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| 34 | - obj - pointer to parent object
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| 35 | - id - data identifier
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| 36 |
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| 37 | - data[] - storage for shadow data
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| 38 |
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| 39 | It is important to note that the klp_shadow_alloc() and
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| 40 | klp_shadow_get_or_alloc() are zeroing the variable by default.
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| 41 | They also allow to call a custom constructor function when a non-zero
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| 42 | value is needed. Callers should provide whatever mutual exclusion
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| 43 | is required.
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| 44 |
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| 45 | Note that the constructor is called under klp_shadow_lock spinlock. It allows
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| 46 | to do actions that can be done only once when a new variable is allocated.
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| 47 |
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| 48 | * klp_shadow_get() - retrieve a shadow variable data pointer
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| 49 | - search hashtable for <obj, id> pair
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| 50 |
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| 51 | * klp_shadow_alloc() - allocate and add a new shadow variable
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| 52 | - search hashtable for <obj, id> pair
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| 53 |
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| 54 | - if exists
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| 55 |
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| 56 | - WARN and return NULL
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| 57 |
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| 58 | - if <obj, id> doesn't already exist
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| 59 |
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| 60 | - allocate a new shadow variable
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| 61 | - initialize the variable using a custom constructor and data when provided
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| 62 | - add <obj, id> to the global hashtable
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| 63 |
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| 64 | * klp_shadow_get_or_alloc() - get existing or alloc a new shadow variable
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| 65 | - search hashtable for <obj, id> pair
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| 66 |
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| 67 | - if exists
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| 68 |
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| 69 | - return existing shadow variable
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| 70 |
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| 71 | - if <obj, id> doesn't already exist
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| 72 |
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| 73 | - allocate a new shadow variable
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| 74 | - initialize the variable using a custom constructor and data when provided
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| 75 | - add <obj, id> pair to the global hashtable
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| 76 |
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| 77 | * klp_shadow_free() - detach and free a <obj, id> shadow variable
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| 78 | - find and remove a <obj, id> reference from global hashtable
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| 79 |
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| 80 | - if found
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| 81 |
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| 82 | - call destructor function if defined
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| 83 | - free shadow variable
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| 84 |
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| 85 | * klp_shadow_free_all() - detach and free all <_, id> shadow variables
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| 86 | - find and remove any <_, id> references from global hashtable
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| 87 |
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| 88 | - if found
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| 89 |
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| 90 | - call destructor function if defined
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| 91 | - free shadow variable
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| 92 |
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| 93 |
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| 94 | 2. Use cases
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| 95 | ============
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| 96 |
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| 97 | (See the example shadow variable livepatch modules in samples/livepatch/
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| 98 | for full working demonstrations.)
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| 99 |
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| 100 | For the following use-case examples, consider commit 1d147bfa6429
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| 101 | ("mac80211: fix AP powersave TX vs. wakeup race"), which added a
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| 102 | spinlock to net/mac80211/sta_info.h :: struct sta_info. Each use-case
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| 103 | example can be considered a stand-alone livepatch implementation of this
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| 104 | fix.
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| 105 |
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| 106 |
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| 107 | Matching parent's lifecycle
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| 108 | ---------------------------
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| 109 |
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| 110 | If parent data structures are frequently created and destroyed, it may
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| 111 | be easiest to align their shadow variables lifetimes to the same
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| 112 | allocation and release functions. In this case, the parent data
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| 113 | structure is typically allocated, initialized, then registered in some
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| 114 | manner. Shadow variable allocation and setup can then be considered
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| 115 | part of the parent's initialization and should be completed before the
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| 116 | parent "goes live" (ie, any shadow variable get-API requests are made
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| 117 | for this <obj, id> pair.)
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| 118 |
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| 119 | For commit 1d147bfa6429, when a parent sta_info structure is allocated,
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| 120 | allocate a shadow copy of the ps_lock pointer, then initialize it::
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| 121 |
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| 122 | #define PS_LOCK 1
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| 123 | struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata,
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| 124 | const u8 *addr, gfp_t gfp)
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| 125 | {
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| 126 | struct sta_info *sta;
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| 127 | spinlock_t *ps_lock;
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| 128 |
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| 129 | /* Parent structure is created */
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| 130 | sta = kzalloc(sizeof(*sta) + hw->sta_data_size, gfp);
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| 131 |
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| 132 | /* Attach a corresponding shadow variable, then initialize it */
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| 133 | ps_lock = klp_shadow_alloc(sta, PS_LOCK, sizeof(*ps_lock), gfp,
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| 134 | NULL, NULL);
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| 135 | if (!ps_lock)
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| 136 | goto shadow_fail;
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| 137 | spin_lock_init(ps_lock);
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| 138 | ...
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| 139 |
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| 140 | When requiring a ps_lock, query the shadow variable API to retrieve one
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| 141 | for a specific struct sta_info:::
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| 142 |
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| 143 | void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
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| 144 | {
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| 145 | spinlock_t *ps_lock;
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| 146 |
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| 147 | /* sync with ieee80211_tx_h_unicast_ps_buf */
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| 148 | ps_lock = klp_shadow_get(sta, PS_LOCK);
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| 149 | if (ps_lock)
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| 150 | spin_lock(ps_lock);
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| 151 | ...
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| 152 |
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| 153 | When the parent sta_info structure is freed, first free the shadow
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| 154 | variable::
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| 155 |
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| 156 | void sta_info_free(struct ieee80211_local *local, struct sta_info *sta)
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| 157 | {
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| 158 | klp_shadow_free(sta, PS_LOCK, NULL);
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| 159 | kfree(sta);
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| 160 | ...
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| 161 |
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| 162 |
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| 163 | In-flight parent objects
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| 164 | ------------------------
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| 165 |
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| 166 | Sometimes it may not be convenient or possible to allocate shadow
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| 167 | variables alongside their parent objects. Or a livepatch fix may
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| 168 | require shadow variables for only a subset of parent object instances.
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| 169 | In these cases, the klp_shadow_get_or_alloc() call can be used to attach
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| 170 | shadow variables to parents already in-flight.
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| 171 |
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| 172 | For commit 1d147bfa6429, a good spot to allocate a shadow spinlock is
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| 173 | inside ieee80211_sta_ps_deliver_wakeup()::
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| 174 |
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| 175 | int ps_lock_shadow_ctor(void *obj, void *shadow_data, void *ctor_data)
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| 176 | {
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| 177 | spinlock_t *lock = shadow_data;
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| 178 |
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| 179 | spin_lock_init(lock);
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| 180 | return 0;
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| 181 | }
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| 182 |
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| 183 | #define PS_LOCK 1
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| 184 | void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
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| 185 | {
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| 186 | spinlock_t *ps_lock;
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| 187 |
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| 188 | /* sync with ieee80211_tx_h_unicast_ps_buf */
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| 189 | ps_lock = klp_shadow_get_or_alloc(sta, PS_LOCK,
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| 190 | sizeof(*ps_lock), GFP_ATOMIC,
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| 191 | ps_lock_shadow_ctor, NULL);
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| 192 |
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| 193 | if (ps_lock)
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| 194 | spin_lock(ps_lock);
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| 195 | ...
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| 196 |
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| 197 | This usage will create a shadow variable, only if needed, otherwise it
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| 198 | will use one that was already created for this <obj, id> pair.
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| 199 |
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| 200 | Like the previous use-case, the shadow spinlock needs to be cleaned up.
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| 201 | A shadow variable can be freed just before its parent object is freed,
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| 202 | or even when the shadow variable itself is no longer required.
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| 203 |
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| 204 |
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| 205 | Other use-cases
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| 206 | ---------------
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| 207 |
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| 208 | Shadow variables can also be used as a flag indicating that a data
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| 209 | structure was allocated by new, livepatched code. In this case, it
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| 210 | doesn't matter what data value the shadow variable holds, its existence
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| 211 | suggests how to handle the parent object.
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| 212 |
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| 213 |
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| 214 | 3. References
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| 215 | =============
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| 216 |
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| 217 | * https://github.com/dynup/kpatch
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| 218 |
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| 219 | The livepatch implementation is based on the kpatch version of shadow
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| 220 | variables.
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| 221 |
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| 222 | * http://files.mkgnu.net/files/dynamos/doc/papers/dynamos_eurosys_07.pdf
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| 223 |
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| 224 | Dynamic and Adaptive Updates of Non-Quiescent Subsystems in Commodity
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| 225 | Operating System Kernels (Kritis Makris, Kyung Dong Ryu 2007) presented
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| 226 | a datatype update technique called "shadow data structures".
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