1 ================ 1 ================
2 Shadow Variables 2 Shadow Variables
3 ================ 3 ================
4 4
5 Shadow variables are a simple way for livepatc 5 Shadow variables are a simple way for livepatch modules to associate
6 additional "shadow" data with existing data st 6 additional "shadow" data with existing data structures. Shadow data is
7 allocated separately from parent data structur 7 allocated separately from parent data structures, which are left
8 unmodified. The shadow variable API described 8 unmodified. The shadow variable API described in this document is used
9 to allocate/add and remove/free shadow variabl 9 to allocate/add and remove/free shadow variables to/from their parents.
10 10
11 The implementation introduces a global, in-ker 11 The implementation introduces a global, in-kernel hashtable that
12 associates pointers to parent objects and a nu 12 associates pointers to parent objects and a numeric identifier of the
13 shadow data. The numeric identifier is a simp 13 shadow data. The numeric identifier is a simple enumeration that may be
14 used to describe shadow variable version, clas 14 used to describe shadow variable version, class or type, etc. More
15 specifically, the parent pointer serves as the 15 specifically, the parent pointer serves as the hashtable key while the
16 numeric id subsequently filters hashtable quer 16 numeric id subsequently filters hashtable queries. Multiple shadow
17 variables may attach to the same parent object 17 variables may attach to the same parent object, but their numeric
18 identifier distinguishes between them. 18 identifier distinguishes between them.
19 19
20 20
21 1. Brief API summary 21 1. Brief API summary
22 ==================== 22 ====================
23 23
24 (See the full API usage docbook notes in livep 24 (See the full API usage docbook notes in livepatch/shadow.c.)
25 25
26 A hashtable references all shadow variables. 26 A hashtable references all shadow variables. These references are
27 stored and retrieved through a <obj, id> pair. 27 stored and retrieved through a <obj, id> pair.
28 28
29 * The klp_shadow variable data structure encap 29 * The klp_shadow variable data structure encapsulates both tracking
30 meta-data and shadow-data: 30 meta-data and shadow-data:
31 31
32 - meta-data 32 - meta-data
33 33
34 - obj - pointer to parent object 34 - obj - pointer to parent object
35 - id - data identifier 35 - id - data identifier
36 36
37 - data[] - storage for shadow data 37 - data[] - storage for shadow data
38 38
39 It is important to note that the klp_shadow_al 39 It is important to note that the klp_shadow_alloc() and
40 klp_shadow_get_or_alloc() are zeroing the vari 40 klp_shadow_get_or_alloc() are zeroing the variable by default.
41 They also allow to call a custom constructor f 41 They also allow to call a custom constructor function when a non-zero
42 value is needed. Callers should provide whatev 42 value is needed. Callers should provide whatever mutual exclusion
43 is required. 43 is required.
44 44
45 Note that the constructor is called under klp_ 45 Note that the constructor is called under klp_shadow_lock spinlock. It allows
46 to do actions that can be done only once when 46 to do actions that can be done only once when a new variable is allocated.
47 47
48 * klp_shadow_get() - retrieve a shadow variabl 48 * klp_shadow_get() - retrieve a shadow variable data pointer
49 - search hashtable for <obj, id> pair 49 - search hashtable for <obj, id> pair
50 50
51 * klp_shadow_alloc() - allocate and add a new 51 * klp_shadow_alloc() - allocate and add a new shadow variable
52 - search hashtable for <obj, id> pair 52 - search hashtable for <obj, id> pair
53 53
54 - if exists 54 - if exists
55 55
56 - WARN and return NULL 56 - WARN and return NULL
57 57
58 - if <obj, id> doesn't already exist 58 - if <obj, id> doesn't already exist
59 59
60 - allocate a new shadow variable 60 - allocate a new shadow variable
61 - initialize the variable using a custom c 61 - initialize the variable using a custom constructor and data when provided
62 - add <obj, id> to the global hashtable 62 - add <obj, id> to the global hashtable
63 63
64 * klp_shadow_get_or_alloc() - get existing or 64 * klp_shadow_get_or_alloc() - get existing or alloc a new shadow variable
65 - search hashtable for <obj, id> pair 65 - search hashtable for <obj, id> pair
66 66
67 - if exists 67 - if exists
68 68
69 - return existing shadow variable 69 - return existing shadow variable
70 70
71 - if <obj, id> doesn't already exist 71 - if <obj, id> doesn't already exist
72 72
73 - allocate a new shadow variable 73 - allocate a new shadow variable
74 - initialize the variable using a custom c 74 - initialize the variable using a custom constructor and data when provided
75 - add <obj, id> pair to the global hashtab 75 - add <obj, id> pair to the global hashtable
76 76
77 * klp_shadow_free() - detach and free a <obj, 77 * klp_shadow_free() - detach and free a <obj, id> shadow variable
78 - find and remove a <obj, id> reference from 78 - find and remove a <obj, id> reference from global hashtable
79 79
80 - if found 80 - if found
81 81
82 - call destructor function if defined 82 - call destructor function if defined
83 - free shadow variable 83 - free shadow variable
84 84
85 * klp_shadow_free_all() - detach and free all 85 * klp_shadow_free_all() - detach and free all <_, id> shadow variables
86 - find and remove any <_, id> references fro 86 - find and remove any <_, id> references from global hashtable
87 87
88 - if found 88 - if found
89 89
90 - call destructor function if defined 90 - call destructor function if defined
91 - free shadow variable 91 - free shadow variable
92 92
93 93
94 2. Use cases 94 2. Use cases
95 ============ 95 ============
96 96
97 (See the example shadow variable livepatch mod 97 (See the example shadow variable livepatch modules in samples/livepatch/
98 for full working demonstrations.) 98 for full working demonstrations.)
99 99
100 For the following use-case examples, consider 100 For the following use-case examples, consider commit 1d147bfa6429
101 ("mac80211: fix AP powersave TX vs. wakeup ra 101 ("mac80211: fix AP powersave TX vs. wakeup race"), which added a
102 spinlock to net/mac80211/sta_info.h :: struct 102 spinlock to net/mac80211/sta_info.h :: struct sta_info. Each use-case
103 example can be considered a stand-alone livepa 103 example can be considered a stand-alone livepatch implementation of this
104 fix. 104 fix.
105 105
106 106
107 Matching parent's lifecycle 107 Matching parent's lifecycle
108 --------------------------- 108 ---------------------------
109 109
110 If parent data structures are frequently creat 110 If parent data structures are frequently created and destroyed, it may
111 be easiest to align their shadow variables lif 111 be easiest to align their shadow variables lifetimes to the same
112 allocation and release functions. In this cas 112 allocation and release functions. In this case, the parent data
113 structure is typically allocated, initialized, 113 structure is typically allocated, initialized, then registered in some
114 manner. Shadow variable allocation and setup 114 manner. Shadow variable allocation and setup can then be considered
115 part of the parent's initialization and should 115 part of the parent's initialization and should be completed before the
116 parent "goes live" (ie, any shadow variable ge 116 parent "goes live" (ie, any shadow variable get-API requests are made
117 for this <obj, id> pair.) 117 for this <obj, id> pair.)
118 118
119 For commit 1d147bfa6429, when a parent sta_inf 119 For commit 1d147bfa6429, when a parent sta_info structure is allocated,
120 allocate a shadow copy of the ps_lock pointer, 120 allocate a shadow copy of the ps_lock pointer, then initialize it::
121 121
122 #define PS_LOCK 1 122 #define PS_LOCK 1
123 struct sta_info *sta_info_alloc(struct ieee8 123 struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata,
124 const u8 *ad 124 const u8 *addr, gfp_t gfp)
125 { 125 {
126 struct sta_info *sta; 126 struct sta_info *sta;
127 spinlock_t *ps_lock; 127 spinlock_t *ps_lock;
128 128
129 /* Parent structure is created */ 129 /* Parent structure is created */
130 sta = kzalloc(sizeof(*sta) + hw->sta_d 130 sta = kzalloc(sizeof(*sta) + hw->sta_data_size, gfp);
131 131
132 /* Attach a corresponding shadow varia 132 /* Attach a corresponding shadow variable, then initialize it */
133 ps_lock = klp_shadow_alloc(sta, PS_LOC 133 ps_lock = klp_shadow_alloc(sta, PS_LOCK, sizeof(*ps_lock), gfp,
134 NULL, NULL) 134 NULL, NULL);
135 if (!ps_lock) 135 if (!ps_lock)
136 goto shadow_fail; 136 goto shadow_fail;
137 spin_lock_init(ps_lock); 137 spin_lock_init(ps_lock);
138 ... 138 ...
139 139
140 When requiring a ps_lock, query the shadow var 140 When requiring a ps_lock, query the shadow variable API to retrieve one
141 for a specific struct sta_info::: 141 for a specific struct sta_info:::
142 142
143 void ieee80211_sta_ps_deliver_wakeup(struct 143 void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
144 { 144 {
145 spinlock_t *ps_lock; 145 spinlock_t *ps_lock;
146 146
147 /* sync with ieee80211_tx_h_unicast_ps 147 /* sync with ieee80211_tx_h_unicast_ps_buf */
148 ps_lock = klp_shadow_get(sta, PS_LOCK) 148 ps_lock = klp_shadow_get(sta, PS_LOCK);
149 if (ps_lock) 149 if (ps_lock)
150 spin_lock(ps_lock); 150 spin_lock(ps_lock);
151 ... 151 ...
152 152
153 When the parent sta_info structure is freed, f 153 When the parent sta_info structure is freed, first free the shadow
154 variable:: 154 variable::
155 155
156 void sta_info_free(struct ieee80211_local *l 156 void sta_info_free(struct ieee80211_local *local, struct sta_info *sta)
157 { 157 {
158 klp_shadow_free(sta, PS_LOCK, NULL); 158 klp_shadow_free(sta, PS_LOCK, NULL);
159 kfree(sta); 159 kfree(sta);
160 ... 160 ...
161 161
162 162
163 In-flight parent objects 163 In-flight parent objects
164 ------------------------ 164 ------------------------
165 165
166 Sometimes it may not be convenient or possible 166 Sometimes it may not be convenient or possible to allocate shadow
167 variables alongside their parent objects. Or 167 variables alongside their parent objects. Or a livepatch fix may
168 require shadow variables for only a subset of 168 require shadow variables for only a subset of parent object instances.
169 In these cases, the klp_shadow_get_or_alloc() 169 In these cases, the klp_shadow_get_or_alloc() call can be used to attach
170 shadow variables to parents already in-flight. 170 shadow variables to parents already in-flight.
171 171
172 For commit 1d147bfa6429, a good spot to alloca 172 For commit 1d147bfa6429, a good spot to allocate a shadow spinlock is
173 inside ieee80211_sta_ps_deliver_wakeup():: 173 inside ieee80211_sta_ps_deliver_wakeup()::
174 174
175 int ps_lock_shadow_ctor(void *obj, void *sha 175 int ps_lock_shadow_ctor(void *obj, void *shadow_data, void *ctor_data)
176 { 176 {
177 spinlock_t *lock = shadow_data; 177 spinlock_t *lock = shadow_data;
178 178
179 spin_lock_init(lock); 179 spin_lock_init(lock);
180 return 0; 180 return 0;
181 } 181 }
182 182
183 #define PS_LOCK 1 183 #define PS_LOCK 1
184 void ieee80211_sta_ps_deliver_wakeup(struct 184 void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
185 { 185 {
186 spinlock_t *ps_lock; 186 spinlock_t *ps_lock;
187 187
188 /* sync with ieee80211_tx_h_unicast_ps 188 /* sync with ieee80211_tx_h_unicast_ps_buf */
189 ps_lock = klp_shadow_get_or_alloc(sta, 189 ps_lock = klp_shadow_get_or_alloc(sta, PS_LOCK,
190 sizeof(*ps_lock), GFP_ 190 sizeof(*ps_lock), GFP_ATOMIC,
191 ps_lock_shadow_ctor, N 191 ps_lock_shadow_ctor, NULL);
192 192
193 if (ps_lock) 193 if (ps_lock)
194 spin_lock(ps_lock); 194 spin_lock(ps_lock);
195 ... 195 ...
196 196
197 This usage will create a shadow variable, only 197 This usage will create a shadow variable, only if needed, otherwise it
198 will use one that was already created for this 198 will use one that was already created for this <obj, id> pair.
199 199
200 Like the previous use-case, the shadow spinloc 200 Like the previous use-case, the shadow spinlock needs to be cleaned up.
201 A shadow variable can be freed just before its 201 A shadow variable can be freed just before its parent object is freed,
202 or even when the shadow variable itself is no 202 or even when the shadow variable itself is no longer required.
203 203
204 204
205 Other use-cases 205 Other use-cases
206 --------------- 206 ---------------
207 207
208 Shadow variables can also be used as a flag in 208 Shadow variables can also be used as a flag indicating that a data
209 structure was allocated by new, livepatched co 209 structure was allocated by new, livepatched code. In this case, it
210 doesn't matter what data value the shadow vari 210 doesn't matter what data value the shadow variable holds, its existence
211 suggests how to handle the parent object. 211 suggests how to handle the parent object.
212 212
213 213
214 3. References 214 3. References
215 ============= 215 =============
216 216
217 * https://github.com/dynup/kpatch 217 * https://github.com/dynup/kpatch
218 218
219 The livepatch implementation is based on the 219 The livepatch implementation is based on the kpatch version of shadow
220 variables. 220 variables.
221 221
222 * http://files.mkgnu.net/files/dynamos/doc/pap 222 * http://files.mkgnu.net/files/dynamos/doc/papers/dynamos_eurosys_07.pdf
223 223
224 Dynamic and Adaptive Updates of Non-Quiescen 224 Dynamic and Adaptive Updates of Non-Quiescent Subsystems in Commodity
225 Operating System Kernels (Kritis Makris, Kyu 225 Operating System Kernels (Kritis Makris, Kyung Dong Ryu 2007) presented
226 a datatype update technique called "shadow d 226 a datatype update technique called "shadow data structures".
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