From 034dd340b08be1f2f0477ad16131d609f9dbd53c Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 30 Aug 2026 09:22:00 -0700 Subject: Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace Pull tracing fixes from Steven Rostedt: - Fix error output of boot instance creation failure Currently if a boot instance creation fails, instead of printing out the name of the instance that failed, it prints "(null)". That is because it prints "cur_str" that had already been processed by strsep(). Print the saved name instead. While at it, print the error code of the failure. - Fix use-after-free for same named historgrams Histograms can be named so that they can be used in multiple events. But if the named histogram has a variable attached, the second event that uses the named histogram which duplicates it and needs to free the original after duplication leaves the old variable in place and still visible. If another histogram uses than variable, it will use the stale one which will try to reference the freed duplicate histogram and crash the kernel. Free the duplicate variables along with the duplicated histogram data. - Check return value of kthread_run() in event self test The events self tests uses a kthread for testing but does not check if it succeeded in creating a kthread. If the kthread creation were to fail, the code will still try to call kthread_stop() on the error returned. - Fix race between reading trace_pipe and updating subbuffer size If a user is reading the trace_pipe file at the same time they update the ring buffer sub-buffer size, can cause the trace_pipe read to read stale data. Add trace_access_lock() around updating the ring buffer sub-buffer size. - Fix eventfs_inode on failure path in creation of the events directory In the creation of the "events" directory, if after allocating the eventfs_inode a failure is detected, it calls cleanup_ei() which calls free_ei(). The free_ei() will test if eventfs_inode being freed has no children. It is a bug if it does. But on the failure case of the creation of the "events" directory, the children lists have not yet been initialized and the free will trigger a warning because list_empty() on an uninitialized list returns false. Move the initialization into init_ei() where it makes more sense and makes sure that a created eventfs_inode has its lists initialized upon creation. - Check return value of kthread_run() in ftrace direct sample code The sample code that shows how to use the ftrace direct calls does not test the return of kthread_run() to see if it succeeds. Return a failure if the kthread_run() doesn't succeed. - Clear user events state on fork in case of alloc failure On fork, the child gets a pointer to the parent's user events state. It makes a copy of it then updates the child's pointer to it. But if the allocation fails, the duplication function leaves the child with a pointer to its parent's descriptor. When the child cleans up its data, it will free the parent's descriptor while the parent is still using it. In the duplication function, set the child's user_event_mm to NULL before testing if the allocation succeeded, and when it exits it will not free the parent's descriptor. - Fix retry exhaustion in simple ring buffer reader swap simple_ring_buffer_swap_reader_page() starts with retry set to 8 and post-decrements it only after a failed link replacement. On the final attempt, a successful replacement leaves retry at zero, while a failed replacement leaves it at -1. But the check for success expects the retry value to be non-zero and exits with an error on zero. This is the opposite result. Fix it. - Fail nicely when the remote swap_reader_page() returns an error Currently, if the swap_reader_page() of a remote buffer fails, it triggers a WARN_ON_ONCE() and continues normally. Instead, have it exit with an error and a pr_warn() print instead of a full WARNING. * tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: ring-buffer: Stop remote reader update when page swap fails tracing: Fix retry exhaustion in simple ring buffer reader swap tracing/user_events: Clear copied tracing state before fork duplication samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify eventfs: Initialize ei->children and ei->list in init_ei() tracing: Fix use-after-free in trace_pipe read on sub-buffer order change tracing: Fix crash passing ERR_PTR to kthread_stop() tracing: Fix use-after-free with same-name named triggers tracing: Fix logged instance name on creation failure --- 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 v1.3.1