Because kpatch-build is relatively easy to use, it can be easy to assume that a successful patch module build means that the patch is safe to apply. But in fact that's a very dangerous assumption.
There are many pitfalls that can be encountered when creating a live patch. This document attempts to guide the patch creation process. It's a work in progress. If you find it useful, please contribute!
kpatch provides some guarantees, but it does not guarantee that all patches are safe to apply. Every patch must also be analyzed in-depth by a human.
The most important point here cannot be stressed enough. Here comes the bold:
Do not blindly apply patches. There is no substitute for human analysis and reasoning on a per-patch basis. All patches must be thoroughly analyzed by a human kernel expert who completely understands the patch and the affected code and how they relate to the live patching environment.
This document assumes that the kpatch-build tool is being used to create livepatch kernel modules. Other live patching systems may have different consistency models, their own guarantees, and other subtle differences. The guidance in this document applies only to kpatch-build generated livepatches.
Due to potential unexpected interactions between patches, it's highly recommended that when patching a system which has already been patched, the second patch should be a cumulative upgrade which is a superset of the first patch.
kpatch patches functions, not data. If the original patch involves a change to a data structure, the patch will require some rework, as changes to data structures are not allowed by default.
Usually you have to get creative. There are several possible ways to handle this:
Sometimes, instead of changing the data structure itself, you can change the code which uses it.
For example, consider this patch. which has the following hunk:
@@ -3270,6 +3277,7 @@ static int (*const svm_exit_handlers[])(struct vcpu_svm *svm) = {
[SVM_EXIT_EXCP_BASE + PF_VECTOR] = pf_interception,
[SVM_EXIT_EXCP_BASE + NM_VECTOR] = nm_interception,
[SVM_EXIT_EXCP_BASE + MC_VECTOR] = mc_interception,
+ [SVM_EXIT_EXCP_BASE + AC_VECTOR] = ac_interception,
[SVM_EXIT_INTR] = intr_interception,
[SVM_EXIT_NMI] = nmi_interception,
[SVM_EXIT_SMI] = nop_on_interception,
svm_exit_handlers[] is an array of function pointers. The patch adds a
ac_interception function pointer to the array at index [SVM_EXIT_EXCP_BASE +
AC_VECTOR]. That change is incompatible with kpatch.
Looking at the source file, we can see that this function pointer is only
accessed by a single function, handle_exit():
if (exit_code >= ARRAY_SIZE(svm_exit_handlers)
|| !svm_exit_handlers[exit_code]) {
WARN_ONCE(1, "svm: unexpected exit reason 0x%x\n", exit_code);
kvm_queue_exception(vcpu, UD_VECTOR);
return 1;
}
return svm_exit_handlers[exit_code](svm);
So an easy solution here is to just change the code to manually check for the new case before looking in the data structure:
@@ -3580,6 +3580,9 @@ static int handle_exit(struct kvm_vcpu *vcpu)
return 1;
}
+ if (exit_code == SVM_EXIT_EXCP_BASE + AC_VECTOR)
+ return ac_interception(svm);
+
return svm_exit_handlers[exit_code](svm);
}
Not only is this an easy solution, it's also safer than touching data since
svm_exit_handlers[] may be in use by tasks that haven't been patched
yet.
Kpatch supports the kernel's livepatch (Un)patching
callbacks.
The kernel API requires callback registration through struct klp_callbacks,
but to do so through kpatch-build, kpatch-macros.h defines the following:
KPATCH_PRE_PATCH_CALLBACK - executed before patchingKPATCH_POST_PATCH_CALLBACK - executed after patchingKPATCH_PRE_UNPATCH_CALLBACK - executed before unpatching, complements the
post-patch callback.KPATCH_POST_UNPATCH_CALLBACK - executed after unpatching, complements the
pre-patch callback.A pre-patch callback routine has the following signature:
static int callback(patch_object *obj) { }
KPATCH_PRE_PATCH_CALLBACK(callback);
and any non-zero return status indicates failure to the kernel. For more information on pre-patch callback failure, see the Pre-patch return status section below.
Post-patch, pre-unpatch, and post-unpatch callback routines all share the following signature:
static void callback(patch_object *obj) { }
KPATCH_POST_PATCH_CALLBACK(callback); /* or */
KPATCH_PRE_UNPATCH_CALLBACK(callback); /* or */
KPATCH_POST_UNPATCH_CALLBACK(callback);
Generally pre-patch callbacks are paired with post-unpatch callbacks, meaning that anything the former allocates or sets up should be torn down by the former callback. Likewise for post-patch and pre-unpatch callbacks.
If kpatch is currently patching already loaded objects (vmlinux always by definition as well as any currently loaded kernel modules), a non-zero pre-patch callback status stops the current patch in progress. The kpatch-module is rejected, completely reverted, and unloaded.
If an already loaded kpatch is patching an incoming kernel module, then a failing pre-patch callback will result in the kernel module loader rejecting the new module.
In both cases, if a pre-patch callback fails, none of its other associated callbacks will be executed.
For patches to vmlinux or already loaded kernel modules, callback functions
will be run around the livepatch transitions in the klp_enable_patch()
callchain. This is executed automatically on kpatch module init.
For patches to kernel modules which haven't been loaded yet, a
module-notifier will execute callbacks when the module is loaded into
the MODULE_STATE_COMING state. The pre and post-patch callbacks are
called before any module_init code.
Example: a kpatch fix for CVE-2016-5389 could utilize the
KPATCH_PRE_PATCH_CALLBACK and KPATCH_POST_UNPATCH_CALLBACK macros to modify
variable sysctl_tcp_challenge_ack_limit in-place:
+#include "kpatch-macros.h"
+
+static bool kpatch_write = false;
+static int kpatch_pre_patch_tcp_send_challenge_ack(patch_object *obj)
+{
+ if (sysctl_tcp_challenge_ack_limit == 100) {
+ sysctl_tcp_challenge_ack_limit = 1000;
+ kpatch_write = true;
+ }
+ return 0;
+}
static void kpatch_post_unpatch_tcp_send_challenge_ack(patch_object *obj)
+{
+ if (kpatch_write && sysctl_tcp_challenge_ack_limit == 1000)
+ sysctl_tcp_challenge_ack_limit = 100;
+}
+KPATCH_PRE_PATCH_CALLBACK(kpatch_pre_patch_tcp_send_challenge_ack);
+KPATCH_POST_UNPATCH_CALLBACK(kpatch_post_unpatch_tcp_send_challenge_ack);
Don't forget to protect access to data as needed. Spinlocks and mutexes /
sleeping locks may be used (this is a change of behavior from when kpatch
relied on the kpatch.ko support module and stop_machine() context.)
Be careful when upgrading. If patch A has a pre/post-patch callback which writes to X, and then you load patch B which is a superset of A, in some cases you may want to prevent patch B from writing to X, if A is already loaded.
If you need to add a field to an existing data structure, or even many existing data structures, you can use the kernel's Shadow Variable API.
Example: The shadow-newpid.patch integration test employs shadow variables
to add a rolling counter to the new struct task_struct instances. A
simplified version is presented here.
A shadow PID variable is allocated in do_fork(): it is associated with the
current struct task_struct *p value, given an ID of KPATCH_SHADOW_NEWPID,
sized accordingly, and allocated as per GFP_KERNEL flag rules. Note that
the shadow variable <obj, id> association is global -- hence it is best to
provide unique ID enumerations per kpatch as needed.
klp_shadow_alloc() returns a pointer to the shadow variable, so we can
dereference and make assignments as usual. In this patch chunk, the shadow
newpid is allocated then assigned to a rolling ctr counter value:
diff --git a/kernel/fork.c b/kernel/fork.c
index 9bff3b28c357..18374fd35bd9 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1751,6 +1751,8 @@ struct task_struct *fork_idle(int cpu)
return task;
}
+#include <linux/livepatch.h>
+#define KPATCH_SHADOW_NEWPID 0
/*
* Ok, this is the main fork-routine.
*
@@ -1794,6 +1796,14 @@ long do_fork(unsigned long clone_flags,
if (!IS_ERR(p)) {
struct completion vfork;
struct pid *pid;
+ int *newpid;
+ static int ctr = 0;
+
+ newpid = klp_shadow_get_or_alloc(p, KPATCH_SHADOW_NEWPID,
+ sizeof(*newpid), GFP_KERNEL,
+ NULL, NULL);
+ if (newpid)
+ *newpid = ctr++;
trace_sched_process_fork(current, p);
A shadow variable may be accessed via klp_shadow_get(). Here the patch
modifies task_context_switch_counts() to fetch the shadow variable
associated with the current struct task_struct *p object and a
KPATCH_SHADOW_NEWPID ID. As in the previous patch chunk, the shadow
variable pointer may be accessed as an ordinary pointer type:
diff --git a/fs/proc/array.c b/fs/proc/array.c
index 39684c79e8e2..fe0259d057a3 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -394,13 +394,19 @@ static inline void task_seccomp(struct seq_file *m, struct task_struct *p)
seq_putc(m, '\n');
}
+#include <linux/livepatch.h>
+#define KPATCH_SHADOW_NEWPID 0
static inline void task_context_switch_counts(struct seq_file *m,
struct task_struct *p)
{
+ int *newpid;
seq_printf(m, "voluntary_ctxt_switches:\t%lu\n"
"nonvoluntary_ctxt_switches:\t%lu\n",
p->nvcsw,
p->nivcsw);
+ newpid = klp_shadow_get(p, KPATCH_SHADOW_NEWPID);
+ if (newpid)
+ seq_printf(m, "newpid:\t%d\n", *newpid);
}
static void task_cpus_allowed(struct seq_file *m, struct task_struct *task)
A shadow variable is freed by calling klp_shadow_free() and providing
the object / enum ID combination. Once freed, the shadow variable is no
longer safe to access:
diff --git a/kernel/exit.c b/kernel/exit.c
index 148a7842928d..44b6fe61e912 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -791,6 +791,8 @@ static void check_stack_usage(void)
static inline void check_stack_usage(void) {}
#endif
+#include <linux/livepatch.h>
+#define KPATCH_SHADOW_NEWPID 0
void do_exit(long code)
{
struct task_struct *tsk = current;
@@ -888,6 +890,8 @@ void do_exit(long code)
check_stack_usage();
exit_thread();
+ klp_shadow_free(tsk, KPATCH_SHADOW_NEWPID, NULL);
+
/*
* Flush inherited counters to the parent - before the parent
* gets woken up by child-exit notifications.
Notes:
klp_shadow_alloc() and klp_shadow_get_or_alloc() initialize only shadow
variable metadata. They allocate variable storage via kmalloc with the
gfp_t flags given, but otherwise leave the area untouched. Initialization
of a shadow variable is the responsibility of the caller.
As soon as klp_shadow_alloc() or klp_shadow_get_or_alloc() create a shadow
variable, its presence will be reported by klp_shadow_get(). Care should be
taken to avoid any potential race conditions between a kernel thread that
allocates a shadow variable and concurrent threads that may attempt to use
it.
* Patches may need to call klp_shadow_free_all() from a post-unpatch handler
to safely cleanup any shadow variables of a particular ID. From post-unpatch
context, unloading kpatch module code (aside from .exit) should be
completely inactive. As long as these shadow variables were only accessed by
the unloaded kpatch, they are be safe to release.
Part of the stable-tree backport
to fix CVE-2014-0206 changed the reference count semantic of struct
kioctx.reqs_active. Associating a shadow variable to new instances of this
structure can be used by patched code to handle both new (post-patch) and
existing (pre-patch) instances.
(Note: this example is trimmed to highlight this use-case. Boilerplate code is
also required to allocate/free a shadow variable with enum ID
KPATCH_SHADOW_REQS_ACTIVE_V2 whenever a new struct kioctx is
created/released. No values are ever assigned to the shadow variable.)
diff --git a/fs/aio.c b/fs/aio.c
index ebd06fd0de89..6a33b73c9107 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -280,6 +280,8 @@ static void free_ioctx_rcu(struct rcu_head *head)
* and ctx->users has dropped to 0, so we know no more kiocbs can be submitted -
* now it's safe to cancel any that need to be.
*/
+#include <linux/livepatch.h>
+#define KPATCH_SHADOW_REQS_ACTIVE_V2 1
static void free_ioctx(struct kioctx *ctx)
{
struct aio_ring *ring;
Shadow variable existence can be verified before applying the new data semantic of the associated object:
@@ -678,6 +681,8 @@ void aio_complete(struct kiocb *iocb, long res, long res2)
put_rq:
/* everything turned out well, dispose of the aiocb. */
aio_put_req(iocb);
+ if (klp_shadow_get(ctx, KPATCH_SHADOW_REQS_ACTIVE_V2))
+ atomic_dec(&ctx->reqs_active);
/*
* We have to order our ring_info tail store above and test
Likewise, shadow variable non-existence can be tested to continue applying the old data semantic:
@@ -310,7 +312,8 @@ static void free_ioctx(struct kioctx *ctx)
avail = (head <= ctx->tail ? ctx->tail : ctx->nr_events) - head;
- atomic_sub(avail, &ctx->reqs_active);
+ if (!klp_shadow_get(ctx, KPATCH_SHADOW_REQS_ACTIVE_V2))
+ atomic_sub(avail, &ctx->reqs_active);
head += avail;
head %= ctx->nr_events;
}
@@ -757,6 +762,8 @@ static long aio_read_events_ring(struct kioctx *ctx,
pr_debug("%li h%u t%u\n", ret, head, ctx->tail);
atomic_sub(ret, &ctx->reqs_active);
+ if (!klp_shadow_get(ctx, KPATCH_SHADOW_REQS_ACTIVE_V2))
+ atomic_sub(ret, &ctx->reqs_active);
out:
mutex_unlock(&ctx->ring_lock);
The previous example can be extended to use shadow variable storage to handle
locking semantic changes. Consider the upstream fix
for CVE-2014-2706, which added a ps_lock to struct sta_info to protect
critical sections throughout net/mac80211/sta_info.c.
When allocating a new struct sta_info, allocate a corresponding shadow
variable large enough to hold a spinlock_t instance, then initialize the
spinlock:
diff --git a/net/mac80211/sta_info.c b/net/mac80211/sta_info.c
index decd30c1e290..758533dda4d8 100644
--- a/net/mac80211/sta_info.c
+++ b/net/mac80211/sta_info.c
@@ -287,6 +287,8 @@ static int sta_prepare_rate_control(struct ieee80211_local *local,
return 0;
}
+#include <linux/livepatch.h>
+#define KPATCH_SHADOW_PS_LOCK 2
struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata,
const u8 *addr, gfp_t gfp)
{
@@ -295,6 +297,7 @@ struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata,
struct timespec uptime;
struct ieee80211_tx_latency_bin_ranges *tx_latency;
int i;
+ spinlock_t *ps_lock;
sta = kzalloc(sizeof(*sta) + local->hw.sta_data_size, gfp);
if (!sta)
@@ -330,6 +333,10 @@ struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata,
rcu_read_unlock();
spin_lock_init(&sta->lock);
+ ps_lock = klp_shadow_alloc(sta, KPATCH_SHADOW_PS_LOCK,
+ sizeof(*ps_lock), gfp, NULL, NULL);
+ if (ps_lock)
+ spin_lock_init(ps_lock);
INIT_WORK(&sta->drv_unblock_wk, sta_unblock);
INIT_WORK(&sta->ampdu_mlme.work, ieee80211_ba_session_work);
mutex_init(&sta->ampdu_mlme.mtx);
Patched code can reference the shadow variable associated with a given struct
sta_info to determine and apply the correct locking semantic for that
instance:
diff --git a/net/mac80211/tx.c b/net/mac80211/tx.c
index 97a02d3f7d87..0edb0ed8dc60 100644
--- a/net/mac80211/tx.c
+++ b/net/mac80211/tx.c
@@ -459,12 +459,15 @@ static int ieee80211_use_mfp(__le16 fc, struct sta_info *sta,
return 1;
}
+#include <linux/livepatch.h>
+#define KPATCH_SHADOW_PS_LOCK 2
static ieee80211_tx_result
ieee80211_tx_h_unicast_ps_buf(struct ieee80211_tx_data *tx)
{
struct sta_info *sta = tx->sta;
struct ieee80211_tx_info *info = IEEE80211_SKB_CB(tx->skb);
struct ieee80211_local *local = tx->local;
+ spinlock_t *ps_lock;
if (unlikely(!sta))
return TX_CONTINUE;
@@ -478,6 +481,23 @@ ieee80211_tx_h_unicast_ps_buf(struct ieee80211_tx_data *tx)
sta->sta.addr, sta->sta.aid, ac);
if (tx->local->total_ps_buffered >= TOTAL_MAX_TX_BUFFER)
purge_old_ps_buffers(tx->local);
+
+ /* sync with ieee80211_sta_ps_deliver_wakeup */
+ ps_lock = klp_shadow_get(sta, KPATCH_SHADOW_PS_LOCK);
+ if (ps_lock) {
+ spin_lock(ps_lock);
+ /*
+ * STA woke up the meantime and all the frames on ps_tx_buf have
+ * been queued to pending queue. No reordering can happen, go
+ * ahead and Tx the packet.
+ */
+ if (!test_sta_flag(sta, WLAN_STA_PS_STA) &&
+ !test_sta_flag(sta, WLAN_STA_PS_DRIVER)) {
+ spin_unlock(ps_lock);
+ return TX_CONTINUE;
+ }
+ }
+
if (skb_queue_len(&sta->ps_tx_buf[ac]) >= STA_MAX_TX_BUFFER) {
struct sk_buff *old = skb_dequeue(&sta->ps_tx_buf[ac]);
ps_dbg(tx->sdata,
Any code which runs in an __init function or during module or device
initialization is problematic, as it may have already run before the patch was
applied. The patch may require a pre-patch callback which detects whether such
init code has run, and which rewrites or changes the original initialization to
force it into the desired state. Some changes involving hardware init are
inherently incompatible with live patching.
When changing header files, be extra careful. If data is being changed, you probably need to modify the patch. See "Data struct changes" above.
If a function prototype is being changed, make sure it's not an exported function. Otherwise it could break out-of-tree modules. One way to workaround this is to define an entirely new copy of the function (with updated code) and patch in-tree callers to invoke it rather than the deprecated version.
Many header file changes result in a complete rebuild of the kernel tree, which makes kpatch-build have to compare every .o file in the kernel. It slows the build down a lot, and can even fail to build if kpatch-build has any bugs lurking. If it's a trivial header file change, like adding a macro, it's advisable to just move that macro into the .c file where it's needed to avoid changing the header file at all.
In general, it's best to patch as minimally as possible. If kpatch-build is reporting some unexpected function changes, it's always a good idea to try to figure out why it thinks they changed. In many cases you can change the source patch so that they no longer change.
Some examples:
If a changed function was inlined, then the callers which inlined the function will also change. In this case there's nothing you can do to prevent the extra changes.
If a changed function was originally inlined, but turned into a callable
function after patching, consider adding __always_inline to the function
definition. Likewise, if a function is only inlined after patching,
consider using noinline to prevent the compiler from doing so.
If your patch adds a call to a function where the original version of the function's ELF symbol has a .constprop or .isra suffix, and the corresponding patched function doesn't, that means the patch caused gcc to no longer perform an interprocedural optimization, which affects the function and all its callers. If you want to prevent this from happening, copy/paste the function with a new name and call the new function from your patch.
Moving around source code lines can introduce unique instructions if any
__LINE__ preprocessor macros are in use. This can be mitigated by adding
any new functions to the bottom of source files, using newline whitespace to
maintain original line counts, etc. A more exact fix can be employed by
modifying the source code that invokes __LINE__ and hard-coding the
original line number in place. This occurred in issue #1124 for example.
Removing references to static locals will fail to patch unless extra steps are taken. Static locals are basically global variables because they outlive the function's scope. They need to be correlated so that the new function will use the old static local. That way patching the function doesn't inadvertently reset the variable to zero; instead the variable keeps its old value.
To work around this limitation one needs to retain the reference to the static local. This might be as simple as adding the variable back in the patched function in a non-functional way and ensuring the compiler doesn't optimize it away.
Some fixes may replace or completely remove functions and references to them. Remember that kpatch modules can only add new functions and redirect existing functions, so "removed" functions will continue to exist in kernel address space as effectively dead code.
That means this patch (source code removal of cmdline_proc_show):
diff -Nupr src.orig/fs/proc/cmdline.c src/fs/proc/cmdline.c
--- src.orig/fs/proc/cmdline.c 2016-11-30 19:39:49.317737234 +0000
+++ src/fs/proc/cmdline.c 2016-11-30 19:39:52.696737234 +0000
@@ -3,15 +3,15 @@
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
-static int cmdline_proc_show(struct seq_file *m, void *v)
-{
- seq_printf(m, "%s\n", saved_command_line);
- return 0;
-}
+static int cmdline_proc_show_v2(struct seq_file *m, void *v)
+{
+ seq_printf(m, "%s kpatch\n", saved_command_line);
+ return 0;
+}
static int cmdline_proc_open(struct inode *inode, struct file *file)
{
- return single_open(file, cmdline_proc_show, NULL);
+ return single_open(file, cmdline_proc_show_v2, NULL);
}
static const struct file_operations cmdline_proc_fops = {
will generate an equivalent kpatch module to this patch (dead
cmdline_proc_show left in source):
diff -Nupr src.orig/fs/proc/cmdline.c src/fs/proc/cmdline.c
--- src.orig/fs/proc/cmdline.c 2016-11-30 19:39:49.317737234 +0000
+++ src/fs/proc/cmdline.c 2016-11-30 19:39:52.696737234 +0000
@@ -9,9 +9,15 @@ static int cmdline_proc_show(struct seq_
return 0;
}
+static int cmdline_proc_show_v2(struct seq_file *m, void *v)
+{
+ seq_printf(m, "%s kpatch\n", saved_command_line);
+ return 0;
+}
+
static int cmdline_proc_open(struct inode *inode, struct file *file)
{
- return single_open(file, cmdline_proc_show, NULL);
+ return single_open(file, cmdline_proc_show_v2, NULL);
}
static const struct file_operations cmdline_proc_fops = {
In both versions, kpatch-build will determine that only
cmdline_proc_open has changed and that cmdline_proc_show_v2 is a
new function.
In some patching cases it might be necessary to completely remove the original function to avoid the compiler complaining about a defined, but unused function. This will depend on symbol scope and kernel build options.
When adding a call to printk_once(), pr_warn_once(), or any other "once"
variation of printk(), you'll get the following eror:
ERROR: vmx.o: 1 unsupported section change(s) vmx.o: WARNING: unable to correlate static local variable __print_once.60588 used by vmx_update_pi_irte, assuming variable is new vmx.o: changed function: vmx_update_pi_irte vmx.o: data section .data..read_mostly selected for inclusion /usr/lib/kpatch/create-diff-object: unreconcilable difference
This error occurs because the printk_once() adds a static local variable to
the .data..read_mostly section. kpatch-build strict disallows any changes to
that section, because in some cases a change to this section indicates a bug.
To work around this issue, you'll need to manually implement your own "once"
logic which doesn't store the static variable in the .data..read_mostly
section.
For example, a pr_warn_once() can be replaced with:
static bool print_once;
...
if (!print_once) {
print_once = true;
pr_warn("...");
}
The linux kernel defines its own version of "inline" in include/linux/compiler_types.h which includes "notrace" as well:
#if !defined(CONFIG_OPTIMIZE_INLINING)
#define inline inline __attribute__((__always_inline__)) __gnu_inline \
__inline_maybe_unused notrace
#else
#define inline inline __gnu_inline \
__inline_maybe_unused notrace
#endif
With the implicit "notrace", use of "inline" in patch sources may lead to kpatch-build errors like the following:
__tcp_mtu_to_mss() is marked as inline:net/ipv4/tcp_output.c:
/* Calculate MSS not accounting any TCP options. */
static inline int __tcp_mtu_to_mss(struct sock *sk, int pmtu)
{
% kpatch-build ... tcp_output.o: function __tcp_mtu_to_mss has no fentry/mcount call, unable to patch
Disassembly of section .text.__tcp_mtu_to_mss: 0000000000000000 <__tcp_mtu_to_mss>: 0: 48 8b 87 60 05 00 00 mov 0x560(%rdi),%rax 7: 0f b7 50 30 movzwl 0x30(%rax),%edx b: 0f b7 40 32 movzwl 0x32(%rax),%eax f: 29 d6 sub %edx,%esi 11: 83 ee 14 sub $0x14,%esi ...
This could be a little confusing since one might have expected to see
changes to all of __tcp_mtu_to_mss() callers (ie, it was inlined as
requested). In this case, a simple workaround is to specify
__tcp_mtu_to_mss() as __always_inline to force the compiler to do so.
When modifying a function that contains a jump label, kpatch-build may
return an error like: ERROR: oom_kill.o: kpatch_regenerate_special_section: 2109: Found a jump label at out_of_memory()+0x10a, using key cpusets_enabled_key. Jump labels aren't currently supported. Use static_key_enabled() instead.
This is due to a limitation in the kernel to process static key livepatch relocations (resolved by late-module patching). Older versions of kpatch-build may have reported successfully building kpatch module, but issue #931 revealed potentially dangerous behavior if the static key value had been modified from its compiled default.
The current workaround is to remove the jump label by explictly checking the static key:
DEFINE_STATIC_KEY_TRUE(true_key); DEFINE_STATIC_KEY_FALSE(false_key); /* unsupported */ if (static_key_true(&true_key)) if (static_key_false(&false_key)) if (static_branch_likely(&key)) /* supported */ if (static_key_enabled(&true_key)) if (static_key_enabled(&false_key)) if (likely(static_key_enabled(&key)))
GCC may generate sibling calls that are incompatible with kpatch, resulting in
an error like: ERROR("Found an unsupported sibling call at foo()+0x123. Add __attribute__((optimize("-fno-optimize-sibling-calls"))) to foo() definition."
For example, if function A() calls function B() at the end of A() and both return similar data-types, GCC may deem them "sibling calls" and apply a tail call optimization in which A() restores the stack to is callee state before setting up B()'s arguments and jumping to B().
This may be an issue for kpatches on PowerPC which modify only A() or B() and the function call crosses a kernel module boundary: the sibling call optimization has changed expected calling conventions and (un)patched code may not be similarly modified.
Commit 8b952bd77130 ("create-diff-object/ppc64le: Don't allow sibling calls") contains an excellent example and description of this problem with annotated disassembly.
Adding __attribute__((optimize("-fno-optimize-sibling-calls"))) instructs
GCC to turn off the optimization for the given function.
CONFIG_MODVERSIONS enables an ABI check between exported kernel symbols and
modules referencing those symbols, enforced on module load. When building the
kernel, preprocessor output from gcc -E for each source file is passed to
scripts/genksyms. The genksyms script recursively expands each exported symbol
to its basic types. A hash is generated for each symbol as it traverses back up
the symbol tree. The end result is a CRC for each exported function in
the Module.symvers file and embedded in the vmlinux kernel object itself.
A similar checksumming is performed when building modules: referenced exported
symbol CRCs are stored in the module’s __versions section (you can also find
these in plain-text intermediate *.mod.c files.)
When the kernel loads a module, the symbol CRCs found in its __versions are
compared to those of the kernel, if the two do not match, the kernel will refuse
to load it:
<module>: disagrees about version of symbol <symbol> <module>: Unknown symbol <symbol> (err -22)
After building the original and patched sources, kpatch-build compares the newly calculated Module.symvers against the original. Discrepancies are reported:
ERROR: Version disagreement for symbol <symbol>
These reports should be addressed to ensure that the resulting kpatch module can be loaded.
It is rare, but possible for a kpatch to introduce inadvertent symbol CRC changes that are not true ABI changes. The following conditions must occur:
The kpatch must modify the definition of an exported symbol. For example,
introducing a new header file may further define an opaque data type:
Before the kpatch, compilation unit U from the original kernel build only
knew about a struct S declaration (not its complete type). At the same
time, U contains function F, which has an interface that references S. If
the kpatch adds a header file to U that now fully defines struct S { int
a, b, c; }, its symbol type graph changes, CRCs generated for F are updated,
but its ABI remains consistent.
The kpatch must introduce either a change or reference to F such that it is
included in the resulting kpatch module. This will force a __version
entry based on the new CRC.
Note: if a kpatch doesn't change or reference F such that it is not
included in the resulting kpatch module, the new CRC value won't be added
to the module's __version table. However, if a future accumulative patch
does add a new change or reference to F, the new CRC will become a problem.
Kpatches should introduce new #include directives sparingly. Whenever
possible, extract the required definitions from header filers into kpatched
compilation units directly.
If additional header files or symbol definitions cannot be avoided, consider
surrounding the offending include/definitions in an #ifndef __GENKSYMS__
macro. The genksyms script will skip over those blocks when performing its CRC
calculations.
If a kpatch introduces a true ABI change, each of calling functions would
consequently need to be updated in the kpatch module. For unexported functions,
this may be handled safely if the kpatch does indeed update all callers.
However, since motivation behind CONFIG_MODVERSIONS is to provide basic ABI
verification between the kernel and modules for exported functions, kpatch
cannot safely change this ABI without worrying about breaking other out-of-tree
drivers. Those drivers have been built against the reference kernel's original
set of CRCs and expect the original ABI.
To track down specifically what caused a symbol CRC change, tools like
kabi-dw can be employed to produce a
detailed symbol definition report. For a kpatch-build, kabi-dw can be modified
to operate on .o object files (not just .ko and vmlinux files) and the
$CACHEDIR/tmp/{orig, patched} directories compared.