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Subject[PATCH] x86: Add a list for custom page fault handlers.
From: Pekka Paalanen <pq@iki.fi>

Provides kernel modules a way to register custom page fault handlers.
On every page fault, except those handled in vmalloc_fault(), this will
call a list of registered functions. The functions may handle the fault
and force do_page_fault() to return immediately.

This functionality is similar to the now removed page fault notifiers.
Custom page fault handlers are used by debugging and reverse engineering
tools. Mmio-trace is one such tool and a patch to add it into the tree
will follow.

The custom page fault handlers are called from the exact same points in
do_page_fault() as the page fault notifiers were.

Signed-off-by: Pekka Paalanen <pq@iki.fi>
---

This change is x86 specific purely because my intended in-tree user of
it is mmio-trace, which AFAIK is really useful only on x86 for reverse
engineering binary drivers. If someone later wishes to merge this with
notify_page_fault() from kprobes, they can do that.

I am not subscribed to LKML, so please keep me in CC.

I will reply to this email with the next patch adding mmio-trace into
kernel.

Thanks,
pq

arch/x86/Kconfig.debug | 9 ++++++
arch/x86/mm/fault_32.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++
arch/x86/mm/fault_64.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++
include/asm-x86/kdebug.h | 9 ++++++
4 files changed, 146 insertions(+), 0 deletions(-)

diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index 761ca7b..920a63d 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -112,4 +112,13 @@ config IOMMU_LEAK
Add a simple leak tracer to the IOMMU code. This is useful when you
are debugging a buggy device driver that leaks IOMMU mappings.

+config PAGE_FAULT_HANDLERS
+ bool "Custom page fault handlers"
+ depends on DEBUG_KERNEL
+ help
+ Allow the use of custom page fault handlers. A kernel module may
+ register a function that is called on every page fault not handled
+ for vmalloc. Custom handlers are used by some debugging and reverse
+ engineering tools.
+
endmenu
diff --git a/arch/x86/mm/fault_32.c b/arch/x86/mm/fault_32.c
index a2273d4..19e16b0 100644
--- a/arch/x86/mm/fault_32.c
+++ b/arch/x86/mm/fault_32.c
@@ -33,6 +33,64 @@

extern void die(const char *,struct pt_regs *,long);

+#ifdef CONFIG_PAGE_FAULT_HANDLERS
+static HLIST_HEAD(pf_handlers); /* protected by RCU */
+static DEFINE_SPINLOCK(pf_handlers_writer);
+
+void register_page_fault_handler(struct pf_handler *new_pfh)
+{
+ spin_lock(&pf_handlers_writer);
+ hlist_add_head_rcu(&new_pfh->hlist, &pf_handlers);
+ spin_unlock(&pf_handlers_writer);
+}
+EXPORT_SYMBOL_GPL(register_page_fault_handler);
+
+void unregister_page_fault_handler(struct pf_handler *old_pfh)
+{
+ might_sleep();
+ spin_lock(&pf_handlers_writer);
+ hlist_del_rcu(&old_pfh->hlist);
+ spin_unlock(&pf_handlers_writer);
+ synchronize_rcu();
+}
+EXPORT_SYMBOL_GPL(unregister_page_fault_handler);
+
+/* returns non-zero if do_page_fault() should return */
+static int call_pf_handlers(struct pt_regs *regs, unsigned long error_code,
+ unsigned long address)
+{
+ int ret = 0;
+ struct pf_handler *cur;
+ struct hlist_node *ncur;
+
+ rcu_read_lock();
+ hlist_for_each_entry_rcu(cur, ncur, &pf_handlers, hlist) {
+ ret = cur->handler(regs, error_code, address);
+ if (ret)
+ break;
+ }
+ rcu_read_unlock();
+ return ret;
+}
+
+static inline int custom_pf_handlers(void)
+{
+ return !hlist_empty(&pf_handlers);
+}
+#else
+static inline int call_pf_handlers(struct pt_regs *regs,
+ unsigned long error_code,
+ unsigned long address)
+{
+ return 0;
+}
+
+static inline int custom_pf_handlers(void)
+{
+ return 0;
+}
+#endif /* CONFIG_PAGE_FAULT_HANDLERS */
+
#ifdef CONFIG_KPROBES
static inline int notify_page_fault(struct pt_regs *regs)
{
@@ -333,6 +391,9 @@ fastcall void __kprobes do_page_fault(struct pt_regs *regs,
return;
if (notify_page_fault(regs))
return;
+ if (unlikely(custom_pf_handlers()))
+ if (call_pf_handlers(regs, error_code, address))
+ return;
/*
* Don't take the mm semaphore here. If we fixup a prefetch
* fault we could otherwise deadlock.
@@ -342,6 +403,9 @@ fastcall void __kprobes do_page_fault(struct pt_regs *regs,

if (notify_page_fault(regs))
return;
+ if (unlikely(custom_pf_handlers()))
+ if (call_pf_handlers(regs, error_code, address))
+ return;

/* It's safe to allow irq's after cr2 has been saved and the vmalloc
fault has been handled. */
diff --git a/arch/x86/mm/fault_64.c b/arch/x86/mm/fault_64.c
index 0e26230..11f15cc 100644
--- a/arch/x86/mm/fault_64.c
+++ b/arch/x86/mm/fault_64.c
@@ -41,6 +41,64 @@
#define PF_RSVD (1<<3)
#define PF_INSTR (1<<4)

+#ifdef CONFIG_PAGE_FAULT_HANDLERS
+static HLIST_HEAD(pf_handlers); /* protected by RCU */
+static DEFINE_SPINLOCK(pf_handlers_writer);
+
+void register_page_fault_handler(struct pf_handler *new_pfh)
+{
+ spin_lock(&pf_handlers_writer);
+ hlist_add_head_rcu(&new_pfh->hlist, &pf_handlers);
+ spin_unlock(&pf_handlers_writer);
+}
+EXPORT_SYMBOL_GPL(register_page_fault_handler);
+
+void unregister_page_fault_handler(struct pf_handler *old_pfh)
+{
+ might_sleep();
+ spin_lock(&pf_handlers_writer);
+ hlist_del_rcu(&old_pfh->hlist);
+ spin_unlock(&pf_handlers_writer);
+ synchronize_rcu();
+}
+EXPORT_SYMBOL_GPL(unregister_page_fault_handler);
+
+/* returns non-zero if do_page_fault() should return */
+static int call_pf_handlers(struct pt_regs *regs, unsigned long error_code,
+ unsigned long address)
+{
+ int ret = 0;
+ struct pf_handler *cur;
+ struct hlist_node *ncur;
+
+ rcu_read_lock();
+ hlist_for_each_entry_rcu(cur, ncur, &pf_handlers, hlist) {
+ ret = cur->handler(regs, error_code, address);
+ if (ret)
+ break;
+ }
+ rcu_read_unlock();
+ return ret;
+}
+
+static inline int custom_pf_handlers(void)
+{
+ return !hlist_empty(&pf_handlers);
+}
+#else
+static inline int call_pf_handlers(struct pt_regs *regs,
+ unsigned long error_code,
+ unsigned long address)
+{
+ return 0;
+}
+
+static inline int custom_pf_handlers(void)
+{
+ return 0;
+}
+#endif /* CONFIG_PAGE_FAULT_HANDLERS */
+
#ifdef CONFIG_KPROBES
static inline int notify_page_fault(struct pt_regs *regs)
{
@@ -345,6 +403,9 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
}
if (notify_page_fault(regs))
return;
+ if (unlikely(custom_pf_handlers()))
+ if (call_pf_handlers(regs, error_code, address))
+ return;
/*
* Don't take the mm semaphore here. If we fixup a prefetch
* fault we could otherwise deadlock.
@@ -354,6 +415,9 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,

if (notify_page_fault(regs))
return;
+ if (unlikely(custom_pf_handlers()))
+ if (call_pf_handlers(regs, error_code, address))
+ return;

if (likely(regs->eflags & X86_EFLAGS_IF))
local_irq_enable();
diff --git a/include/asm-x86/kdebug.h b/include/asm-x86/kdebug.h
index e2f9b62..7449ad2 100644
--- a/include/asm-x86/kdebug.h
+++ b/include/asm-x86/kdebug.h
@@ -30,4 +30,13 @@ extern void dump_pagetable(unsigned long);
extern unsigned long oops_begin(void);
extern void oops_end(unsigned long);

+struct pf_handler {
+ struct hlist_node hlist;
+ int (*handler)(struct pt_regs *regs, unsigned long error_code,
+ unsigned long address);
+};
+
+extern void register_page_fault_handler(struct pf_handler *new_pfh);
+extern void unregister_page_fault_handler(struct pf_handler *old_pfh);
+
#endif
--
1.5.3.7


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