kprobes: Remove kprobe::fault_handler

The reason for kprobe::fault_handler(), as given by their comment:

 * We come here because instructions in the pre/post
 * handler caused the page_fault, this could happen
 * if handler tries to access user space by
 * copy_from_user(), get_user() etc. Let the
 * user-specified handler try to fix it first.

Is just plain bad. Those other handlers are ran from non-preemptible
context and had better use _nofault() functions. Also, there is no
upstream usage of this.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20210525073213.561116662@infradead.org

Documentation/trace/kprobes.rst

@@ -362,14 +362,11 @@ register_kprobe
 	#include <linux/kprobes.h>
 	int register_kprobe(struct kprobe *kp);
 
-Sets a breakpoint at the address kp->addr.  When the breakpoint is
+Sets a breakpoint at the address kp->addr.  When the breakpoint is hit, Kprobes
-hit, Kprobes calls kp->pre_handler.  After the probed instruction
+calls kp->pre_handler.  After the probed instruction is single-stepped, Kprobe
-is single-stepped, Kprobe calls kp->post_handler.  If a fault
+calls kp->post_handler.  Any or all handlers can be NULL. If kp->flags is set
-occurs during execution of kp->pre_handler or kp->post_handler,
+KPROBE_FLAG_DISABLED, that kp will be registered but disabled, so, its handlers
-or during single-stepping of the probed instruction, Kprobes calls
+aren't hit until calling enable_kprobe(kp).
-kp->fault_handler.  Any or all handlers can be NULL. If kp->flags
-is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
-so, its handlers aren't hit until calling enable_kprobe(kp).
 
 .. note::
 
@@ -415,17 +412,6 @@ User's post-handler (kp->post_handler)::
 p and regs are as described for the pre_handler.  flags always seems
 to be zero.
 
-User's fault-handler (kp->fault_handler)::
-
-	#include <linux/kprobes.h>
-	#include <linux/ptrace.h>
-	int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
-
-p and regs are as described for the pre_handler.  trapnr is the
-architecture-specific trap number associated with the fault (e.g.,
-on i386, 13 for a general protection fault or 14 for a page fault).
-Returns 1 if it successfully handled the exception.
-
 register_kretprobe
 ------------------
 

arch/arc/kernel/kprobes.c

@@ -323,16 +323,6 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned long trapnr)
 		 */
 		kprobes_inc_nmissed_count(cur);
 
-		/*
-		 * We come here because instructions in the pre/post
-		 * handler caused the page_fault, this could happen
-		 * if handler tries to access user space by
-		 * copy_from_user(), get_user() etc. Let the
-		 * user-specified handler try to fix it first.
-		 */
-		if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
-			return 1;
-
 		/*
 		 * In case the user-specified fault handler returned zero,
 		 * try to fix up.

arch/arm/probes/kprobes/core.c

@@ -358,15 +358,6 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
 		 */
 		kprobes_inc_nmissed_count(cur);
 
-		/*
-		 * We come here because instructions in the pre/post
-		 * handler caused the page_fault, this could happen
-		 * if handler tries to access user space by
-		 * copy_from_user(), get_user() etc. Let the
-		 * user-specified handler try to fix it.
-		 */
-		if (cur->fault_handler && cur->fault_handler(cur, regs, fsr))
-			return 1;
 		break;
 
 	default:

arch/arm64/kernel/probes/kprobes.c

@@ -283,16 +283,6 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
 		 */
 		kprobes_inc_nmissed_count(cur);
 
-		/*
-		 * We come here because instructions in the pre/post
-		 * handler caused the page_fault, this could happen
-		 * if handler tries to access user space by
-		 * copy_from_user(), get_user() etc. Let the
-		 * user-specified handler try to fix it first.
-		 */
-		if (cur->fault_handler && cur->fault_handler(cur, regs, fsr))
-			return 1;
-
 		/*
 		 * In case the user-specified fault handler returned
 		 * zero, try to fix up.

arch/csky/kernel/probes/kprobes.c

@@ -301,16 +301,6 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int trapnr)
 		 */
 		kprobes_inc_nmissed_count(cur);
 
-		/*
-		 * We come here because instructions in the pre/post
-		 * handler caused the page_fault, this could happen
-		 * if handler tries to access user space by
-		 * copy_from_user(), get_user() etc. Let the
-		 * user-specified handler try to fix it first.
-		 */
-		if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
-			return 1;
-
 		/*
 		 * In case the user-specified fault handler returned
 		 * zero, try to fix up.

arch/ia64/kernel/kprobes.c

@@ -850,15 +850,6 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, int trapnr)
 		 */
 		kprobes_inc_nmissed_count(cur);
 
-		/*
-		 * We come here because instructions in the pre/post
-		 * handler caused the page_fault, this could happen
-		 * if handler tries to access user space by
-		 * copy_from_user(), get_user() etc. Let the
-		 * user-specified handler try to fix it first.
-		 */
-		if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
-			return 1;
 		/*
 		 * In case the user-specified fault handler returned
 		 * zero, try to fix up.

arch/mips/kernel/kprobes.c

@@ -403,9 +403,6 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
 	struct kprobe *cur = kprobe_running();
 	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
 
-	if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
-		return 1;
-
 	if (kcb->kprobe_status & KPROBE_HIT_SS) {
 		resume_execution(cur, regs, kcb);
 		regs->cp0_status |= kcb->kprobe_old_SR;

arch/powerpc/kernel/kprobes.c

@@ -508,16 +508,6 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
 		 */
 		kprobes_inc_nmissed_count(cur);
 
-		/*
-		 * We come here because instructions in the pre/post
-		 * handler caused the page_fault, this could happen
-		 * if handler tries to access user space by
-		 * copy_from_user(), get_user() etc. Let the
-		 * user-specified handler try to fix it first.
-		 */
-		if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
-			return 1;
-
 		/*
 		 * In case the user-specified fault handler returned
 		 * zero, try to fix up.

arch/riscv/kernel/probes/kprobes.c

@@ -283,16 +283,6 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int trapnr)
 		 */
 		kprobes_inc_nmissed_count(cur);
 
-		/*
-		 * We come here because instructions in the pre/post
-		 * handler caused the page_fault, this could happen
-		 * if handler tries to access user space by
-		 * copy_from_user(), get_user() etc. Let the
-		 * user-specified handler try to fix it first.
-		 */
-		if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
-			return 1;
-
 		/*
 		 * In case the user-specified fault handler returned
 		 * zero, try to fix up.

arch/s390/kernel/kprobes.c

@@ -452,16 +452,6 @@ static int kprobe_trap_handler(struct pt_regs *regs, int trapnr)
 		 */
 		kprobes_inc_nmissed_count(p);
 
-		/*
-		 * We come here because instructions in the pre/post
-		 * handler caused the page_fault, this could happen
-		 * if handler tries to access user space by
-		 * copy_from_user(), get_user() etc. Let the
-		 * user-specified handler try to fix it first.
-		 */
-		if (p->fault_handler && p->fault_handler(p, regs, trapnr))
-			return 1;
-
 		/*
 		 * In case the user-specified fault handler returned
 		 * zero, try to fix up.

arch/sh/kernel/kprobes.c

@@ -389,16 +389,6 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, int trapnr)
 		 */
 		kprobes_inc_nmissed_count(cur);
 
-		/*
-		 * We come here because instructions in the pre/post
-		 * handler caused the page_fault, this could happen
-		 * if handler tries to access user space by
-		 * copy_from_user(), get_user() etc. Let the
-		 * user-specified handler try to fix it first.
-		 */
-		if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
-			return 1;
-
 		/*
 		 * In case the user-specified fault handler returned
 		 * zero, try to fix up.

arch/sparc/kernel/kprobes.c

@@ -352,16 +352,6 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, int trapnr)
 		 */
 		kprobes_inc_nmissed_count(cur);
 
-		/*
-		 * We come here because instructions in the pre/post
-		 * handler caused the page_fault, this could happen
-		 * if handler tries to access user space by
-		 * copy_from_user(), get_user() etc. Let the
-		 * user-specified handler try to fix it first.
-		 */
-		if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
-			return 1;
-
 		/*
 		 * In case the user-specified fault handler returned
 		 * zero, try to fix up.

arch/x86/kernel/kprobes/core.c

@@ -1110,16 +1110,6 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
 		 * these specific fault cases.
 		 */
 		kprobes_inc_nmissed_count(cur);
-
-		/*
-		 * We come here because instructions in the pre/post
-		 * handler caused the page_fault, this could happen
-		 * if handler tries to access user space by
-		 * copy_from_user(), get_user() etc. Let the
-		 * user-specified handler try to fix it first.
-		 */
-		if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
-			return 1;
 	}
 
 	return 0;

include/linux/kprobes.h

@@ -54,8 +54,6 @@ struct kretprobe_instance;
 typedef int (*kprobe_pre_handler_t) (struct kprobe *, struct pt_regs *);
 typedef void (*kprobe_post_handler_t) (struct kprobe *, struct pt_regs *,
 				       unsigned long flags);
-typedef int (*kprobe_fault_handler_t) (struct kprobe *, struct pt_regs *,
-				       int trapnr);
 typedef int (*kretprobe_handler_t) (struct kretprobe_instance *,
 				    struct pt_regs *);
 
@@ -83,12 +81,6 @@ struct kprobe {
 	/* Called after addr is executed, unless... */
 	kprobe_post_handler_t post_handler;
 
-	/*
-	 * ... called if executing addr causes a fault (eg. page fault).
-	 * Return 1 if it handled fault, otherwise kernel will see it.
-	 */
-	kprobe_fault_handler_t fault_handler;
-
 	/* Saved opcode (which has been replaced with breakpoint) */
 	kprobe_opcode_t opcode;
 

kernel/kprobes.c

@@ -1183,23 +1183,6 @@ static void aggr_post_handler(struct kprobe *p, struct pt_regs *regs,
 }
 NOKPROBE_SYMBOL(aggr_post_handler);
 
-static int aggr_fault_handler(struct kprobe *p, struct pt_regs *regs,
-			      int trapnr)
-{
-	struct kprobe *cur = __this_cpu_read(kprobe_instance);
-
-	/*
-	 * if we faulted "during" the execution of a user specified
-	 * probe handler, invoke just that probe's fault handler
-	 */
-	if (cur && cur->fault_handler) {
-		if (cur->fault_handler(cur, regs, trapnr))
-			return 1;
-	}
-	return 0;
-}
-NOKPROBE_SYMBOL(aggr_fault_handler);
-
 /* Walks the list and increments nmissed count for multiprobe case */
 void kprobes_inc_nmissed_count(struct kprobe *p)
 {
@@ -1330,7 +1313,6 @@ static void init_aggr_kprobe(struct kprobe *ap, struct kprobe *p)
 	ap->addr = p->addr;
 	ap->flags = p->flags & ~KPROBE_FLAG_OPTIMIZED;
 	ap->pre_handler = aggr_pre_handler;
-	ap->fault_handler = aggr_fault_handler;
 	/* We don't care the kprobe which has gone. */
 	if (p->post_handler && !kprobe_gone(p))
 		ap->post_handler = aggr_post_handler;
@@ -2014,7 +1996,6 @@ int register_kretprobe(struct kretprobe *rp)
 
 	rp->kp.pre_handler = pre_handler_kretprobe;
 	rp->kp.post_handler = NULL;
-	rp->kp.fault_handler = NULL;
 
 	/* Pre-allocate memory for max kretprobe instances */
 	if (rp->maxactive <= 0) {

samples/kprobes/kprobe_example.c

@@ -94,26 +94,11 @@ static void __kprobes handler_post(struct kprobe *p, struct pt_regs *regs,
 #endif
 }
 
-/*
- * fault_handler: this is called if an exception is generated for any
- * instruction within the pre- or post-handler, or when Kprobes
- * single-steps the probed instruction.
- */
-static int handler_fault(struct kprobe *p, struct pt_regs *regs, int trapnr)
-{
-	pr_info("fault_handler: p->addr = 0x%p, trap #%dn", p->addr, trapnr);
-	/* Return 0 because we don't handle the fault. */
-	return 0;
-}
-/* NOKPROBE_SYMBOL() is also available */
-NOKPROBE_SYMBOL(handler_fault);
-
 static int __init kprobe_init(void)
 {
 	int ret;
 	kp.pre_handler = handler_pre;
 	kp.post_handler = handler_post;
-	kp.fault_handler = handler_fault;
 
 	ret = register_kprobe(&kp);
 	if (ret < 0) {