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Subject[RFC][PATCH] kprobes: Remove kprobe::fault_handler

Somewhat related.. I had this pending.

---
Subject: kprobes: Remove kprobe::fault_handler
From: Peter Zijlstra <peterz@infradead.org>
Date: Tue Feb 2 10:43:41 CET 2021

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.

If 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>
---
Documentation/trace/kprobes.rst | 24 +++++-------------------
arch/arc/kernel/kprobes.c | 10 ----------
arch/arm/probes/kprobes/core.c | 9 ---------
arch/arm64/kernel/probes/kprobes.c | 10 ----------
arch/csky/kernel/probes/kprobes.c | 10 ----------
arch/ia64/kernel/kprobes.c | 9 ---------
arch/mips/kernel/kprobes.c | 3 ---
arch/powerpc/kernel/kprobes.c | 10 ----------
arch/s390/kernel/kprobes.c | 10 ----------
arch/sh/kernel/kprobes.c | 10 ----------
arch/sparc/kernel/kprobes.c | 10 ----------
arch/x86/kernel/kprobes/core.c | 10 ----------
include/linux/kprobes.h | 8 --------
kernel/kprobes.c | 19 -------------------
samples/kprobes/kprobe_example.c | 15 ---------------
15 files changed, 5 insertions(+), 162 deletions(-)

--- a/Documentation/trace/kprobes.rst
+++ b/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
-hit, Kprobes calls kp->pre_handler. After the probed instruction
-is single-stepped, Kprobe calls kp->post_handler. If a fault
-occurs during execution of kp->pre_handler or kp->post_handler,
-or during single-stepping of the probed instruction, Kprobes calls
-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).
+Sets a breakpoint at the address kp->addr. When the breakpoint is hit, Kprobes
+calls kp->pre_handler. After the probed instruction is single-stepped, Kprobe
+calls kp->post_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 @@ the breakpoint was hit. Return 0 here u
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
------------------

--- a/arch/arc/kernel/kprobes.c
+++ b/arch/arc/kernel/kprobes.c
@@ -324,16 +324,6 @@ int __kprobes kprobe_fault_handler(struc
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.
*/
--- a/arch/arm/probes/kprobes/core.c
+++ b/arch/arm/probes/kprobes/core.c
@@ -358,15 +358,6 @@ int __kprobes kprobe_fault_handler(struc
*/
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:
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c
@@ -283,16 +283,6 @@ int __kprobes kprobe_fault_handler(struc
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.
*/
--- a/arch/csky/kernel/probes/kprobes.c
+++ b/arch/csky/kernel/probes/kprobes.c
@@ -302,16 +302,6 @@ int __kprobes kprobe_fault_handler(struc
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.
*/
--- a/arch/ia64/kernel/kprobes.c
+++ b/arch/ia64/kernel/kprobes.c
@@ -851,15 +851,6 @@ int __kprobes kprobe_fault_handler(struc
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.
*/
--- a/arch/mips/kernel/kprobes.c
+++ b/arch/mips/kernel/kprobes.c
@@ -403,9 +403,6 @@ int kprobe_fault_handler(struct pt_regs
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;
--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c
@@ -509,16 +509,6 @@ int kprobe_fault_handler(struct pt_regs
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.
*/
--- a/arch/s390/kernel/kprobes.c
+++ b/arch/s390/kernel/kprobes.c
@@ -453,16 +453,6 @@ static int kprobe_trap_handler(struct pt
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.
*/
--- a/arch/sh/kernel/kprobes.c
+++ b/arch/sh/kernel/kprobes.c
@@ -390,16 +390,6 @@ int __kprobes kprobe_fault_handler(struc
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.
*/
--- a/arch/sparc/kernel/kprobes.c
+++ b/arch/sparc/kernel/kprobes.c
@@ -353,16 +353,6 @@ int __kprobes kprobe_fault_handler(struc
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.
*/
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -962,16 +962,6 @@ int kprobe_fault_handler(struct pt_regs
* 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;
--- a/include/linux/kprobes.h
+++ b/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;

--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1183,23 +1183,6 @@ static void aggr_post_handler(struct kpr
}
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 kpro
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;
@@ -1991,7 +1973,6 @@ int register_kretprobe(struct kretprobe

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) {
--- a/samples/kprobes/kprobe_example.c
+++ b/samples/kprobes/kprobe_example.c
@@ -79,26 +79,11 @@ static void __kprobes handler_post(struc
#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) {
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