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Subject[RFC PATCH -tip 7/9] x86: kprobes: Do not disable preempt on int3 path
Date
Since int3 and debug exception(for singlestep) are run with
IRQ disabled and while running single stepping we drop IF
from regs->flags, that path must not be preemptible. So we
can remove the preempt disable/enable calls from that path.

Note that, this changes the behavior of execution path override
which is done by modifying regs->ip in pre_handler().
Previously it requires reset_current_kprobe(), enable preempt
and return !0.
With this change, preempt count is not changed on int3 path, so
user no need to enable preempt. To fit this behavior, this
modifies ftrace-based kprobe too. In total, pre_handler() does
not need to recover preempt count even if it changes regs->ip.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Suggested-by: Ingo Molnar <mingo@kernel.org>
---
Documentation/kprobes.txt | 11 +++++------
arch/x86/kernel/kprobes/core.c | 14 +++-----------
arch/x86/kernel/kprobes/ftrace.c | 5 ++---
arch/x86/kernel/kprobes/opt.c | 1 -
4 files changed, 10 insertions(+), 21 deletions(-)

diff --git a/Documentation/kprobes.txt b/Documentation/kprobes.txt
index 94df43224c6c..679cba3380e4 100644
--- a/Documentation/kprobes.txt
+++ b/Documentation/kprobes.txt
@@ -566,12 +566,11 @@ the same handler) may run concurrently on different CPUs.
Kprobes does not use mutexes or allocate memory except during
registration and unregistration.

-Probe handlers are run with preemption disabled. Depending on the
-architecture and optimization state, handlers may also run with
-interrupts disabled (e.g., kretprobe handlers and optimized kprobe
-handlers run without interrupt disabled on x86/x86-64). In any case,
-your handler should not yield the CPU (e.g., by attempting to acquire
-a semaphore).
+Probe handlers are run with preemption disabled or interrupt disabled,
+which depends on the architecture and optimization state. (e.g.,
+kretprobe handlers and optimized kprobe handlers run without interrupt
+disabled on x86/x86-64). In any case, your handler should not yield
+the CPU (e.g., by attempting to acquire a semaphore, or waiting I/O).

Since a return probe is implemented by replacing the return
address with the trampoline's address, stack backtraces and calls
diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 7663f23aa860..b771a01d51f4 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -592,7 +592,6 @@ static void setup_singlestep(struct kprobe *p, struct pt_regs *regs,
* stepping.
*/
regs->ip = (unsigned long)p->ainsn.insn;
- preempt_enable_no_resched();
return;
}
#endif
@@ -665,12 +664,10 @@ int kprobe_int3_handler(struct pt_regs *regs)

addr = (kprobe_opcode_t *)(regs->ip - sizeof(kprobe_opcode_t));
/*
- * We don't want to be preempted for the entire
- * duration of kprobe processing. We conditionally
- * re-enable preemption at the end of this function,
- * and also in reenter_kprobe() and setup_singlestep().
+ * We don't want to be preempted for the entire duration of kprobe
+ * processing. Since int3 and debug trap disables irqs and we clear
+ * IF while singlestepping, it must be no preemptible.
*/
- preempt_disable();

kcb = get_kprobe_ctlblk();
p = get_kprobe(addr);
@@ -704,11 +701,9 @@ int kprobe_int3_handler(struct pt_regs *regs)
* the original instruction.
*/
regs->ip = (unsigned long)addr;
- preempt_enable_no_resched();
return 1;
} /* else: not a kprobe fault; let the kernel handle it */

- preempt_enable_no_resched();
return 0;
}
NOKPROBE_SYMBOL(kprobe_int3_handler);
@@ -959,8 +954,6 @@ int kprobe_debug_handler(struct pt_regs *regs)
}
reset_current_kprobe();
out:
- preempt_enable_no_resched();
-
/*
* if somebody else is singlestepping across a probe point, flags
* will have TF set, in which case, continue the remaining processing
@@ -1007,7 +1000,6 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
restore_previous_kprobe(kcb);
else
reset_current_kprobe();
- preempt_enable_no_resched();
} else if (kcb->kprobe_status == KPROBE_HIT_ACTIVE ||
kcb->kprobe_status == KPROBE_HIT_SSDONE) {
/*
diff --git a/arch/x86/kernel/kprobes/ftrace.c b/arch/x86/kernel/kprobes/ftrace.c
index c8696f2a583f..f1ac65d6b352 100644
--- a/arch/x86/kernel/kprobes/ftrace.c
+++ b/arch/x86/kernel/kprobes/ftrace.c
@@ -67,10 +67,9 @@ void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
preempt_disable();
__this_cpu_write(current_kprobe, p);
kcb->kprobe_status = KPROBE_HIT_ACTIVE;
- if (!p->pre_handler || !p->pre_handler(p, regs)) {
+ if (!p->pre_handler || !p->pre_handler(p, regs))
skip_singlestep(p, regs, kcb, orig_ip);
- preempt_enable_no_resched();
- }
+ preempt_enable_no_resched();
/*
* If pre_handler returns !0, it sets regs->ip and
* resets current kprobe, and keep preempt count +1.
diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
index 203d398802a3..eaf02f2e7300 100644
--- a/arch/x86/kernel/kprobes/opt.c
+++ b/arch/x86/kernel/kprobes/opt.c
@@ -491,7 +491,6 @@ int setup_detour_execution(struct kprobe *p, struct pt_regs *regs, int reenter)
regs->ip = (unsigned long)op->optinsn.insn + TMPL_END_IDX;
if (!reenter)
reset_current_kprobe();
- preempt_enable_no_resched();
return 1;
}
return 0;
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