[PATCH RFC 07/13] x86/kprobes: Maintain Tasks RCU trampoline nesting in the optprobe template

[Josef Bacik <josef@toxicpanda.com>]

The jump-optimized kprobe template calls optimized_callback() with
preemption still enabled for its first few instructions, so bracket the
call with an increment/decrement of current->rcu_tramp_nesting.  The
template lives in .rodata and is memcpy()d into each optinsn slot without
relocation processing, so the per-CPU reference to current_task must be
an absolute %gs: address (R_X86_64_32S, relocated for KASLR like any
other) rather than %rip-relative.  %rax has already been saved by
SAVE_REGS_STRING and is dead after the call.

The slot itself is dynamically allocated text, so the instructions before
the increment and after the decrement are covered by the irq-exit IP
check.  64-bit only; 32-bit x86 does not take part.

Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 arch/x86/kernel/kprobes/opt.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
index 3f8fea52619f..f722520bb989 100644
--- a/arch/x86/kernel/kprobes/opt.c
+++ b/arch/x86/kernel/kprobes/opt.c
@@ -31,6 +31,7 @@
 #include <asm/set_memory.h>
 #include <asm/sections.h>
 #include <asm/nospec-branch.h>
+#include <asm/asm-offsets.h>
 
 #include "common.h"
 
@@ -101,6 +102,23 @@ static void synthesize_set_arg1(kprobe_opcode_t *addr, unsigned long val)
 	*(unsigned long *)addr = val;
 }
 
+/*
+ * Tasks RCU trampoline nesting, see rcu_tasks_trampoline_enter().  The
+ * template is memcpy()d into the slot without relocation processing, so the
+ * per-CPU reference must be absolute, not %rip-relative.
+ */
+#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_X86_64)
+#define OPTPROBE_RCU_TASKS_ENTER					\
+			"	movq %gs:current_task, %rax\n"		\
+			"	incl " __stringify(TASK_rcu_tramp_nesting) "(%rax)\n"
+#define OPTPROBE_RCU_TASKS_EXIT					\
+			"	movq %gs:current_task, %rax\n"		\
+			"	decl " __stringify(TASK_rcu_tramp_nesting) "(%rax)\n"
+#else
+#define OPTPROBE_RCU_TASKS_ENTER
+#define OPTPROBE_RCU_TASKS_EXIT
+#endif
+
 asm (
 			".pushsection .rodata\n"
 			".global optprobe_template_entry\n"
@@ -114,6 +132,7 @@ asm (
 			"optprobe_template_clac:\n"
 			ASM_NOP3
 			SAVE_REGS_STRING
+			OPTPROBE_RCU_TASKS_ENTER
 			"	movq %rsp, %rsi\n"
 			".global optprobe_template_val\n"
 			"optprobe_template_val:\n"
@@ -122,6 +141,7 @@ asm (
 			".global optprobe_template_call\n"
 			"optprobe_template_call:\n"
 			ASM_NOP5
+			OPTPROBE_RCU_TASKS_EXIT
 			/* Copy 'regs->flags' into 'regs->ss'. */
 			"	movq 18*8(%rsp), %rdx\n"
 			"	movq %rdx, 20*8(%rsp)\n"

-- 
2.55.0