[PATCH RFC v3 08/13] x86/kprobes: Take a Tasks Trace reader in the optprobe template

[Josef Bacik]

The jump-optimized kprobe template calls optimized_callback() from a
dynamically allocated slot with preemption enabled, and only Tasks RCU
keeps that slot alive under a task preempted in the callback.  For
HAVE_RCU_TRAMPOLINE_READERS that means the template must be a Tasks
Trace reader across the call, so open-code rcu_read_lock_trace() and
rcu_read_unlock_trace() around it as ftrace_64.S does.  The template
lives in .rodata and is memcpy()d into each slot without relocation
processing, so the references to current_task and
rcu_tasks_trace_srcu_struct are absolute (R_X86_64_32S, relocated for
KASLR like any other) rather than %rip-relative.  %rax and %rcx have
already been saved by SAVE_REGS_STRING and are dead after the call.

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

Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
 arch/x86/kernel/kprobes/opt.c | 44 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
index 3f8fea52619f..68a5de6cdabe 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 +102,47 @@ static void synthesize_set_arg1(kprobe_opcode_t *addr, unsigned long val)
 	*(unsigned long *)addr = val;
 }
 
+/*
+ * Open-coded rcu_read_lock_trace() / rcu_read_unlock_trace() around the call
+ * to optimized_callback(), see CONFIG_HAVE_RCU_TRAMPOLINE_READERS and the
+ * equivalent macros in ftrace_64.S.  The template is memcpy()d into the slot
+ * without relocation processing, so memory references must be absolute
+ * rather than %rip-relative.  %rax and %rcx are free at both points.
+ */
+#ifdef CONFIG_TASKS_RCU_TRAMPOLINE_READERS
+#ifndef CONFIG_TASKS_TRACE_RCU_NO_MB
+#define OPTPROBE_TRACE_RCU_MB	"	lock addl $0, -4(%rsp)\n"
+#else
+#define OPTPROBE_TRACE_RCU_MB
+#endif
+#define OPTPROBE_TRACE_RCU_READ_LOCK						\
+		"	movq %gs:current_task, %rcx\n"				\
+		"	movl " __stringify(TASK_trc_reader_nesting) "(%rcx), %eax\n"	\
+		"	incl " __stringify(TASK_trc_reader_nesting) "(%rcx)\n"	\
+		"	testl %eax, %eax\n"						\
+		"	jnz 1f\n"							\
+		"	movq rcu_tasks_trace_srcu_struct+" __stringify(SRCU_srcu_ctrp) ", %rax\n" \
+		"	incq %gs:" __stringify(SRCU_CTR_srcu_locks) "(%rax)\n"		\
+		"	movq %rax, " __stringify(TASK_trc_reader_scp) "(%rcx)\n"	\
+		OPTPROBE_TRACE_RCU_MB						\
+		"1:\n"
+#define OPTPROBE_TRACE_RCU_READ_UNLOCK						\
+		"	movq %gs:current_task, %rcx\n"				\
+		"	movl " __stringify(TASK_trc_reader_nesting) "(%rcx), %eax\n"	\
+		"	subl $1, %eax\n"						\
+		"	jnz 2f\n"							\
+		"	movq " __stringify(TASK_trc_reader_scp) "(%rcx), %rax\n"	\
+		"	movl $0, " __stringify(TASK_trc_reader_nesting) "(%rcx)\n"	\
+		OPTPROBE_TRACE_RCU_MB						\
+		"	incq %gs:" __stringify(SRCU_CTR_srcu_unlocks) "(%rax)\n"	\
+		"	jmp 3f\n"							\
+		"2:	movl %eax, " __stringify(TASK_trc_reader_nesting) "(%rcx)\n"	\
+		"3:\n"
+#else
+#define OPTPROBE_TRACE_RCU_READ_LOCK
+#define OPTPROBE_TRACE_RCU_READ_UNLOCK
+#endif
+
 asm (
 			".pushsection .rodata\n"
 			".global optprobe_template_entry\n"
@@ -114,6 +156,7 @@ asm (
 			"optprobe_template_clac:\n"
 			ASM_NOP3
 			SAVE_REGS_STRING
+			OPTPROBE_TRACE_RCU_READ_LOCK
 			"	movq %rsp, %rsi\n"
 			".global optprobe_template_val\n"
 			"optprobe_template_val:\n"
@@ -122,6 +165,7 @@ asm (
 			".global optprobe_template_call\n"
 			"optprobe_template_call:\n"
 			ASM_NOP5
+			OPTPROBE_TRACE_RCU_READ_UNLOCK
 			/* Copy 'regs->flags' into 'regs->ss'. */
 			"	movq 18*8(%rsp), %rdx\n"
 			"	movq %rdx, 20*8(%rsp)\n"

-- 
2.55.0