[PATCH 2/3] x86_64: Allow breakpoints to emulate call instructions

From: Steven Rostedt
Date: Tue May 07 2019 - 22:01:07 EST

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

In order to allow breakpoints to emulate call instructions, they need to push
the return address onto the stack. The x86_64 int3 handler adds a small gap
to allow the stack to grow some. Use this gap to add the return address to
be able to emulate a call instruction at the breakpoint location.

These helper functions are added:

int3_emulate_jmp(): changes the location of the regs->ip to return there.

(The next two are only for x86_64)
int3_emulate_push(): to push the address onto the gap in the stack
int3_emulate_call(): push the return address and change regs->ip

Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Nicolai Stange <nstange@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: the arch/x86 maintainers <x86@xxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Jiri Kosina <jikos@xxxxxxxxxx>
Cc: Miroslav Benes <mbenes@xxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Mimi Zohar <zohar@xxxxxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Nayna Jain <nayna@xxxxxxxxxxxxx>
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Cc: Joerg Roedel <jroedel@xxxxxxx>
Cc: "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@xxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: b700e7f03df5 ("livepatch: kernel: add support for live patching")
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
[ Modified to only work for x86_64 and added comment to int3_emulate_push() ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
arch/x86/include/asm/text-patching.h | 28 ++++++++++++++++++++++++++++
1 file changed, 28 insertions(+)

diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
index e85ff65c43c3..05861cc08787 100644
--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -39,4 +39,32 @@ extern int poke_int3_handler(struct pt_regs *regs);
extern void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler);
extern int after_bootmem;

+static inline void int3_emulate_jmp(struct pt_regs *regs, unsigned long ip)
+ regs->ip = ip;
+#define INT3_INSN_SIZE 1
+#define CALL_INSN_SIZE 5
+#ifdef CONFIG_X86_64
+static inline void int3_emulate_push(struct pt_regs *regs, unsigned long val)
+ /*
+ * The int3 handler in entry_64.S adds a gap between the
+ * stack where the break point happened, and the saving of
+ * pt_regs. We can extend the original stack because of
+ * this gap. See the idtentry macro's create_gap option.
+ */
+ regs->sp -= sizeof(unsigned long);
+ *(unsigned long *)regs->sp = val;
+static inline void int3_emulate_call(struct pt_regs *regs, unsigned long func)
+ int3_emulate_push(regs, regs->ip - INT3_INSN_SIZE + CALL_INSN_SIZE);
+ int3_emulate_jmp(regs, func);
#endif /* _ASM_X86_TEXT_PATCHING_H */