From 3049fc4b5f1d2320a84e2902b3ac5a735f60ca04 Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Wed, 17 Sep 2025 09:03:18 -0700 Subject: [PATCH] x86/alternative: Refactor INT3 call emulation selftest The INT3 call emulation selftest is a bit fragile as it relies on the compiler not inserting any extra instructions before the int3_selftest_ip() definition. Also, the int3_selftest_ip() symbol overlaps with the int3_selftest symbol(), which can confuse objtool. Fix those issues by slightly reworking the functionality and moving int3_selftest_ip() to a separate asm function. While at it, improve the naming. Acked-by: Petr Mladek Tested-by: Joe Lawrence Signed-off-by: Josh Poimboeuf --- arch/x86/kernel/alternative.c | 51 +++++++++++++++++++---------------- 1 file changed, 28 insertions(+), 23 deletions(-) diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index 8ee5ff547357a..d19a3fd7cf04f 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -2244,21 +2244,34 @@ int alternatives_text_reserved(void *start, void *end) * See entry_{32,64}.S for more details. */ -/* - * We define the int3_magic() function in assembly to control the calling - * convention such that we can 'call' it from assembly. - */ +extern void int3_selftest_asm(unsigned int *ptr); -extern void int3_magic(unsigned int *ptr); /* defined in asm */ +asm ( +" .pushsection .init.text, \"ax\", @progbits\n" +" .type int3_selftest_asm, @function\n" +"int3_selftest_asm:\n" + ANNOTATE_NOENDBR + /* + * INT3 padded with NOP to CALL_INSN_SIZE. The INT3 triggers an + * exception, then the int3_exception_nb notifier emulates a call to + * int3_selftest_callee(). + */ +" int3; nop; nop; nop; nop\n" + ASM_RET +" .size int3_selftest_asm, . - int3_selftest_asm\n" +" .popsection\n" +); + +extern void int3_selftest_callee(unsigned int *ptr); asm ( " .pushsection .init.text, \"ax\", @progbits\n" -" .type int3_magic, @function\n" -"int3_magic:\n" +" .type int3_selftest_callee, @function\n" +"int3_selftest_callee:\n" ANNOTATE_NOENDBR -" movl $1, (%" _ASM_ARG1 ")\n" +" movl $0x1234, (%" _ASM_ARG1 ")\n" ASM_RET -" .size int3_magic, .-int3_magic\n" +" .size int3_selftest_callee, . - int3_selftest_callee\n" " .popsection\n" ); @@ -2267,7 +2280,7 @@ extern void int3_selftest_ip(void); /* defined in asm below */ static int __init int3_exception_notify(struct notifier_block *self, unsigned long val, void *data) { - unsigned long selftest = (unsigned long)&int3_selftest_ip; + unsigned long selftest = (unsigned long)&int3_selftest_asm; struct die_args *args = data; struct pt_regs *regs = args->regs; @@ -2282,7 +2295,7 @@ int3_exception_notify(struct notifier_block *self, unsigned long val, void *data if (regs->ip - INT3_INSN_SIZE != selftest) return NOTIFY_DONE; - int3_emulate_call(regs, (unsigned long)&int3_magic); + int3_emulate_call(regs, (unsigned long)&int3_selftest_callee); return NOTIFY_STOP; } @@ -2298,19 +2311,11 @@ static noinline void __init int3_selftest(void) BUG_ON(register_die_notifier(&int3_exception_nb)); /* - * Basically: int3_magic(&val); but really complicated :-) - * - * INT3 padded with NOP to CALL_INSN_SIZE. The int3_exception_nb - * notifier above will emulate CALL for us. + * Basically: int3_selftest_callee(&val); but really complicated :-) */ - asm volatile ("int3_selftest_ip:\n\t" - ANNOTATE_NOENDBR - " int3; nop; nop; nop; nop\n\t" - : ASM_CALL_CONSTRAINT - : __ASM_SEL_RAW(a, D) (&val) - : "memory"); - - BUG_ON(val != 1); + int3_selftest_asm(&val); + + BUG_ON(val != 0x1234); unregister_die_notifier(&int3_exception_nb); } -- 2.47.3