[RFC 01/31] x86/alternative: Refactor INT3 call emulation selftest

Josh Poimboeuf
2024-09-03T04:00:29Z

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 tooling like 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.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
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 89de61243272..56fca5e6ba23 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1546,21 +1546,34 @@ 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_magic(unsigned int *ptr); /* defined in asm */
+extern void int3_selftest_asm(unsigned int *ptr);

asm (
" .pushsection .init.text, \"ax\", @progbits\n"
-" .type int3_magic, @function\n"
-"int3_magic:\n"
+" .type int3_selftest_asm, @function\n"
+"int3_selftest_asm:\n"
ANNOTATE_NOENDBR
-" movl $1, (%" _ASM_ARG1 ")\n"
+ /*
+ * 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_magic, .-int3_magic\n"
+" .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_selftest_callee, @function\n"
+"int3_selftest_callee:\n"
+ ANNOTATE_NOENDBR
+" movl $0x1234, (%" _ASM_ARG1 ")\n"
+ ASM_RET
+" .size int3_selftest_callee, . - int3_selftest_callee\n"
" .popsection\n"
);

@@ -1569,7 +1582,7 @@ 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;

@@ -1584,7 +1597,7 @@ 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;
}

@@ -1600,19 +1613,11 @@ 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");
+ int3_selftest_asm(&val);

- BUG_ON(val != 1);
+ BUG_ON(val != 0x1234);

unregister_die_notifier(&int3_exception_nb);
}
--
2.45.2