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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 <pmladek@suse.com>
Tested-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
This commit is contained in:
Josh Poimboeuf 2025-09-17 09:03:18 -07:00
parent 4109043bff
commit 3049fc4b5f
1 changed files with 28 additions and 23 deletions
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51
arch/x86/kernel/alternative.c
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@ -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_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"
);
@ -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");
int3_selftest_asm(&val);
BUG_ON(val != 1);
BUG_ON(val != 0x1234);
unregister_die_notifier(&int3_exception_nb);
}