解决LKM编译报错:include/linux/compiler_types.h:258:20: warning: ‘asm’ operand 2 probably does not match constraints

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背景交代

在进行linux LKM（Loadable Kernel Module）开发过程中，make时发生了莫名奇妙的报错。
该报错问题拦了我3个小时，主要原因在于，报错代码不是我的代码，而是报系统源码的问题，报错内容如下：

 CC [M]  /root/mysrc/testproject/testproject_main.o
In file included from ./include/linux/filter.h:25,
                 from ./include/net/sock.h:59,
                 from ./include/linux/tcp.h:19,
                 from /root/mysrc/testproject/testproject_main.c:8:
./include/net/sch_generic.h: In function ‘qdisc_cb_private_validate’:
./include/net/sch_generic.h:508:30: warning: unused variable ‘qcb’ [-Wunused-variable]
  508 |         struct qdisc_skb_cb *qcb;
      |                              ^~~
In file included from <command-line>:
./include/linux/thread_info.h: In function ‘copy_overflow’:
././include/linux/compiler_types.h:258:20: warning: ‘asm’ operand 2 probably does not match constraints
  258 | #define asm_inline asm __inline
      |                    ^~~
./arch/x86/include/asm/bug.h:27:9: note: in expansion of macro ‘asm_inline’
   27 |         asm_inline volatile("1:\t" ins "\n"                             \
      |         ^~~~~~~~~~
./arch/x86/include/asm/bug.h:82:9: note: in expansion of macro ‘_BUG_FLAGS’
   82 |         _BUG_FLAGS(ASM_UD2, __flags, ASM_REACHABLE);            \
      |         ^~~~~~~~~~
./include/asm-generic/bug.h:100:17: note: in expansion of macro ‘__WARN_FLAGS’
  100 |                 __WARN_FLAGS(BUGFLAG_NO_CUT_HERE | BUGFLAG_TAINT(taint));\
      |                 ^~~~~~~~~~~~
./include/asm-generic/bug.h:132:17: note: in expansion of macro ‘__WARN_printf’
  132 |                 __WARN_printf(TAINT_WARN, format);                      \
      |                 ^~~~~~~~~~~~~
./include/linux/thread_info.h:200:9: note: in expansion of macro ‘WARN’
  200 |         WARN(1, "Buffer overflow detected (%d < %lu)!\n", size, count);
      |         ^~~~
././include/linux/compiler_types.h:258:20: error: impossible constraint in ‘asm’
  258 | #define asm_inline asm __inline
      |                    ^~~
./arch/x86/include/asm/bug.h:27:9: note: in expansion of macro ‘asm_inline’
   27 |         asm_inline volatile("1:\t" ins "\n"                             \
      |         ^~~~~~~~~~
./arch/x86/include/asm/bug.h:82:9: note: in expansion of macro ‘_BUG_FLAGS’
   82 |         _BUG_FLAGS(ASM_UD2, __flags, ASM_REACHABLE);            \
      |         ^~~~~~~~~~
./include/asm-generic/bug.h:100:17: note: in expansion of macro ‘__WARN_FLAGS’
  100 |                 __WARN_FLAGS(BUGFLAG_NO_CUT_HERE | BUGFLAG_TAINT(taint));\
      |                 ^~~~~~~~~~~~
./include/asm-generic/bug.h:132:17: note: in expansion of macro ‘__WARN_printf’
  132 |                 __WARN_printf(TAINT_WARN, format);                      \
      |                 ^~~~~~~~~~~~~
./include/linux/thread_info.h:200:9: note: in expansion of macro ‘WARN’
  200 |         WARN(1, "Buffer overflow detected (%d < %lu)!\n", size, count);
      |         ^~~~
./include/linux/thread_info.h: In function ‘hook_getdents64’:
././include/linux/compiler_types.h:258:20: warning: ‘asm’ operand 2 probably does not match constraints
  258 | #define asm_inline asm __inline
      |                    ^~~
./arch/x86/include/asm/bug.h:27:9: note: in expansion of macro ‘asm_inline’
   27 |         asm_inline volatile("1:\t" ins "\n"                             \
      |         ^~~~~~~~~~
./arch/x86/include/asm/bug.h:82:9: note: in expansion of macro ‘_BUG_FLAGS’
   82 |         _BUG_FLAGS(ASM_UD2, __flags, ASM_REACHABLE);            \
      |         ^~~~~~~~~~
./include/asm-generic/bug.h:106:17: note: in expansion of macro ‘__WARN_FLAGS’
  106 |                 __WARN_FLAGS(BUGFLAG_ONCE |                     \
      |                 ^~~~~~~~~~~~
./include/linux/thread_info.h:216:13: note: in expansion of macro ‘WARN_ON_ONCE’
  216 |         if (WARN_ON_ONCE(bytes > INT_MAX))
      |             ^~~~~~~~~~~~
make[1]: *** [scripts/Makefile.build:285: /root/mysrc/testproject/testproject_main.o] Error 1
make: *** [Makefile:1875: /root/mysrc/testproject] Error 2
make: Leaving directory '/usr/src/linux-headers-5.15.0-40-generic'

由于是将我的低版本内核代码移植到高版本内核(5.15)后导致的。在花了很多时间解决掉我的代码的移植问题后，我的代码终于不报错了，但诡异的是，系统的源码开始报编译错误。

一开始将报错内容甩给各个AI，无法分析出问题，最后通过二分法确定，是我在调用copy_from_user/copy_to_user的时候才会出现该编译问题。于是查看了该两个函数在5.15中的的定义：

static __always_inline unsigned long __must_check
copy_from_user(void *to, const void __user *from, unsigned long n)
{
	if (likely(check_copy_size(to, n, false)))
		n = _copy_from_user(to, from, n);
	return n;
}

static __always_inline unsigned long __must_check
copy_to_user(void __user *to, const void *from, unsigned long n)
{
	if (likely(check_copy_size(from, n, true)))
		n = _copy_to_user(to, from, n);
	return n;
}

这两个函数都是强制内联+必须检查返回值。根据这些信息再次跟AI多轮沟通，最终给到我两个原因。

可能的原因

内存访问越界：copy_from_user 和 copy_to_user 函数在复制数据时，若源地址或者目标地址无效，或者复制的长度超出了可用内存范围，就会引发错误。
编译器优化问题：编译器对内联汇编代码进行优化时，可能会产生不匹配的约束。

对于第一个原因，我这边对函数的参数和返回值都通过if判断进行了合理性检查，所以排除。
那就剩下编译器优化问题，我的项目是LKM，所以编译规则都放在Makefile里，我的Makefile如下：

obj-m := testproject.o
CC = gcc -ggdb -O0
KDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)

all:
	$(MAKE) -C $(KDIR) M=$(PWD) modules

clean:
	$(MAKE) -C $(KDIR) M=$(PWD) clean

而问题就出在编译上面，由于我要进行LKM调试，所以打开了调试选项，并且禁止优化(-O0)。

这种情况下，在调用被"__always_inline"修饰的函数的时候会因为强制内联时汇编代码不匹配从而报错。所以，关闭掉-O0选项，问题即可解决。

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