Re: [XEN PATCH v2 2/3] x86/uaccess: replace __{get,put}_user_bad() with STATIC_ASSERT_UNREACHABLE()

[Federico Serafini <federico.serafini@xxxxxxxxxxx>]

On 15/02/24 09:10, Jan Beulich wrote:
> On 14.02.2024 17:11, Federico Serafini wrote:
> > I did some tries with example programs
> > and the assembler error always points to file and line
> > of the most enclosing function that caused the failure.
> > If I am not missing something, using __FILE__ and __LINE__ does not add
> > any information.
> >
> > Therefore, if the new macro is used within the body of other macros,
> > then the resulting assembler error will point to the source of
> > the problem (e.g., the site of a bogus call to put_guest()).
> >
> > In my opinion, converting put_guest() &Co. to inline functions is not
> > convenient: the assembler error will point to the most enclosing
> > function that would be put_unsafe_size(), instead of pointing to the
> > source of the problem.
> The assembler error will point to where the inline function was expanded,
> sure. __FILE__ / __LINE__ ought to point to that inline function (where
> the macro was used) then, though?
This is what I get:

federico@Dell:~$ cat m.c
#define STRINGIFY(arg) #arg
#define STATIC_ASSERT_UNREACHABLE(file, line) \
  asm(".error \"static assertion failed: " file ": " STRINGIFY(line) "\"")

static inline __attribute__((always_inline)) void g(int x) {
  switch(x) {
    case 0:
      STATIC_ASSERT_UNREACHABLE(__FILE__, __LINE__);
  }
}

static inline __attribute__((always_inline)) void f(int x) {
  g(x);
}

int main(void) {
  f(0);
  return 0;
}
federico@Dell:~$ gcc -O3 m.c
m.c: Assembler messages:
m.c:8: Error: static assertion failed: m.c: 8


Note that the linker behaves differently:

federico@Dell:~$ cat m.c
extern void __put_user_bad(void);

static inline __attribute__((always_inline)) void g(int x) {
  switch(x) {
    case 0:
      __put_user_bad();
  }
}

static inline __attribute__((always_inline)) void f(int x) {
  g(x);
}

int main(void) {
  f(0);
  return 0;
}
federico@Dell:~$ gcc -O3 m.c
/usr/bin/ld: /tmp/ccv9KHJD.o: in function `main':
m.c:(.text.startup+0x9): undefined reference to `__put_user_bad'
collect2: error: ld returned 1 exit status

--
Federico Serafini, M.Sc.

Software Engineer, BUGSENG (http://bugseng.com)