[PATCH 2/3] x32: Properly pass long to syscall [BZ #25810]

[Florian Weimer]

* H. J. Lu:

> On Mon, Apr 13, 2020 at 7:43 AM Florian Weimer <fw@deneb.enyo.de> wrote:
>>
>> * H. J. Lu via Libc-alpha:
>>
>> > diff --git a/sysdeps/unix/sysv/linux/x86_64/sysdep.h b/sysdeps/unix/sysv/linux/x86_64/sysdep.h
>> > index 06e39212ee..93dad97c8f 100644
>> > --- a/sysdeps/unix/sysv/linux/x86_64/sysdep.h
>> > +++ b/sysdeps/unix/sysv/linux/x86_64/sysdep.h
>> > @@ -219,12 +219,14 @@
>> >  /* Registers clobbered by syscall.  */
>> >  # define REGISTERS_CLOBBERED_BY_SYSCALL "cc", "r11", "cx"
>> >
>> > -/* Create a variable 'name' based on type 'X' to avoid explicit types.
>> > -   This is mainly used set use 64-bits arguments in x32.   */
>> > -#define TYPEFY(X, name) __typeof__ ((X) - (X)) name
>> > +/* This also works when X is an array.  */
>> > +#define TYPEFY1(X) __typeof__ ((X) - (X))
>>
>> I think the comment is now misleading.  For an array X, (X) - (X) is
>> of type ptrdiff_t, which is signed, so it triggers sign extension, not
>
> I will update the comments.

Thanks.

>> zero extension, in the x32 case.  I think this is the reason why my
>> proposed construct went astray in your experiments.
>>
>> Is it really necessary to support arrays for system call arguments?
>
> int
> fexecve (int fd, char *const argv[], char *const envp[])
> {
>   if (fd < 0 || argv == NULL || envp == NULL)
>     {
>       __set_errno (EINVAL);
>       return -1;
>     }
>
> #ifdef __NR_execveat
>   /* Avoid implicit array coercion in syscall macros.  */
>   INLINE_SYSCALL (execveat, 5, fd, "", &argv[0], &envp[0], AT_EMPTY_PATH);
>
> "" is an array.

If that were the only reason for enforcing pointer decay, I'd say we'd
change the fexecve implementation.  But see below.

>> > diff --git a/sysdeps/unix/sysv/linux/x86_64/x32/sysdep.h b/sysdeps/unix/sysv/linux/x86_64/x32/sysdep.h
>> > index 2b5cf0c0ea..f3bc7e89df 100644
>> > --- a/sysdeps/unix/sysv/linux/x86_64/x32/sysdep.h
>> > +++ b/sysdeps/unix/sysv/linux/x86_64/x32/sysdep.h
>> > @@ -44,6 +44,24 @@
>> >  # define ZERO_EXTEND_5 movl %r8d, %r8d;
>> >  # undef ZERO_EXTEND_6
>> >  # define ZERO_EXTEND_6 movl %r9d, %r9d;
>> > +#else /*! __ASSEMBLER__ */
>> > +# undef ARGIFY
>> > +/* For x32, cast
>> > +   1. pointer to unsigned long (32 bit).
>> > +   2. 32-bit unsigned integer to unsigned long long (64 bit).
>> > +   3. 32-bit integer to long long (64 bit).
>> > + */
>> > +# define ARGIFY(X) \
>> > + ({                                                                  \
>> > +    _Pragma ("GCC diagnostic push");                                 \
>> > +    _Pragma ("GCC diagnostic ignored \"-Wpointer-to-int-cast\"");    \
>> > +    (__builtin_classify_type (X) == 5                                        \
>> > +     ? (unsigned long) (X)                                           \
>> > +     : ((sizeof (X) <= 4 && (TYPEFY1 (X)) 0 < (TYPEFY1 (X)) -1)              \
>> > +     ? (unsigned long long) (X)                                      \
>> > +     : (long long) (X)));                                            \
>> > +    _Pragma ("GCC diagnostic pop");                                  \
>> > +  })
>> >  #endif       /* __ASSEMBLER__ */
>>
>> This should use long int instead long everywhere.
>
> I will update.
>
>> __builtin_classify_type is undocumented.  I'm not sure if its behavior
>> and the constant 5 is actually stable across GCC versions, but the
>> powerpc uses it as well, for a slightly difference purpose (to
>> recognize array arguments, it seems).
>
> include/libc-pointer-arith.h has
>
> /* 1 if 'type' is a pointer type, 0 otherwise.  */
> # define __pointer_type(type) (__builtin_classify_type ((type) 0) == 5)
>
> If __builtin_classify_type ever changes, glibc won't build correctly.

I see.  In this case, can we solely rely on __builtin_classify_type?
I dont't think the inner ternary operator is necessary.

/* Enforce zero-extension for pointers and array system call
   arguments.  For integer types, extend to long long int (the full
   register) using a regular cast, resulting in zero or sign extension
   based on the signed-ness of the original type.  */
#define ARGIFY(X)                                                    \
  ({                                                                 \
    _Pragma ("GCC diagnostic push");                                 \
    _Pragma ("GCC diagnostic ignored \"-Wpointer-to-int-cast\"");    \
    (__builtin_classify_type (X) == 5                                \
     ? (long long int) (uintptr_t) (X)                               \
     : (long long int) (X)));                                        \
    _Pragma ("GCC diagnostic pop");                                  \
  })

(Untested, sorry.)