Message-ID: <CACVxJT_=GLvA0D9LGsXnOcj+r8VgFMP4pcL5oM6cQ2CcT2ZNVA@mail.gmail.com>
Date:	Mon, 4 May 2015 17:32:50 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/10] Add parse_integer() (replacement for simple_strto*())

On Mon, May 4, 2015 at 4:24 PM, Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
> On Sat, May 02 2015, Alexey Dobriyan <adobriyan@...il.com> wrote:

>> Enter parse_integer().
>>
>>       int parse_integer(const char *s, unsigned int base, T *val);
>>
>
> I like the general idea. Few nits below (and in reply to other patches).
>
> First: Could you tell me what tree I can commit this on top of, to see
> what gcc makes of it.

Any recent kernel should be OK, code it quite self contained.
I've just applied first two patches on top 4.1-rc2.
BTW the correct order is

1) [PATCH 01/10] Add parse_integer() (replacement for simple_strto*())
*** 2) [PATCH CORRECT 03/10] parse_integer: convert sscanf()
3) [PATCH 03/10] parse_integer: convert sscanf()
4) [PATCH 04/10] sscanf: fix overflow
 ...
10) [PATCH 10/10] ext2, ext3, ext4: convert to parse_integer()/kstrto*()

I've copied patch #2 twice, so it won't apply and resent it
with subject from patch #3 to confuse everyone even more.

>> +#define parse_integer(s, base, val)  \
>> +({                                   \
>> +     const char *_s = (s);           \
>> +     unsigned int _base = (base);    \
>> +     typeof(val) _val = (val);       \
>> +                                     \
>> +     __builtin_choose_expr(                                          \
>> +     __builtin_types_compatible_p(typeof(_val), signed char *),      \
>> +     _parse_integer_sc(_s, _base, (void *)_val),
>> \
>
> Why the (void*) cast? Isn't _val supposed to have precisely the type
> expected by _parse_integer_sc at this point?
>
>> +     __builtin_choose_expr(                                          \
>> +     __builtin_types_compatible_p(typeof(_val), long *) && sizeof(long) == 4,\
>> +     _parse_integer_i(_s, _base, (void *)_val),                      \
>> +     __builtin_choose_expr(                                          \
>> +     __builtin_types_compatible_p(typeof(_val), long *) && sizeof(long) == 8,\
>> +     _parse_integer_ll(_s, _base, (void *)_val),                     \
>> +     __builtin_choose_expr(                                          \
>> +     __builtin_types_compatible_p(typeof(_val), unsigned long *) && sizeof(unsigned long) == 4,\
>> +     _parse_integer_u(_s, _base, (void *)_val),                      \
>> +     __builtin_choose_expr(                                          \
>> +     __builtin_types_compatible_p(typeof(_val), unsigned long *) && sizeof(unsigned long) == 8,\
>> +     _parse_integer_ull(_s, _base, (void *)_val),                    \
>
> Ah, I see. In these cases, one probably has to do a cast to pass a
> (long*) as either (int*) or (long long*) - but why not cast to the type
> actually expected by _parse_integer_* instead of the launder-anything (void*)?

First macro was written without casts at all naively thinking
that gcc will only typecheck in chosen __builtin_choose_expr branch.
But it doesn't do that, remove casts and observe million of warnings.
So I shut it up with "void *". Branch is chosen base on __b_t_c_p
expression and I don't think it is possible to sneak in incorrect pointer.

> Another thing: It may be slightly confusing that this can't be used with
> an array passed as val. This won't work:
>
> long x[1];
> rv = parse_integer(s, 0, x);
> One could argue that one should pass &x[0] instead, but since this is a
> macro, gcc doesn't really give a very helpful error (I just get "error:
> invalid initializer"). I think it can be fixed simply by declaring _val
> using typeof(&val[0]).

I'd say &x[0] is way more clear that x in this case, but objection taken.
kstrto*() works in exactly same situation after all.

>> +int _parse_integer_ull(const char *s, unsigned int base, unsigned long long *val)
>> +{
>> +     int rv;
>> +
>> +     if (*s == '-') {
>> +             return -EINVAL;
>> +     } else if (*s == '+') {
>> +             rv = __parse_integer(s + 1, base, val);
>> +             if (rv < 0)
>> +                     return rv;
>> +             return rv + 1;
>> +     } else
>> +             return __parse_integer(s, base, val);
>> +}
>> +EXPORT_SYMBOL(_parse_integer_ull);
>> +
>> +int _parse_integer_ll(const char *s, unsigned int base, long long *val)
>> +{
>> +     unsigned long long tmp;
>> +     int rv;
>> +
>> +     if (*s == '-') {
>> +             rv = __parse_integer(s + 1, base, &tmp);
>> +             if (rv < 0)
>> +                     return rv;
>> +             if ((long long)-tmp >= 0)
>> +                     return -ERANGE;
>
> Is there any reason to disallow "-0"?

No! -0 is not accepted because code is copied from kstrtoll()
which doesn't accept "-0". It is even in the testsuite:

  static void __init test_kstrtoll_fail(void)
  {
  ...
/* negative zero isn't an integer in Linux */
{"-0",  0},
{"-0",  8},
{"-0",  10},
{"-0",  16},

Frankly I don't even remember why it does that, and
no one noticed until now. libc functions accept "-0".
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