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Re: [PATCH] Add a character size parameter to c_strlen/get_range_strlen
On 08/18/18 20:01, Martin Sebor wrote:
> On 08/17/2018 05:01 PM, Bernd Edlinger wrote:
>> On 08/17/18 22:17, Martin Sebor wrote:
>>> On 08/17/2018 12:44 PM, Bernd Edlinger wrote:
>>>> On 08/17/18 20:23, Martin Sebor wrote:
>>>>> On 08/17/2018 06:14 AM, Joseph Myers wrote:
>>>>>> On Fri, 17 Aug 2018, Jeff Law wrote:
>>>>>>
>>>>>>> On 08/16/2018 05:01 PM, Joseph Myers wrote:
>>>>>>>> On Thu, 16 Aug 2018, Jeff Law wrote:
>>>>>>>>
>>>>>>>>> restores previous behavior. The sprintf bits want to count element
>>>>>>>>> sized chunks, which for wchars is 4 bytes (that count will then be
>>>>>>>>
>>>>>>>>> /* Compute the range the argument's length can be in. */
>>>>>>>>> - fmtresult slen = get_string_length (arg);
>>>>>>>>> + int count_by = dir.specifier == 'S' || dir.modifier == FMT_LEN_l ? 4 : 1;
>>>>>>>>
>>>>>>>> I don't see how a hardcoded 4 is correct here. Surely you need to example
>>>>>>>> wchar_type_node to determine its actual size for this target.
>>>>>>> We did kick this around a little. IIRC Martin didn't think that it was
>>>>>>> worth handling the 2 byte wchar case.
>>>>>
>>>>> Sorry, I think we may have miscommunicated -- I didn't think it
>>>>> was useful to pass a size of the character type to the function.
>>>>> I agree that passing in a hardcoded constant doesn't seem right
>>>>> (even if GCC's wchar_t were always 4 bytes wide).
>>>>>
>>>>> I'm still not sure I see the benefit of passing in the expected
>>>>> element size given that the patch causes c_strlen() fail when
>>>>> the actual element size doesn't match ELTSIZE. If the caller
>>>>> asks for the number of bytes in a const wchar_t array it should
>>>>> get back the number bytes. (I could see it fail if the caller
>>>>> asked for the number of words in a char array whose size was
>>>>> not evenly divisible by wordsize.)
>>>>>
>>>>
>>>> I think in this case c_strlen should use the type which the %S format
>>>> uses at runtime, otherwise it will not have anything to do with
>>>> the reality.
>>>
>>> %S is not what I'm talking about.
>>>
>>> Failing in the case I described (a caller asking for the size
>>> in bytes of a constant object whose type is larger) prevents
>>> callers that don't care about the type from detecting the actual
>>> size of a constant.
>>>
>>> Specifically for sprintf, it means that the buffer overflow
>>> below is no longer diagnosed:
>>>
>>> struct S { char a[2]; void (*pf)(void); };
>>>
>>> void f (struct S *p)
>>> {
>>> const char *q = (char*)L"\x41424344\x45464748";
>>>
>>> sprintf (p->a, "%s", q);
>>> }
>>>
>>> There may be no other analyses that would benefit from this
>>> ability today but there easily could be. There certainly
>>> are optimizations that depend on c_getstr() returning
>>> a pointer to the constant object regardless of its type
>>> (memchr being one of them).
>>>
>>
>> Yes, I agree.
>>
>> Coincidentally that is exactly what in my follow-up patch implements.
>> See: https://gcc.gnu.org/ml/gcc-patches/2018-08/msg01005.html
>>
>> If you call c_getstr(x) you get a valid zero-terminated single-byte
>> string or nothing.
>>
>> If you call c_getstr(x, &memsize) you get a pointer to a memory
>> of the specified memsize, regardless of the underlying type.
>> and whether or not zero terminated.
>
> Most functions in GCC call c_strlen() at some point to determine
> the length of a string. They need to be able to detect the missing
> nul -- it would double the amount of processing to have them also
> call c_getstr() when c_strlen() fails to see if the failure happens
> to be due to a missing nul -- it the overwhelming majority of cases
> it won't be.
>
Well, yes, if the c_strlen function fails, it would be cheap to have some
error code for the failure reason.
But that is something for later.
Bernd.