[PATCH] remove attribute access from regexec
Martin Sebor
msebor@gmail.com
Sat Aug 14 20:08:11 GMT 2021
On 8/13/21 4:34 PM, Paul Eggert wrote:
> On 8/13/21 2:30 PM, Martin Sebor wrote:
>> Attached is a revised patch with this approach.
>
> The revised patch is to include/regex.h but the original patch was to
> posix/regex.h. Is that intentional?
Yes, they need to be consistent, otherwise GCC issues -Wvla-parameter.
(That's to help detect inadvertent array/VLA mismatches as well as
mismatches in the VLA parameter bounds.)
>
> We need to check whether __STDC_VERSION__ is defined. Also, no need for
> parens around arg of 'defined'. Something like this perhaps:
>
> #if (defined __STDC_VERSION__ && 199901L <= __STDC_VERSION__ \
> && !defined __STDC_NO_VLA__)
>
> Also, the duplication of the declarations make the headers harder to
> read and encourage typos (I noticed one typo: "_Restrict_arr" without
> the trailing "_"). Instead, I suggest something like this:
>
> #if (defined __STDC_VERSION__ && 199901L <= __STDC_VERSION__ \
> && !defined __STDC_NO_VLA__)
> # define _REGEX_VLA(arg) arg
> #else
> # define _REGEX_VLA(arg)
> #endif
>
> That way, we can simply change "regmatch_t __pmatch[_Restrict_arr_]" to
> "regmatch_t __pmatch[_Restrict_arr_ _REGEX_VLA (__nmatch)]" without
> having to duplicate the entire function declaration.
Sounds good. I've defined the macro in cdefs.h and mamed it _VLA_ARG
to make it usable in other contexts. Please see the attached revision.
>
>> PS POSIX says regexec() ignores pnmatch when REG_NOSUB is set, so
>> strictly speaking, warning for such calls to it in that case is
>> also a false positive.
>
> Ouch, this casts doubt on the entire exercise. It's not simply about
> warnings: it's about the code being generated for the matcher. For
> example, for:
>
> int
> f (_Bool flag, unsigned long n, int a[n])
> {
> return n == 0 ? 0 : flag ? a[n - 1] : a[0];
> }
>
> a compiler is allowed to generate code that loads a[n - 1] even when
> FLAG is false. Similarly, if we add this VLA business to regexec, the
> generated machine code could dereference pmatch unconditionally even if
> our source code makes the dereferencing conditional on REG_NOSUB, and
> the resulting behavior would fail to conform to POSIX.
The VLA bound by itself doesn't affect codegen. I suspect you're
thinking of a[static n]? With just a[n], without static, there
is no requirement that a point to an array with n elements. It
simply declares an ordinary pointer, same as [] or *.
Martin
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