Re: [PATCH v3 4/5] string: Add [C]QChar_of() and QVoid_of()

[Alejandro Colomar <alx@kernel.org>]

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> Date: 2026-11-10 15:45:13+0000
> From: sashiko-bot@kernel.org
>
> Thank you for your contribution! Sashiko AI review found 2 potential
> issues to consider:
> - [High] string: void pointer subscript in QVoid_of() breaks Clang
>   builds

It seems I can't build with LLVM in the way that sashiko suggested in
the other mail, so I can't reproduce this:

	$ make LLVM=1
	  SYNC    include/config/auto.conf
	  HOSTCC  scripts/basic/fixdep
	clang: error: invalid linker name in argument '-fuse-ld=lld'
	make[3]: *** [scripts/Makefile.host:114: scripts/basic/fixdep] Error 1
	make[2]: *** [Makefile:682: scripts_basic] Error 2
	make[1]: *** [/srv/alx/src/linux/linux/llvm/Makefile:912: include/config/auto.conf] Error 2
	make[1]: *** [include/config/auto.conf] Deleting file 'include/generated/rustc_cfg'
	make[1]: *** [include/config/auto.conf] Deleting file 'include/generated/autoconf.h'
	make: *** [Makefile:248: __sub-make] Error 2

But I see no reason why Clang shouldn't build this.  I've written
a small test program just to see, and Clang seems to just work well:

	alx@devuan:~/tmp$ cat const.c 
	#include <stddef.h>

	#define QVoid_of(p)       typeof((__QVoidptr_of(p)){0}[0])
	#define __QVoidptr_of(p)  typeof(1?(p):(void*){0})

	int
	main(void)
	{
		const void  *p;
		QVoid_of(p) *q;  // const void *
	}
	alx@devuan:~/tmp$ gcc -Wall -Wextra -Wno-unused const.c 
	alx@devuan:~/tmp$ clang -Wall -Wextra -Wno-unused const.c 
	alx@devuan:~/tmp$ 

Unless anyone can actually reproduce the build issue, I'll assume this
report is nonsense.

> - [Medium] string: QChar_of() fails to compile when pointee has
>   qualifiers other than Q

That's actually desirable.  You don't want to call strchr(3) with
a pointer to volatile.

> --
> 
> commit cea37634307d89ede7b6ca6128f006d50a90391c
> Author: Alejandro Colomar <alx@kernel.org>
> 
> string: Add [C]QChar_of() and QVoid_of()
> 
> This commit introduces the QVoid_of(), QChar_of(), and CQChar_of() helper
> macros in include/linux/string.h to support qualifier-generic string macros.
> 
> > diff --git a/include/linux/string.h b/include/linux/string.h
[...]
> 
> [Severity: High]
> Does applying the array subscript [0] to a void pointer in QVoid_of()
> break Clang builds?

I don't see why it would.  I can't use LLVM myself, so can't reproduce,
but a small test program shows that Clang works just fine with it.

> __QVoidptr_of(p) produces a pointer to (qualified) void.  Because array
> subscripting requires a pointer to a complete object type, Clang rejects
> (__QVoidptr_of(p)){0}[0] even inside typeof() with an incomplete type
> error, and GCC emits a -Wpointer-arith warning when pointer arithmetic
> warnings are enabled.  Using unary * instead of [0], such as
> typeof(*(__QVoidptr_of(p)){0}), avoids both issues.

This is a lie, unless anyone can confirm.

[...]
> [Severity: Medium]
> Can QChar_of() fail to compile when the pointed-to type of p carries a
> qualifier other than Q?

Indeed, a desirable property.

> QVoid_of(p) preserves all qualifiers on the target of p, but the _Generic
> selection only matches exact Q void * and unqualified void *.  Passing a
> volatile char * or const volatile char * to CQChar_of(p) produces a
> volatile void * or const volatile void * controlling expression, which
> matches neither association and fails at compile time.  Matching against
> Q QVoid_of(p) *: (Q char){0} and default: (char){0} would test specifically
> for Q regardless of other qualifiers.

You certainly don't want to pass a volatile char* to strchr(3).


Alex

-- 
<https://www.alejandro-colomar.es>

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