linux-kernel - Re: [PATCH next 10/14] bits: Fix assmebler expansions of GENMASK_Uxx() and BIT

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Message-ID: <aYj-LdTWwvGLJP4O@yury>
Date: Sun, 8 Feb 2026 16:20:45 -0500
From: Yury Norov <ynorov@...dia.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Nathan Chancellor <nathan@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, Yury Norov <yury.norov@...il.com>,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Jani Nikula <jani.nikula@...el.com>,
	Vincent Mailhol <mailhol.vincent@...adoo.fr>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH next 10/14] bits: Fix assmebler expansions of
 GENMASK_Uxx() and BIT_Uxx()

On Sun, Feb 08, 2026 at 11:42:14AM +0000, David Laight wrote:
> On Sat, 7 Feb 2026 22:31:34 -0500
> Yury Norov <ynorov@...dia.com> wrote:
> 
> > On Wed, Jan 21, 2026 at 02:57:27PM +0000, david.laight.linux@...il.com wrote:
> > > From: David Laight <david.laight.linux@...il.com>
> > > 
> > > The assembler only supports one type of signed integers, so expressions
> > > using BITS_PER_LONG (etc) cannot be guaranteed to be correct.
> > > 
> > > Use ((2 << (h)) - (1 << (l))) for all assembler GENMASK() expansions and
> > > add definitions of BIT_Uxx() as (1 << (nr)).
> > > 
> > > Note that 64bit results are (probably) only correct for 64bit builds
> > > and 128bits results will never be valid.  
> > 
> > And this important note will sink in git history.
> 
> At least it isn't only in the email archives.
> I can put it in a comment.
> 
> > > Signed-off-by: David Laight <david.laight.linux@...il.com>  
> > 
> > This has been discussed in details when those GENMASK_Uxx() were
> > introduced. Assembler doesn't support C types, and can't provide any
> > guarantees. It may only confuse readers when they see something like
> > GENMASK_U8() in the assembler code, and there's nothing on behalf of
> > that declaration to enforce the limitation.
> 
> It won't be in asm code, the asm code will be expanding a constant
> from a C header file.

It can be included and preprocessed well in any .S file:

#define GENMASK_TYPE(t, h,l) ((2 << (h)) - (1 << (l)))
#define GENMASK(h, l) GENMASK_TYPE(unsigned long, (h), (l))

    .section .rodata
fmt:
    .string "GENMASK(63,60) = 0x%016llx\n"

    .text
    .globl main
    .type main, @function

main:
    push    %rbp
    mov     %rsp, %rbp

    lea     fmt(%rip), %rdi
    mov     $GENMASK(63,60), %rsi
    xor     %rax, %rax
    call    printf@PLT

    mov     $0, %eax
    pop     %rbp
    ret

In C this doesn't work at all as it throws overflow. It doesn't even
work in asm volatile section.
 
> > That's why we didn't add fake C types support in the assembler. Unless
> > we find a way to enforce C types capacity in assembler(s), let's keep
> > those macros C-only.
> 
> But GENMASK_ULL() was already there and would generate invalid values
> (for small values) on 32bit.

You continuously repeat that GENMASK_ULL() generates wrong values, but
never submitted a fix.

Anyways, if you think GENMASK_ULL() is not needed in assembler, it's
even harder to advocate fixed-type flavors.

> The only reason for defining these for assembler is so that .h files
> that use the definitions can be used in .S files.
> As soon as any of the BIT_Unn() get used the asm code is likely to
> try to expand them.

The only reason for fixed-type GENMASK() and BIT() is strict
parameters checking. This is not possible in assembler.

Thanks,
Yury