linux-kernel - Re: [PATCH 0/6] Don't use BIT() macro in UAPI headers

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Message-ID: <cda47735-3448-9450-1bf4-51a8cb9c43b0@redhat.com>
Date:   Thu, 20 May 2021 14:09:47 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Joe Richey <joerichey94@...il.com>, trivial@...nel.org
Cc:     Joe Richey <joerichey@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Mark Rutland <mark.rutland@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Zhangfei Gao <zhangfei.gao@...aro.org>,
        Zhou Wang <wangzhou1@...ilicon.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org,
        linux-accelerators@...ts.ozlabs.org
Subject: Re: [PATCH 0/6] Don't use BIT() macro in UAPI headers

On 20/05/21 12:43, Joe Richey wrote:
> From: Joe Richey <joerichey@...gle.com>
> 
> The BIT(n) macro is used in the kernel as an alias for (1 << n).
> However, it is not defined in the UAPI headers, which means that any
> UAPI header files must be careful not to use it, or else the user
> will get a linker error. For example, compiling the following program:
> 
>      #include <sys/auxv.h>
>      #include <asm/hwcap2.h>
> 
>      // Detect if FSGSBASE instructions are enabled
>      int main() {
>          unsigned long val = getauxval(AT_HWCAP2);
>          return !(val & HWCAP2_FSGSBASE);
>      }
> 
> Results in the following likner error:
> 
>      /usr/bin/ld: /tmp/cceFpAdR.o: in function `main':
>      gs.c:(.text+0x21): undefined reference to `BIT'
> 
> This patch series changes all UAPI uses of BIT() to just be open-coded.
> However, there really should be a check for this in checkpatch.pl
> Currently, the script actually _encourages_ users to use the BIT macro
> even if adding things to UAPI.
> 
> Running `rg "BIT\(" **/uapi/**` shows no more usage of BIT() in any
> UAPI headers. Tested by building a basic kernel. Changes are trivial.
> 
> Joe Richey (6):
>    x86/elf: Don't use BIT() macro in UAPI headers
>    KVM: X86: Don't use BIT() macro in UAPI headers
>    drivers: firmware: psci: Don't use BIT() macro in UAPI headers
>    uacce: Don't use BIT() macro in UAPI headers
>    media: vicodec: Don't use BIT() macro in UAPI headers
>    tools headers UAPI: Sync pkt_sched.h with the kernel sources
> 
>   arch/x86/include/uapi/asm/hwcap2.h   |   2 +-
>   include/uapi/linux/kvm.h             |   4 +-
>   include/uapi/linux/psci.h            |   2 +-
>   include/uapi/linux/v4l2-controls.h   |  22 ++---
>   include/uapi/misc/uacce/uacce.h      |   2 +-
>   tools/include/uapi/linux/kvm.h       |   4 +-
>   tools/include/uapi/linux/pkt_sched.h | 122 ++++++++++++++++++++++++---
>   7 files changed, 130 insertions(+), 28 deletions(-)
> 

Will queue the KVM one, thanks!

Paolo